On 12/2/21 08:35, Christian Brauner wrote:

On Tue, Nov 30, 2021 at 11:06:47AM -0500, Stefan Berger wrote:

quoted

Implement 'securityfs_ns' for support of IMA namespacing so that each
IMA (user) namespace can have its own front-end for showing the currently
active policy, the measurement list, number of violations and so on. This
filesystem shares much of the existing code of SecurityFS but requires a
new API call securityfs_ns_create_mount() for creating a new instance.

The API calls of securityfs_ns have the prefix securityfs_ns_ and take
additional parameters struct vfsmount * and mount_count that allow for
multiple instances of this filesystem to exist.

The filesystem can be mounted to the usual securityfs mount point like
this:

mount -t securityfs_ns /sys/kernel/security /sys/kernel/security

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
---
  include/linux/security.h   |  18 ++++
  include/uapi/linux/magic.h |   1 +
  security/inode.c           | 197 +++++++++++++++++++++++++++++++++++--
  3 files changed, 210 insertions(+), 6 deletions(-)

diff --git a/include/linux/security.h b/include/linux/security.h
index 7e0ba63b5dde..8e479266f544 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h

@@ -1929,6 +1929,24 @@ struct dentry *securityfs_create_symlink(const char *name,
  					 const struct inode_operations *iops);
  extern void securityfs_remove(struct dentry *dentry);
  
+extern struct dentry *securityfs_ns_create_file(const char *name, umode_t mode,
+						struct dentry *parent, void *data,
+						const struct file_operations *fops,
+						const struct inode_operations *iops,
+						struct vfsmount **mount, int *mount_count);
+extern struct dentry *securityfs_ns_create_dir(const char *name, struct dentry *parent,
+					       const struct inode_operations *iops,
+					       struct vfsmount **mount, int *mount_count);
+struct dentry *securityfs_ns_create_symlink(const char *name,
+					    struct dentry *parent,
+					    const char *target,
+					    const struct inode_operations *iops,
+					    struct vfsmount **mount, int *mount_count);
+extern void securityfs_ns_remove(struct dentry *dentry,
+				 struct vfsmount **mount, int *mount_count);
+struct vfsmount *securityfs_ns_create_mount(struct user_namespace *user_ns);
+extern struct vfsmount *securityfs_ns_mount;
+
  #else /* CONFIG_SECURITYFS */
  
  static inline struct dentry *securityfs_create_dir(const char *name,

diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index 35687dcb1a42..5c1cc6088dd2 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h

@@ -11,6 +11,7 @@
  #define CRAMFS_MAGIC_WEND	0x453dcd28	/* magic number with the wrong endianess */
  #define DEBUGFS_MAGIC          0x64626720
  #define SECURITYFS_MAGIC	0x73636673
+#define SECURITYFS_NS_MAGIC	0x73334473
  #define SELINUX_MAGIC		0xf97cff8c
  #define SMACK_MAGIC		0x43415d53	/* "SMAC" */
  #define RAMFS_MAGIC		0x858458f6	/* some random number */

diff --git a/security/inode.c b/security/inode.c
index 429744ff4ab3..8077d1f31489 100644
--- a/security/inode.c
+++ b/security/inode.c

@@ -21,6 +21,7 @@
  #include <linux/security.h>
  #include <linux/lsm_hooks.h>
  #include <linux/magic.h>
+#include <linux/user_namespace.h>
  
  static struct vfsmount *securityfs_mount;
  static int securityfs_mount_count;

@@ -73,6 +74,61 @@ static struct file_system_type securityfs_type = {
  	.kill_sb =	kill_litter_super,
  };
  
+static int securityfs_ns_fill_super(struct super_block *sb, struct fs_context *fc)
+{
+	static const struct tree_descr files[] = {{""}};
+	int error;
+
+	error = simple_fill_super(sb, SECURITYFS_NS_MAGIC, files);
+	if (error)
+		return error;
+
+	sb->s_op = &securityfs_super_operations;
+
+	return 0;
+}
+
+static int securityfs_ns_get_tree(struct fs_context *fc)
+{
+	return get_tree_keyed(fc, securityfs_ns_fill_super, fc->user_ns);
+}
+
+static const struct fs_context_operations securityfs_ns_context_ops = {
+	.get_tree	= securityfs_ns_get_tree,
+};
+
+static int securityfs_ns_init_fs_context(struct fs_context *fc)
+{
+	fc->ops = &securityfs_ns_context_ops;
+	return 0;
+}
+
+static struct file_system_type securityfs_ns_type = {
+	.owner			= THIS_MODULE,
+	.name			= "securityfs_ns",
+	.init_fs_context	= securityfs_ns_init_fs_context,
+	.kill_sb		= kill_litter_super,
+	.fs_flags		= FS_USERNS_MOUNT,
+};
+
+struct vfsmount *securityfs_ns_create_mount(struct user_namespace *user_ns)
+{
+	struct fs_context *fc;
+	struct vfsmount *mnt;
+
+	fc = fs_context_for_mount(&securityfs_ns_type, SB_KERNMOUNT);
+	if (IS_ERR(fc))
+		return ERR_CAST(fc);
+
+	put_user_ns(fc->user_ns);
+	fc->user_ns = get_user_ns(user_ns);
+
+	mnt = fc_mount(fc);
+	put_fs_context(fc);
+	return mnt;
+}
+
+
  /**
   * securityfs_create_dentry - create a dentry in the securityfs filesystem
   *

