[PATCH 0/3] SELinux support for anonymous inodes and UFFD

Userfaultfd in unprivileged contexts could be potentially very
useful. We'd like to harden userfaultfd to make such unprivileged use
less risky. This patch series allows SELinux to manage userfaultfd
file descriptors and in the future, other kinds of
anonymous-inode-based file descriptor.  SELinux policy authors can
apply policy types to anonymous inodes by providing name-based
transition rules keyed off the anonymous inode internal name (
"[userfaultfd]" in the case of userfaultfd(2) file descriptors) and
applying policy to the new SIDs thus produced.

Inside the kernel, a pair of new anon_inodes interface,
anon_inode_getfile_secure and anon_inode_getfd_secure, allow callers
to opt into this SELinux management. In this new "secure" mode,
anon_inodes creates new ephemeral inodes for anonymous file objects
instead of reusing the normal anon_inodes singleton dummy inode. A new
LSM hook gives security modules an opportunity to configure and veto
these ephemeral inodes.

This patch series is one of two fork of [1] and is an
alternative to [2].

The primary difference between the two patch series is that this
partch series creates a unique inode for each "secure" anonymous
inode, while the other patch series ([2]) continues using the
singleton dummy anonymous inode and adds a way to attach SELinux
security information directly to file objects.

I prefer the approach in this patch series because 1) it's a smaller
patch than [2], and 2) it produces a more regular security
architecture: in this patch series, secure anonymous inodes aren't
S_PRIVATE and they maintain the SELinux property that the label for a
file is in its inode. We do need an additional inode per anonymous
file, but per-struct-file inode creation doesn't seem to be a problem
for pipes and sockets.

The previous version of this feature ([1]) created a new SELinux
security class for userfaultfd file descriptors. This version adopts
the generic transition-based approach of [2].

This patch series also differs from [2] in that it doesn't affect all
anonymous inodes right away --- instead requiring anon_inodes callers
to opt in --- but this difference isn't one of basic approach. The
important question to resolve is whether we should be creating new
inodes or enhancing per-file data.

[1] https://lore.kernel.org/lkml/20200211225547.235083-1-dancol@google.com/
[2] https://lore.kernel.org/linux-fsdevel/20200213194157.5877-1-sds@tycho.nsa.gov/

Daniel Colascione (3):
  Add a new LSM-supporting anonymous inode interface
  Teach SELinux about anonymous inodes
  Wire UFFD up to SELinux

 fs/anon_inodes.c            | 196 ++++++++++++++++++++++++++++--------
 fs/userfaultfd.c            |  34 +++++--
 include/linux/anon_inodes.h |  13 +++
 include/linux/lsm_hooks.h   |   9 ++
 include/linux/security.h    |   4 +
 security/security.c         |  10 ++
 security/selinux/hooks.c    |  57 +++++++++++
 7 files changed, 274 insertions(+), 49 deletions(-)

-- 
2.25.0.265.gbab2e86ba0-goog

This change adds two new functions, anon_inode_getfile_secure and
anon_inode_getfd_secure, that create anonymous-node files with
individual non-S_PRIVATE inodes to which security modules can apply
policy. Existing callers continue using the original singleton-inode
kind of anonymous-inode file. We can transition anonymous inode users
to the new kind of anonymous inode in individual patches for the sake
of bisection and review.

The new functions accept an optional context_inode parameter that
callers can use to provide additional contextual information to
security modules, e.g., indicating that one anonymous struct file is a
logical child of another, allowing a security model to propagate
security information from one to the other.

Signed-off-by: Daniel Colascione <redacted>
---
 fs/anon_inodes.c            | 196 ++++++++++++++++++++++++++++--------
 fs/userfaultfd.c            |   4 +-
 include/linux/anon_inodes.h |  13 +++
 include/linux/lsm_hooks.h   |   9 ++
 include/linux/security.h    |   4 +
 security/security.c         |  10 ++
 6 files changed, 191 insertions(+), 45 deletions(-)

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index 89714308c25b..114a04fc1db4 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c

@@ -55,75 +55,135 @@ static struct file_system_type anon_inode_fs_type = {
 	.kill_sb	= kill_anon_super,
 };
 
-/**
- * anon_inode_getfile - creates a new file instance by hooking it up to an
- *                      anonymous inode, and a dentry that describe the "class"
- *                      of the file
- *
- * @name:    [in]    name of the "class" of the new file
- * @fops:    [in]    file operations for the new file
- * @priv:    [in]    private data for the new file (will be file's private_data)
- * @flags:   [in]    flags
- *
- * Creates a new file by hooking it on a single inode. This is useful for files
- * that do not need to have a full-fledged inode in order to operate correctly.
- * All the files created with anon_inode_getfile() will share a single inode,
- * hence saving memory and avoiding code duplication for the file/inode/dentry
- * setup.  Returns the newly created file* or an error pointer.
- */
-struct file *anon_inode_getfile(const char *name,
-				const struct file_operations *fops,
-				void *priv, int flags)
+static struct inode *anon_inode_make_secure_inode(
+	const char *name,
+	const struct inode *context_inode,
+	const struct file_operations *fops)
+{
+	struct inode *inode;
+	const struct qstr qname = QSTR_INIT(name, strlen(name));
+	int error;
+
+	inode = alloc_anon_inode(anon_inode_mnt->mnt_sb);
+	if (IS_ERR(inode))
+		return ERR_PTR(PTR_ERR(inode));
+	inode->i_flags &= ~S_PRIVATE;
+	error =	security_inode_init_security_anon(
+		inode, &qname, fops, context_inode);
+	if (error) {
+		iput(inode);
+		return ERR_PTR(error);
+	}
+	return inode;
+}
+
+struct file *_anon_inode_getfile(const char *name,
+				 const struct file_operations *fops,
+				 void *priv, int flags,
+				 const struct inode *context_inode,
+				 bool secure)
 {
+	struct inode *inode;
 	struct file *file;
 
-	if (IS_ERR(anon_inode_inode))
-		return ERR_PTR(-ENODEV);
+	if (secure) {
+		inode =	anon_inode_make_secure_inode(
+			name, context_inode, fops);
+		if (IS_ERR(inode))
+			return ERR_PTR(PTR_ERR(inode));
+	} else {
+		inode =	anon_inode_inode;
+		if (IS_ERR(inode))
+			return ERR_PTR(-ENODEV);
+		/*
+		 * We know the anon_inode inode count is always
+		 * greater than zero, so ihold() is safe.
+		 */
+		ihold(inode);
+	}
 
-	if (fops->owner && !try_module_get(fops->owner))
-		return ERR_PTR(-ENOENT);
+	if (fops->owner && !try_module_get(fops->owner)) {
+		file = ERR_PTR(-ENOENT);
+		goto err;
+	}
 
-	/*
-	 * We know the anon_inode inode count is always greater than zero,
-	 * so ihold() is safe.
-	 */
-	ihold(anon_inode_inode);
-	file = alloc_file_pseudo(anon_inode_inode, anon_inode_mnt, name,
+	file = alloc_file_pseudo(inode, anon_inode_mnt, name,
 				 flags & (O_ACCMODE | O_NONBLOCK), fops);
 	if (IS_ERR(file))
 		goto err;
 
-	file->f_mapping = anon_inode_inode->i_mapping;
+	file->f_mapping = inode->i_mapping;
 
 	file->private_data = priv;
 
 	return file;
 
 err:
-	iput(anon_inode_inode);
+	iput(inode);
 	module_put(fops->owner);
 	return file;
 }
-EXPORT_SYMBOL_GPL(anon_inode_getfile);
 
 /**
- * anon_inode_getfd - creates a new file instance by hooking it up to an
- *                    anonymous inode, and a dentry that describe the "class"
- *                    of the file
+ * anon_inode_getfile_secure - creates a new file instance by hooking
+ *                             it up to a new anonymous inode and a
+ *                             dentry that describe the "class" of the
+ *                             file.  Make it possible to use security
+ *                             modules to control access to the
+ *                             new file.
  *
  * @name:    [in]    name of the "class" of the new file
  * @fops:    [in]    file operations for the new file
  * @priv:    [in]    private data for the new file (will be file's private_data)
- * @flags:   [in]    flags
+ * @flags:   [in]    flags for the file
+ * @anon_inode_flags: [in] flags for anon_inode*
+ *
+ * Creates a new file by hooking it on an unspecified inode. This is
+ * useful for files that do not need to have a full-fledged inode in
+ * order to operate correctly.  All the files created with
+ * anon_inode_getfile_secure() will have distinct inodes, avoiding
+ * code duplication for the file/inode/dentry setup.  Returns the
+ * newly created file* or an error pointer.
+ */
+struct file *anon_inode_getfile_secure(const char *name,
+				       const struct file_operations *fops,
+				       void *priv, int flags,
+				       const struct inode *context_inode)
+{
+	return _anon_inode_getfile(
+		name, fops, priv, flags, context_inode, true);
+}
+EXPORT_SYMBOL_GPL(anon_inode_getfile_secure);
+
+/**
+ * anon_inode_getfile - creates a new file instance by hooking it up
+ *                      to an anonymous inode and a dentry that
+ *                      describe the "class" of the file.
+ *
+ * @name:    [in]    name of the "class" of the new file
+ * @fops:    [in]    file operations for the new file
+ * @priv:    [in]    private data for the new file (will be file's private_data)
+ * @flags:   [in]    flags for the file
  *
- * Creates a new file by hooking it on a single inode. This is useful for files
+ * Creates a new file by hooking it on an unspecified inode. This is useful for files
  * that do not need to have a full-fledged inode in order to operate correctly.
- * All the files created with anon_inode_getfd() will share a single inode,
+ * All the files created with anon_inode_getfile() will share a single inode,
  * hence saving memory and avoiding code duplication for the file/inode/dentry
- * setup.  Returns new descriptor or an error code.
+ * setup.  Returns the newly created file* or an error pointer.
  */
-int anon_inode_getfd(const char *name, const struct file_operations *fops,
-		     void *priv, int flags)
+struct file *anon_inode_getfile(const char *name,
+				const struct file_operations *fops,
+				void *priv, int flags)
+{
+	return _anon_inode_getfile(name, fops, priv, flags, NULL, false);
+}
+EXPORT_SYMBOL_GPL(anon_inode_getfile);
+
+static int _anon_inode_getfd(const char *name,
+			     const struct file_operations *fops,
+			     void *priv, int flags,
+			     const struct inode *context_inode,
+			     bool secure)
 {
 	int error, fd;
 	struct file *file;

@@ -133,7 +193,8 @@ int anon_inode_getfd(const char *name, const struct file_operations *fops,
 		return error;
 	fd = error;
 
-	file = anon_inode_getfile(name, fops, priv, flags);
+	file = _anon_inode_getfile(name, fops, priv, flags, context_inode,
+				   secure);
 	if (IS_ERR(file)) {
 		error = PTR_ERR(file);
 		goto err_put_unused_fd;