@@ -155,8 +211,8 @@ static struct dentry *securityfs_create_dentry(const char *name, umode_t mode,
  	inode->i_atime = inode->i_mtime = inode->i_ctime = current_time(inode);
  	inode->i_private = data;
  	if (S_ISDIR(mode)) {
-		inode->i_op = &simple_dir_inode_operations;
-		inode->i_fop = &simple_dir_operations;
+		inode->i_op = iops ? iops : &simple_dir_inode_operations;
+		inode->i_fop = fops ? fops : &simple_dir_operations;
  		inc_nlink(inode);
  		inc_nlink(dir);
  	} else if (S_ISLNK(mode)) {

@@ -214,6 +270,41 @@ struct dentry *securityfs_create_file(const char *name, umode_t mode,
  }
  EXPORT_SYMBOL_GPL(securityfs_create_file);
  
+/**
+ * securityfs_ns_create_file - create a file in the securityfs_ns filesystem
+ *
+ * @name: a pointer to a string containing the name of the file to create.
+ * @mode: the permission that the file should have
+ * @parent: a pointer to the parent dentry for this file.  This should be a
+ *          directory dentry if set.  If this parameter is %NULL, then the
+ *          file will be created in the root of the securityfs_ns filesystem.
+ * @data: a pointer to something that the caller will want to get to later
+ *        on.  The inode.i_private pointer will point to this value on
+ *        the open() call.
+ * @fops: a pointer to a struct file_operations that should be used for
+ *        this file.
+ * @mount: Pointer to a pointer of a an existing vfsmount
+ * @mount_count: The mount_count that goes along with the @mount
+ *
+ * This function creates a file in securityfs_ns with the given @name.
+ *
+ * This function returns a pointer to a dentry if it succeeds.  This
+ * pointer must be passed to the securityfs_ns_remove() function when the file
+ * is to be removed (no automatic cleanup happens if your module is unloaded,
+ * you are responsible here).  If an error occurs, the function will return
+ * the error value (via ERR_PTR).
+ */
+struct dentry *securityfs_ns_create_file(const char *name, umode_t mode,
+					 struct dentry *parent, void *data,
+					 const struct file_operations *fops,
+					 const struct inode_operations *iops,
+					 struct vfsmount **mount, int *mount_count)
+{
+	return securityfs_create_dentry(name, mode, parent, data, fops, iops,
+					&securityfs_ns_type, mount, mount_count);
+}
+EXPORT_SYMBOL_GPL(securityfs_ns_create_file);
+
  /**
   * securityfs_create_dir - create a directory in the securityfs filesystem
   *

@@ -240,6 +331,34 @@ struct dentry *securityfs_create_dir(const char *name, struct dentry *parent)
  }
  EXPORT_SYMBOL_GPL(securityfs_create_dir);
  
+/**
+ * securityfs_ns_create_dir - create a directory in the securityfs_ns filesystem
+ *
+ * @name: a pointer to a string containing the name of the directory to
+ *        create.
+ * @parent: a pointer to the parent dentry for this file.  This should be a
+ *          directory dentry if set.  If this parameter is %NULL, then the
+ *          directory will be created in the root of the securityfs_ns filesystem.
+ * @mount: Pointer to a pointer of a an existing vfsmount
+ * @mount_count: The mount_count that goes along with the @mount
+ *
+ * This function creates a directory in securityfs_ns with the given @name.
+ *
+ * This function returns a pointer to a dentry if it succeeds.  This
+ * pointer must be passed to the securityfs_ns_remove() function when the file
+ * is to be removed (no automatic cleanup happens if your module is unloaded,
+ * you are responsible here).  If an error occurs, the function will return
+ * the error value (via ERR_PTR).
+ */
+struct dentry *securityfs_ns_create_dir(const char *name, struct dentry *parent,
+					const struct inode_operations *iops,
+					struct vfsmount **mount, int *mount_count)
+{
+	return securityfs_ns_create_file(name, S_IFDIR | 0755, parent, NULL, NULL,
+					 iops, mount, mount_count);
+}
+EXPORT_SYMBOL_GPL(securityfs_ns_create_dir);
+
  struct dentry *_securityfs_create_symlink(const char *name,
  					  struct dentry *parent,
  					  const char *target,