@@ -146,6 +207,57 @@ int anon_inode_getfd(const char *name, const struct file_operations *fops,
 	put_unused_fd(fd);
 	return error;
 }
+
+/**
+ * anon_inode_getfd_secure - creates a new file instance by hooking it
+ *                           up to a new anonymous inode and a dentry
+ *                           that describe the "class" of the file.
+ *                           Make it possible to use security modules
+ *                           to control access to the new file.
+ *
+ * @name:    [in]    name of the "class" of the new file
+ * @fops:    [in]    file operations for the new file
+ * @priv:    [in]    private data for the new file (will be file's private_data)
+ * @flags:   [in]    flags
+ *
+ * Creates a new file by hooking it on an unspecified inode. This is
+ * useful for files that do not need to have a full-fledged inode in
+ * order to operate correctly.  All the files created with
+ * anon_inode_getfile_secure() will have distinct inodes, avoiding
+ * code duplication for the file/inode/dentry setup.  Returns a newly
+ * created file descriptor or an error code.
+ */
+int anon_inode_getfd_secure(const char *name, const struct file_operations *fops,
+			    void *priv, int flags,
+			    const struct inode *context_inode)
+{
+	return _anon_inode_getfd(name, fops, priv, flags,
+				 context_inode, true);
+}
+EXPORT_SYMBOL_GPL(anon_inode_getfd_secure);
+
+/**
+ * anon_inode_getfd - creates a new file instance by hooking it up to
+ *                    an anonymous inode and a dentry that describe
+ *                    the "class" of the file
+ *
+ * @name:    [in]    name of the "class" of the new file
+ * @fops:    [in]    file operations for the new file
+ * @priv:    [in]    private data for the new file (will be file's private_data)
+ * @flags:   [in]    flags
+ *
+ * Creates a new file by hooking it on an unspecified inode. This is
+ * useful for files that do not need to have a full-fledged inode in
+ * order to operate correctly.  All the files created with
+ * anon_inode_getfile() will use the same singleton inode, reducing
+ * memory use and avoiding code duplication for the file/inode/dentry
+ * setup.  Returns a newly created file descriptor or an error code.
+ */
+int anon_inode_getfd(const char *name, const struct file_operations *fops,
+		     void *priv, int flags)
+{
+	return _anon_inode_getfd(name, fops, priv, flags, NULL, false);
+}
 EXPORT_SYMBOL_GPL(anon_inode_getfd);
 
 static int __init anon_inode_init(void)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 37df7c9eedb1..07b0f6e03849 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c

@@ -1014,8 +1014,6 @@ static __poll_t userfaultfd_poll(struct file *file, poll_table *wait)
 	}
 }
 
-static const struct file_operations userfaultfd_fops;
-
 static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
 				  struct userfaultfd_ctx *new,
 				  struct uffd_msg *msg)

@@ -1920,7 +1918,7 @@ static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
 }
 #endif
 
-static const struct file_operations userfaultfd_fops = {
+const struct file_operations userfaultfd_fops = {
 #ifdef CONFIG_PROC_FS
 	.show_fdinfo	= userfaultfd_show_fdinfo,
 #endif

diff --git a/include/linux/anon_inodes.h b/include/linux/anon_inodes.h
index d0d7d96261ad..67bd85d92dca 100644
--- a/include/linux/anon_inodes.h
+++ b/include/linux/anon_inodes.h

@@ -10,12 +10,25 @@
 #define _LINUX_ANON_INODES_H
 
 struct file_operations;
+struct inode;
+
+struct file *anon_inode_getfile_secure(const char *name,
+				       const struct file_operations *fops,
+				       void *priv, int flags,
+				       const struct inode *context_inode);
 
 struct file *anon_inode_getfile(const char *name,
 				const struct file_operations *fops,
 				void *priv, int flags);
+
+int anon_inode_getfd_secure(const char *name,
+			    const struct file_operations *fops,
+			    void *priv, int flags,
+			    const struct inode *context_inode);
+
 int anon_inode_getfd(const char *name, const struct file_operations *fops,
 		     void *priv, int flags);
 
+
 #endif /* _LINUX_ANON_INODES_H */
 

diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 20d8cf194fb7..de5d37e388df 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h

@@ -215,6 +215,10 @@
  *	Returns 0 if @name and @value have been successfully set,
  *	-EOPNOTSUPP if no security attribute is needed, or
  *	-ENOMEM on memory allocation failure.
+ * @inode_init_security_anon:
+ *      Set up a secure anonymous inode.
+ *	Returns 0 on success. Returns -EPERM if	the security module denies
+ *	the creation of this inode.
  * @inode_create:
  *	Check permission to create a regular file.
  *	@dir contains inode structure of the parent of the new file.

@@ -1552,6 +1556,10 @@ union security_list_options {
 					const struct qstr *qstr,
 					const char **name, void **value,
 					size_t *len);
+	int (*inode_init_security_anon)(struct inode *inode,
+					const struct qstr *name,
+					const struct file_operations *fops,
+					const struct inode *context_inode);
 	int (*inode_create)(struct inode *dir, struct dentry *dentry,
 				umode_t mode);
 	int (*inode_link)(struct dentry *old_dentry, struct inode *dir,

@@ -1884,6 +1892,7 @@ struct security_hook_heads {
 	struct hlist_head inode_alloc_security;
 	struct hlist_head inode_free_security;
 	struct hlist_head inode_init_security;
+	struct hlist_head inode_init_security_anon;
 	struct hlist_head inode_create;
 	struct hlist_head inode_link;
 	struct hlist_head inode_unlink;

diff --git a/include/linux/security.h b/include/linux/security.h
index 64b19f050343..8ea76af0be7a 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h

@@ -320,6 +320,10 @@ void security_inode_free(struct inode *inode);
 int security_inode_init_security(struct inode *inode, struct inode *dir,
 				 const struct qstr *qstr,
 				 initxattrs initxattrs, void *fs_data);
+int security_inode_init_security_anon(struct inode *inode,
+				      const struct qstr *name,
+				      const struct file_operations *fops,
+				      const struct inode *context_inode);
 int security_old_inode_init_security(struct inode *inode, struct inode *dir,
 				     const struct qstr *qstr, const char **name,
 				     void **value, size_t *len);

diff --git a/security/security.c b/security/security.c
index 565bc9b67276..d06f3969c030 100644
--- a/security/security.c
+++ b/security/security.c

@@ -1033,6 +1033,16 @@ int security_inode_init_security(struct inode *inode, struct inode *dir,
 }
 EXPORT_SYMBOL(security_inode_init_security);
 
+int
+security_inode_init_security_anon(struct inode *inode,
+				  const struct qstr *name,
+				  const struct file_operations *fops,
+				  const struct inode *context_inode)
+{
+	return call_int_hook(inode_init_security_anon, 0, inode, name,
+			     fops, context_inode);
+}
+
 int security_old_inode_init_security(struct inode *inode, struct inode *dir,
 				     const struct qstr *qstr, const char **name,
 				     void **value, size_t *len)

-- 
2.25.0.265.gbab2e86ba0-goog

This change uses the anon_inodes and LSM infrastructure introduced in
the previous patch to give SELinux the ability to control
anonymous-inode files that are created using the new _secure()
anon_inodes functions.

A SELinux policy author detects and controls these anonymous inodes by
adding a name-based type_transition rule that assigns a new security
type to anonymous-inode files created in some domain. The name used
for the name-based transition is the name associated with the
anonymous inode for file listings --- e.g., "[userfaultfd]" or
"[perf_event]".

Example:

type uffd_t;
type_transition sysadm_t sysadm_t : file uffd_t "[userfaultfd]";
allow sysadm_t uffd_t:file { create };

(The next patch in this series is necessary for making userfaultfd
support this new interface.  The example above is just
for exposition.)

Signed-off-by: Daniel Colascione <redacted>
---
 security/selinux/hooks.c | 57 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 1659b59fb5d7..6de0892620b3 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c

@@ -2915,6 +2915,62 @@ static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
 	return 0;
 }
 
+static int selinux_inode_init_security_anon(struct inode *inode,
+					    const struct qstr *name,
+					    const struct file_operations *fops,
+					    const struct inode *context_inode)
+{
+	const struct task_security_struct *tsec = selinux_cred(current_cred());
+	struct common_audit_data ad;
+	struct inode_security_struct *isec;
+	int rc;
+
+	if (unlikely(IS_PRIVATE(inode)))
+		return 0;
+
+	if (unlikely(!selinux_state.initialized))
+		return 0;
+
+	isec = selinux_inode(inode);
+
+	/*
+	 * We only get here once per ephemeral inode.  The inode has
+	 * been initialized via inode_alloc_security but is otherwise
+	 * untouched.
+	 */
+
+	if (context_inode) {
+		struct inode_security_struct *context_isec =
+			selinux_inode(context_inode);
+		if (IS_ERR(context_isec))
+			return PTR_ERR(context_isec);
+		isec->sid = context_isec->sid;
+	} else {
+		rc = security_transition_sid(
+			&selinux_state, tsec->sid, tsec->sid,
+			SECCLASS_FILE, name, &isec->sid);
+		if (rc)
+			return rc;
+	}
+
+	isec->initialized = LABEL_INITIALIZED;
+
+	/*
+	 * Now that we've initialized security, check whether we're
+	 * allowed to actually create this type of anonymous inode.
+	 */
+
+	ad.type = LSM_AUDIT_DATA_INODE;
+	ad.u.inode = inode;
+
+	return avc_has_perm(&selinux_state,
+			    tsec->sid,
+			    isec->sid,
+			    isec->sclass,
+			    FILE__CREATE,
+			    &ad);
+}
+
 static int selinux_inode_create(struct inode *dir, struct dentry *dentry, umode_t mode)
 {
 	return may_create(dir, dentry, SECCLASS_FILE);

@@ -6923,6 +6979,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
 
 	LSM_HOOK_INIT(inode_free_security, selinux_inode_free_security),
 	LSM_HOOK_INIT(inode_init_security, selinux_inode_init_security),
+	LSM_HOOK_INIT(inode_init_security_anon, selinux_inode_init_security_anon),
 	LSM_HOOK_INIT(inode_create, selinux_inode_create),
 	LSM_HOOK_INIT(inode_link, selinux_inode_link),
 	LSM_HOOK_INIT(inode_unlink, selinux_inode_unlink),

-- 
2.25.0.265.gbab2e86ba0-goog

On 2/13/20 10:26 PM, Daniel Colascione wrote:

This is not possible since the caller clears S_PRIVATE before calling 
and it would be a bug to call the hook on an inode that was intended to 
be private, so we shouldn't check it here.

+
+	if (unlikely(!selinux_state.initialized))
+		return 0;

Are we doing this to defer initialization until selinux_complete_init() 
- that's normally why we bail in the !initialized case?  Not entirely 
sure what will happen in such a situation since we won't have the 
context_inode or the allocating task information at that time, so we 
certainly won't get the same result - probably they would all be labeled 
with whatever anon_inodefs is assigned via genfscon or 
SECINITSID_UNLABELED by default.  If we instead just drop this test and 
proceed, we'll inherit the context inode SID if specified or we'll call 
security_transition_sid(), which in the !initialized case will just 
return the tsid i.e. tsec->sid, so it will be labeled with the creating 
task SID (SECINITSID_KERNEL prior to initialization).  Then the 
avc_has_perm() call will pass because everything gets allowed until 
initialization. So you could drop this check and userfaultfds created 
before policy load would get the kernel SID, or you can keep it and they 
will get the unlabeled SID.  Preference?

+
+	isec = selinux_inode(inode);
+
+	/*
+	 * We only get here once per ephemeral inode.  The inode has
+	 * been initialized via inode_alloc_security but is otherwise
+	 * untouched.
+	 */
+
+	if (context_inode) {
+		struct inode_security_struct *context_isec =
+			selinux_inode(context_inode);
+		if (IS_ERR(context_isec))
+			return PTR_ERR(context_isec);

This isn't possible AFAICT so you don't need to test for it or handle 
it.  In fact, even the test for NULL in selinux_inode() is bogus and 
should get dropped AFAICT; we always allocate an inode security blob 
even before policy load so it would be a bug if we ever had a NULL there.