@@ -263,6 +382,7 @@ struct dentry *_securityfs_create_symlink(const char *name,
  
  	return dent;
  }
+
  /**
   * securityfs_create_symlink - create a symlink in the securityfs filesystem
   *

@@ -300,6 +420,42 @@ struct dentry *securityfs_create_symlink(const char *name,
  }
  EXPORT_SYMBOL_GPL(securityfs_create_symlink);
  
+/**
+ * securityfs_ns_create_symlink - create a symlink in the securityfs_ns filesystem
+ *
+ * @name: a pointer to a string containing the name of the symlink to
+ *        create.
+ * @parent: a pointer to the parent dentry for the symlink.  This should be a
+ *          directory dentry if set.  If this parameter is %NULL, then the
+ *          directory will be created in the root of the securityfs_ns filesystem.
+ * @target: a pointer to a string containing the name of the symlink's target.
+ *          If this parameter is %NULL, then the @iops parameter needs to be
+ *          setup to handle .readlink and .get_link inode_operations.
+ * @iops: a pointer to the struct inode_operations to use for the symlink. If
+ *        this parameter is %NULL, then the default simple_symlink_inode
+ *        operations will be used.
+ * @mount: Pointer to a pointer of a an existing vfsmount
+ * @mount_count: The mount_count that goes along with the @mount
+ *
+ * This function creates a symlink in securityfs_ns with the given @name.
+ *
+ * This function returns a pointer to a dentry if it succeeds.  This
+ * pointer must be passed to the securityfs_ns_remove() function when the file
+ * is to be removed (no automatic cleanup happens if your module is unloaded,
+ * you are responsible here).  If an error occurs, the function will return
+ * the error value (via ERR_PTR).
+ */
+struct dentry *securityfs_ns_create_symlink(const char *name,
+					    struct dentry *parent,
+					    const char *target,
+					    const struct inode_operations *iops,
+					    struct vfsmount **mount, int *mount_count)
+{
+	return _securityfs_create_symlink(name, parent, target, iops,
+					  &securityfs_ns_type, mount, mount_count);
+}
+EXPORT_SYMBOL_GPL(securityfs_ns_create_symlink);
+
  void _securityfs_remove(struct dentry *dentry, struct vfsmount **mount, int *mount_count)
  {
  	struct inode *dir;

@@ -340,6 +496,27 @@ void securityfs_remove(struct dentry *dentry)
  
  EXPORT_SYMBOL_GPL(securityfs_remove);
  
+/**
+ * securityfs_ns_remove - removes a file or directory from the securityfs_ns filesystem
+ *
+ * @dentry: a pointer to a the dentry of the file or directory to be removed.
+ * @mount: Pointer to a pointer of a an existing vfsmount
+ * @mount_count: The mount_count that goes along with the @mount
+ *
+ * This function removes a file or directory in securityfs_ns that was previously
+ * created with a call to another securityfs_ns function (like
+ * securityfs_ns_create_file() or variants thereof.)
+ *
+ * This function is required to be called in order for the file to be
+ * removed. No automatic cleanup of files will happen when a module is
+ * removed; you are responsible here.
+ */
+void securityfs_ns_remove(struct dentry *dentry, struct vfsmount **mount, int *mount_count)
+{
+	_securityfs_remove(dentry, mount, mount_count);
+}
+EXPORT_SYMBOL_GPL(securityfs_ns_remove);
+
  #ifdef CONFIG_SECURITY
  static struct dentry *lsm_dentry;
  static ssize_t lsm_read(struct file *filp, char __user *buf, size_t count,

@@ -364,14 +541,22 @@ static int __init securityfs_init(void)
  		return retval;
  
  	retval = register_filesystem(&securityfs_type);
-	if (retval) {
-		sysfs_remove_mount_point(kernel_kobj, "security");
-		return retval;
-	}
+	if (retval)
+		goto remove_mount;
+	retval = register_filesystem(&securityfs_ns_type);
+	if (retval)
+		goto unregister_filesystem;

So you're introducing a new filesystem type securityfs_ns. Ithink that's
simply wrong and feels like a hack. What issues did you run into when
trying to convert the existing securityfs itself?

I primarily didn't want to touch the existing securityfs with its 
existing users and it being a single instance filesystem. So I though 
I'd create something with a new API just for namespaces that is 
multi-instance capable.

I see no immediate reason why a get_tree_keyed() conversion for
securityfs wouldn't work even with the debugfs pin/unpin logic in there
kept for the securityfs mounted in the initial userns.

Ok, let me try to convert securityfs then.