+		isec->sid = context_isec->sid;
+	} else {
+		rc = security_transition_sid(
+			&selinux_state, tsec->sid, tsec->sid,
+			SECCLASS_FILE, name, &isec->sid);
+		if (rc)
+			return rc;
+	}

Since you switched to using security_transition_sid(), you are not using 
the fops parameter anymore nor comparing with userfaultfd_fops, so you 
could drop the parameter from the hook and leave the latter static in 
the first patch.
That's assuming you are ok with having to define these type_transition 
rules for the userfaultfd case instead of having your own separate 
security class.  Wondering how many different anon inode names/classes 
there are in the kernel today and how much they change over time; for a 
small, relatively stable set, separate classes might be ok; for a large, 
dynamic set, type transitions should scale better.  We might still need 
to create a mapping table in SELinux from the names to some stable 
identifier for the policy lookup if we can't rely on the names being stable.

On Fri, Feb 14, 2020 at 8:38 AM Stephen Smalley [off-list ref] wrote:

On 2/13/20 10:26 PM, Daniel Colascione wrote:

quoted

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 1659b59fb5d7..6de0892620b3 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c

@@ -2915,6 +2915,62 @@ static int selinux_inode_init_security(struct inode *inode, struct inode *dir,
      return 0;
  }

+static int selinux_inode_init_security_anon(struct inode *inode,
+                                         const struct qstr *name,
+                                         const struct file_operations *fops,
+                                         const struct inode *context_inode)
+{
+     const struct task_security_struct *tsec = selinux_cred(current_cred());
+     struct common_audit_data ad;
+     struct inode_security_struct *isec;
+     int rc;
+
+     if (unlikely(IS_PRIVATE(inode)))
+             return 0;

This is not possible since the caller clears S_PRIVATE before calling
and it would be a bug to call the hook on an inode that was intended to
be private, so we shouldn't check it here.

quoted

+
+     if (unlikely(!selinux_state.initialized))
+             return 0;

Are we doing this to defer initialization until selinux_complete_init()
- that's normally why we bail in the !initialized case?  Not entirely
sure what will happen in such a situation since we won't have the
context_inode or the allocating task information at that time, so we
certainly won't get the same result - probably they would all be labeled
with whatever anon_inodefs is assigned via genfscon or
SECINITSID_UNLABELED by default.
If we instead just drop this test and
proceed, we'll inherit the context inode SID if specified or we'll call
security_transition_sid(), which in the !initialized case will just
return the tsid i.e. tsec->sid, so it will be labeled with the creating
task SID (SECINITSID_KERNEL prior to initialization).  Then the
avc_has_perm() call will pass because everything gets allowed until
initialization. So you could drop this check and userfaultfds created
before policy load would get the kernel SID, or you can keep it and they
will get the unlabeled SID.  Preference?

The kernel SID seems safer. Thanks for the explanation!

quoted

+
+     isec = selinux_inode(inode);
+
+     /*
+      * We only get here once per ephemeral inode.  The inode has
+      * been initialized via inode_alloc_security but is otherwise
+      * untouched.
+      */
+
+     if (context_inode) {
+             struct inode_security_struct *context_isec =
+                     selinux_inode(context_inode);
+             if (IS_ERR(context_isec))
+                     return PTR_ERR(context_isec);

This isn't possible AFAICT so you don't need to test for it or handle
it.  In fact, even the test for NULL in selinux_inode() is bogus and
should get dropped AFAICT; we always allocate an inode security blob
even before policy load so it would be a bug if we ever had a NULL there.

Thanks. Will fix.

quoted

+             isec->sid = context_isec->sid;
+     } else {
+             rc = security_transition_sid(
+                     &selinux_state, tsec->sid, tsec->sid,
+                     SECCLASS_FILE, name, &isec->sid);
+             if (rc)
+                     return rc;
+     }

Since you switched to using security_transition_sid(), you are not using
the fops parameter anymore nor comparing with userfaultfd_fops, so you
could drop the parameter from the hook and leave the latter static in
the first patch.

That's true, but I figured different LSMs might want different rules
that depend on the fops. I'm also okay with removing this parameter
for now, since we're not using it.

That's assuming you are ok with having to define these type_transition
rules for the userfaultfd case instead of having your own separate
security class.  Wondering how many different anon inode names/classes
there are in the kernel today and how much they change over time; for a
small, relatively stable set, separate classes might be ok; for a large,
dynamic set, type transitions should scale better.

I think we can get away without a class per anonymous-inode-type. I do
wonder whether we need a class for all anonymous inodes, though: if we
just give them the file class and use the anon inode type name for the
type_transition rule, isn't it possible that the type_transition rule
might also fire for plain files with the same names in the last path
component and the same originating sid? (Maybe I'm not understanding
type_transition rules properly.) Using a class to encompass all
anonymous inodes would address this problem (assuming the problem
exists in the first place).

We might still need
to create a mapping table in SELinux from the names to some stable
identifier for the policy lookup if we can't rely on the names being stable.

Sure. The anonymous inode type names have historically been stable,
though, so maybe we can just use the names from anon_inodes directly
for now and then add some kind of remapping if we want to change those
names in the core, remaping to the old name for SELinux
type_transition purposes.

On 2/14/20 12:21 PM, Daniel Colascione wrote:

On Fri, Feb 14, 2020 at 8:38 AM Stephen Smalley [off-list ref] wrote:

quoted

That's assuming you are ok with having to define these type_transition
rules for the userfaultfd case instead of having your own separate
security class.  Wondering how many different anon inode names/classes
there are in the kernel today and how much they change over time; for a
small, relatively stable set, separate classes might be ok; for a large,
dynamic set, type transitions should scale better.

I think we can get away without a class per anonymous-inode-type. I do
wonder whether we need a class for all anonymous inodes, though: if we
just give them the file class and use the anon inode type name for the
type_transition rule, isn't it possible that the type_transition rule
might also fire for plain files with the same names in the last path
component and the same originating sid? (Maybe I'm not understanding
type_transition rules properly.) Using a class to encompass all
anonymous inodes would address this problem (assuming the problem
exists in the first place).

It shouldn't fire for non-anon inodes because on a (non-anon) file 
creation, security_transition_sid() is passed the parent directory SID 
as the second argument and we only assign task SIDs to /proc/pid 
directories, which don't support (userspace) file creation anyway.

However, in the absence of a matching type_transition rule, we'll end up 
defaulting to the task SID on the anon inode, and without a separate 
class we won't be able to distinguish it from a /proc/pid inode.  So 
that might justify a separate anoninode or similar class.

This however reminded me that for the context_inode case, we not only 
want to inherit the SID but also the sclass from the context_inode. 
That is so that anon inodes created via device node ioctls inherit the 
same SID/class pair as the device node and a single allowx rule can 
govern all ioctl commands on that device.

On 2/14/20 1:02 PM, Stephen Smalley wrote:

It shouldn't fire for non-anon inodes because on a (non-anon) file 
creation, security_transition_sid() is passed the parent directory SID 
as the second argument and we only assign task SIDs to /proc/pid 
directories, which don't support (userspace) file creation anyway.

However, in the absence of a matching type_transition rule, we'll end up 
defaulting to the task SID on the anon inode, and without a separate 
class we won't be able to distinguish it from a /proc/pid inode.  So 
that might justify a separate anoninode or similar class.

This however reminded me that for the context_inode case, we not only 
want to inherit the SID but also the sclass from the context_inode. That 
is so that anon inodes created via device node ioctls inherit the same 
SID/class pair as the device node and a single allowx rule can govern 
all ioctl commands on that device.

At least that's the way our patch worked with the /dev/kvm example. 
However, if we are introducing a separate anoninode class for the 
type_transition case, maybe we should apply that to all anon inodes 
regardless of how they are labeled (based on context_inode or 
transition) and then we'd need to write two allowx rules, one for ioctls 
on the original device node and one for those on anon inodes created 
from it.  Not sure how Android wants to handle that as the original 
developer and primary user of SELinux ioctl whitelisting.

On 2/14/20 1:08 PM, Stephen Smalley wrote:

On 2/14/20 1:02 PM, Stephen Smalley wrote:

quoted

It shouldn't fire for non-anon inodes because on a (non-anon) file 
creation, security_transition_sid() is passed the parent directory SID 
as the second argument and we only assign task SIDs to /proc/pid 
directories, which don't support (userspace) file creation anyway.

However, in the absence of a matching type_transition rule, we'll end 
up defaulting to the task SID on the anon inode, and without a 
separate class we won't be able to distinguish it from a /proc/pid 
inode.  So that might justify a separate anoninode or similar class.

This however reminded me that for the context_inode case, we not only 
want to inherit the SID but also the sclass from the context_inode. 
That is so that anon inodes created via device node ioctls inherit the 
same SID/class pair as the device node and a single allowx rule can 
govern all ioctl commands on that device.

At least that's the way our patch worked with the /dev/kvm example. 
However, if we are introducing a separate anoninode class for the 
type_transition case, maybe we should apply that to all anon inodes 
regardless of how they are labeled (based on context_inode or 
transition) and then we'd need to write two allowx rules, one for ioctls 
on the original device node and one for those on anon inodes created 
from it.  Not sure how Android wants to handle that as the original 
developer and primary user of SELinux ioctl whitelisting.

I would tentatively argue for inheriting both sclass and SID from the 
context_inode for the sake of sane policy writing.  In the userfaultfd 
case, that will still end up using the new SECCLASS_ANONINODE or 
whatever since the sclass will be initially set to that value for the 
transition SID case and then inherited on fork.  But for /dev/kvm, it 
would be the class from the /dev/kvm inode, i.e. SECCLASS_CHR_FILE.

This change gives userfaultfd file descriptors a real security
context, allowing policy to act on them.

Signed-off-by: Daniel Colascione <redacted>
---
 fs/userfaultfd.c | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 07b0f6e03849..06e92697aba4 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c

@@ -76,6 +76,8 @@ struct userfaultfd_ctx {
 	bool mmap_changing;
 	/* mm with one ore more vmas attached to this userfaultfd_ctx */
 	struct mm_struct *mm;
+	/* The inode that owns this context --- not a strong reference.  */
+	const struct inode *owner;
 };
 
 struct userfaultfd_fork_ctx {

@@ -1020,8 +1022,10 @@ static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
 {
 	int fd;
 
-	fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, new,
-			      O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS));
+	fd = anon_inode_getfd_secure(
+		"[userfaultfd]", &userfaultfd_fops, new,
+		O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS),
+		ctx->owner);
 	if (fd < 0)
 		return fd;
 

@@ -1943,6 +1947,7 @@ static void init_once_userfaultfd_ctx(void *mem)
 
 SYSCALL_DEFINE1(userfaultfd, int, flags)
 {
+	struct file *file;
 	struct userfaultfd_ctx *ctx;
 	int fd;
 

@@ -1972,8 +1977,25 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
 	/* prevent the mm struct to be freed */
 	mmgrab(ctx->mm);
 
-	fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, ctx,
-			      O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
+	file = anon_inode_getfile_secure(
+		"[userfaultfd]", &userfaultfd_fops, ctx,
+		O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS),
+		NULL);
+	if (IS_ERR(file)) {
+		fd = PTR_ERR(file);
+		goto out;
+	}
+
+	fd = get_unused_fd_flags(O_RDONLY | O_CLOEXEC);
+	if (fd < 0) {
+		fput(file);
+		goto out;
+	}
+
+	ctx->owner = file_inode(file);
+	fd_install(fd, file);
+
+out:
 	if (fd < 0) {
 		mmdrop(ctx->mm);
 		kmem_cache_free(userfaultfd_ctx_cachep, ctx);

-- 
2.25.0.265.gbab2e86ba0-goog

Userfaultfd in unprivileged contexts could be potentially very
useful. We'd like to harden userfaultfd to make such unprivileged use
less risky. This patch series allows SELinux to manage userfaultfd
file descriptors and in the future, other kinds of
anonymous-inode-based file descriptor.  SELinux policy authors can
apply policy types to anonymous inodes by providing name-based
transition rules keyed off the anonymous inode internal name (
"[userfaultfd]" in the case of userfaultfd(2) file descriptors) and
applying policy to the new SIDs thus produced.

Inside the kernel, a pair of new anon_inodes interface,
anon_inode_getfile_secure and anon_inode_getfd_secure, allow callers
to opt into this SELinux management. In this new "secure" mode,
anon_inodes creates new ephemeral inodes for anonymous file objects
instead of reusing the normal anon_inodes singleton dummy inode. A new
LSM hook gives security modules an opportunity to configure and veto
these ephemeral inodes.

This patch series is one of two fork of [1] and is an
alternative to [2].

The primary difference between the two patch series is that this
partch series creates a unique inode for each "secure" anonymous
inode, while the other patch series ([2]) continues using the
singleton dummy anonymous inode and adds a way to attach SELinux
security information directly to file objects.

I prefer the approach in this patch series because 1) it's a smaller
patch than [2], and 2) it produces a more regular security
architecture: in this patch series, secure anonymous inodes aren't
S_PRIVATE and they maintain the SELinux property that the label for a
file is in its inode. We do need an additional inode per anonymous
file, but per-struct-file inode creation doesn't seem to be a problem
for pipes and sockets.

The previous version of this feature ([1]) created a new SELinux
security class for userfaultfd file descriptors. This version adopts
the generic transition-based approach of [2].

This patch series also differs from [2] in that it doesn't affect all
anonymous inodes right away --- instead requiring anon_inodes callers
to opt in --- but this difference isn't one of basic approach. The
important question to resolve is whether we should be creating new
inodes or enhancing per-file data.

Changes from the first version of the patch:

  - Removed some error checks
  - Defined a new anon_inode SELinux class to resolve the
    ambiguity in [3]
  - Inherit sclass as well as descriptor from context inode

[1] https://lore.kernel.org/lkml/20200211225547.235083-1-dancol@google.com/
[2] https://lore.kernel.org/linux-fsdevel/20200213194157.5877-1-sds@tycho.nsa.gov/
[3] https://lore.kernel.org/lkml/23f725ca-5b5a-5938-fcc8-5bbbfc9ba9bc@tycho.nsa.gov/

Daniel Colascione (3):
  Add a new LSM-supporting anonymous inode interface
  Teach SELinux about anonymous inodes
  Wire UFFD up to SELinux

 fs/anon_inodes.c                    | 196 ++++++++++++++++++++++------
 fs/userfaultfd.c                    |  30 ++++-
 include/linux/anon_inodes.h         |  13 ++
 include/linux/lsm_hooks.h           |   9 ++
 include/linux/security.h            |   4 +
 security/security.c                 |  10 ++
 security/selinux/hooks.c            |  54 ++++++++
 security/selinux/include/classmap.h |   2 +
 8 files changed, 272 insertions(+), 46 deletions(-)

-- 
2.25.1.696.g5e7596f4ac-goog

This change adds two new functions, anon_inode_getfile_secure and
anon_inode_getfd_secure, that create anonymous-node files with
individual non-S_PRIVATE inodes to which security modules can apply
policy. Existing callers continue using the original singleton-inode
kind of anonymous-inode file. We can transition anonymous inode users
to the new kind of anonymous inode in individual patches for the sake
of bisection and review.

The new functions accept an optional context_inode parameter that
callers can use to provide additional contextual information to
security modules, e.g., indicating that one anonymous struct file is a
logical child of another, allowing a security model to propagate
security information from one to the other.

Signed-off-by: Daniel Colascione <redacted>
---
 fs/anon_inodes.c            | 196 ++++++++++++++++++++++++++++--------
 fs/userfaultfd.c            |   4 +-
 include/linux/anon_inodes.h |  13 +++
 include/linux/lsm_hooks.h   |   9 ++
 include/linux/security.h    |   4 +
 security/security.c         |  10 ++
 6 files changed, 191 insertions(+), 45 deletions(-)

diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
index 89714308c25b..114a04fc1db4 100644
--- a/fs/anon_inodes.c
+++ b/fs/anon_inodes.c

@@ -55,75 +55,135 @@ static struct file_system_type anon_inode_fs_type = {
 	.kill_sb	= kill_anon_super,
 };
 
-/**
- * anon_inode_getfile - creates a new file instance by hooking it up to an
- *                      anonymous inode, and a dentry that describe the "class"
- *                      of the file
- *
- * @name:    [in]    name of the "class" of the new file
- * @fops:    [in]    file operations for the new file
- * @priv:    [in]    private data for the new file (will be file's private_data)
- * @flags:   [in]    flags
- *
- * Creates a new file by hooking it on a single inode. This is useful for files
- * that do not need to have a full-fledged inode in order to operate correctly.
- * All the files created with anon_inode_getfile() will share a single inode,
- * hence saving memory and avoiding code duplication for the file/inode/dentry
- * setup.  Returns the newly created file* or an error pointer.
- */
-struct file *anon_inode_getfile(const char *name,
-				const struct file_operations *fops,
-				void *priv, int flags)
+static struct inode *anon_inode_make_secure_inode(
+	const char *name,
+	const struct inode *context_inode,
+	const struct file_operations *fops)
+{
+	struct inode *inode;
+	const struct qstr qname = QSTR_INIT(name, strlen(name));
+	int error;
+
+	inode = alloc_anon_inode(anon_inode_mnt->mnt_sb);
+	if (IS_ERR(inode))
+		return ERR_PTR(PTR_ERR(inode));
+	inode->i_flags &= ~S_PRIVATE;
+	error =	security_inode_init_security_anon(
+		inode, &qname, fops, context_inode);
+	if (error) {
+		iput(inode);
+		return ERR_PTR(error);
+	}
+	return inode;
+}
+
+struct file *_anon_inode_getfile(const char *name,
+				 const struct file_operations *fops,
+				 void *priv, int flags,
+				 const struct inode *context_inode,
+				 bool secure)
 {
+	struct inode *inode;
 	struct file *file;
 
-	if (IS_ERR(anon_inode_inode))
-		return ERR_PTR(-ENODEV);
+	if (secure) {
+		inode =	anon_inode_make_secure_inode(
+			name, context_inode, fops);
+		if (IS_ERR(inode))
+			return ERR_PTR(PTR_ERR(inode));
+	} else {
+		inode =	anon_inode_inode;
+		if (IS_ERR(inode))
+			return ERR_PTR(-ENODEV);
+		/*
+		 * We know the anon_inode inode count is always
+		 * greater than zero, so ihold() is safe.
+		 */
+		ihold(inode);
+	}
 
-	if (fops->owner && !try_module_get(fops->owner))
-		return ERR_PTR(-ENOENT);
+	if (fops->owner && !try_module_get(fops->owner)) {
+		file = ERR_PTR(-ENOENT);
+		goto err;
+	}
 
-	/*
-	 * We know the anon_inode inode count is always greater than zero,
-	 * so ihold() is safe.
-	 */
-	ihold(anon_inode_inode);
-	file = alloc_file_pseudo(anon_inode_inode, anon_inode_mnt, name,
+	file = alloc_file_pseudo(inode, anon_inode_mnt, name,
 				 flags & (O_ACCMODE | O_NONBLOCK), fops);
 	if (IS_ERR(file))
 		goto err;
 
-	file->f_mapping = anon_inode_inode->i_mapping;
+	file->f_mapping = inode->i_mapping;
 
 	file->private_data = priv;
 
 	return file;
 
 err:
-	iput(anon_inode_inode);
+	iput(inode);
 	module_put(fops->owner);
 	return file;
 }
-EXPORT_SYMBOL_GPL(anon_inode_getfile);
 
 /**
- * anon_inode_getfd - creates a new file instance by hooking it up to an
- *                    anonymous inode, and a dentry that describe the "class"
- *                    of the file
+ * anon_inode_getfile_secure - creates a new file instance by hooking
+ *                             it up to a new anonymous inode and a
+ *                             dentry that describe the "class" of the
+ *                             file.  Make it possible to use security
+ *                             modules to control access to the
+ *                             new file.
  *
  * @name:    [in]    name of the "class" of the new file
  * @fops:    [in]    file operations for the new file
  * @priv:    [in]    private data for the new file (will be file's private_data)
- * @flags:   [in]    flags
+ * @flags:   [in]    flags for the file
+ * @anon_inode_flags: [in] flags for anon_inode*
+ *
+ * Creates a new file by hooking it on an unspecified inode. This is
+ * useful for files that do not need to have a full-fledged inode in
+ * order to operate correctly.  All the files created with
+ * anon_inode_getfile_secure() will have distinct inodes, avoiding
+ * code duplication for the file/inode/dentry setup.  Returns the
+ * newly created file* or an error pointer.
+ */
+struct file *anon_inode_getfile_secure(const char *name,
+				       const struct file_operations *fops,
+				       void *priv, int flags,
+				       const struct inode *context_inode)
+{
+	return _anon_inode_getfile(
+		name, fops, priv, flags, context_inode, true);
+}
+EXPORT_SYMBOL_GPL(anon_inode_getfile_secure);
+
+/**
+ * anon_inode_getfile - creates a new file instance by hooking it up
+ *                      to an anonymous inode and a dentry that
+ *                      describe the "class" of the file.
+ *
+ * @name:    [in]    name of the "class" of the new file
+ * @fops:    [in]    file operations for the new file
+ * @priv:    [in]    private data for the new file (will be file's private_data)
+ * @flags:   [in]    flags for the file
  *
- * Creates a new file by hooking it on a single inode. This is useful for files
+ * Creates a new file by hooking it on an unspecified inode. This is useful for files
  * that do not need to have a full-fledged inode in order to operate correctly.
- * All the files created with anon_inode_getfd() will share a single inode,
+ * All the files created with anon_inode_getfile() will share a single inode,
  * hence saving memory and avoiding code duplication for the file/inode/dentry
- * setup.  Returns new descriptor or an error code.
+ * setup.  Returns the newly created file* or an error pointer.
  */
-int anon_inode_getfd(const char *name, const struct file_operations *fops,
-		     void *priv, int flags)
+struct file *anon_inode_getfile(const char *name,
+				const struct file_operations *fops,
+				void *priv, int flags)
+{
+	return _anon_inode_getfile(name, fops, priv, flags, NULL, false);
+}
+EXPORT_SYMBOL_GPL(anon_inode_getfile);
+
+static int _anon_inode_getfd(const char *name,
+			     const struct file_operations *fops,
+			     void *priv, int flags,
+			     const struct inode *context_inode,
+			     bool secure)
 {
 	int error, fd;
 	struct file *file;