"Serge E. Hallyn" [off-list ref] writes:

Root in a non-initial user ns cannot be trusted to write a traditional
security.capability xattr.  If it were allowed to do so, then any
unprivileged user on the host could map his own uid to root in a private
namespace, write the xattr, and execute the file with privilege on the
host.

However supporting file capabilities in a user namespace is very
desirable.  Not doing so means that any programs designed to run with
limited privilege must continue to support other methods of gaining and
dropping privilege.  For instance a program installer must detect
whether file capabilities can be assigned, and assign them if so but set
setuid-root otherwise.  The program in turn must know how to drop
partial capabilities, and do so only if setuid-root.

This patch introduces v3 of the security.capability xattr.  It builds a
vfs_ns_cap_data struct by appending a uid_t rootid to struct
vfs_cap_data.  This is the absolute uid_t (that is, the uid_t in user
namespace which mounted the filesystem, usually init_user_ns) of the
root id in whose namespaces the file capabilities may take effect.

When a task asks to write a v2 security.capability xattr, if it is
privileged with respect to the userns which mounted the filesystem, then
nothing should change.  Otherwise, the kernel will transparently rewrite
the xattr as a v3 with the appropriate rootid.  Subsequently, any task
executing the file which has the noted kuid as its root uid, or which is
in a descendent user_ns of such a user_ns, will run the file with
capabilities.

Similarly when asking to read file capabilities, a v3 capability will
be presented as v2 if it applies to the caller's namespace.

If a task writes a v3 security.capability, then it can provide a uid for
the xattr so long as the uid is valid in its own user namespace, and it
is privileged with CAP_SETFCAP over its namespace.  The kernel will
translate that rootid to an absolute uid, and write that to disk.  After
this, a task in the writer's namespace will not be able to use those
capabilities (unless rootid was 0), but a task in a namespace where the
given uid is root will.

Only a single security.capability xattr may exist at a time for a given
file.  A task may overwrite an existing xattr so long as it is
privileged over the inode.  Note this is a departure from previous
semantics, which required privilege to remove a security.capability
xattr.  This check can be re-added if deemed useful.

This allows a simple setcap/setxattr to work, should allow tar to work,
and should allow us to support tar in one namespace and untar in another
while preserving the capability, without risking leaking privilege into
a parent namespace.

A patch to linux-test-project adding a new set of tests for this
functionality is in the nsfscaps branch at github.com/hallyn/ltp

Changelog:
   Nov 02 2016: fix invalid check at refuse_fcap_overwrite()
   Nov 07 2016: convert rootid from and to fs user_ns
   (From ebiederm: mar 28 2017)
     commoncap.c: fix typos - s/v4/v3
     get_vfs_caps_from_disk: clarify the fs_ns root access check
     nsfscaps: change the code split for cap_inode_setxattr()
   Apr 09 2017:
       don't return v3 cap for caps owned by current root.
      return a v2 cap for a true v2 cap in non-init ns
   Apr 18 2017:
      . Change the flow of fscap writing to support s_user_ns writing.
      . Remove refuse_fcap_overwrite().  The value of the previous
        xattr doesn't matter.

Overall this looks quite reasonable.

My only big concern was the lack of verifying of magic_etc.  As without
that the code might not be future compatible with new versions of the
capability xattrs.  It it tends to be nice to be able to boot old
kernels when regression testing etc.  Even if they can't make use of
all of the features of a new filesystem.

Why is the conversion in __vfs_setxattr_noperm and not in setattr as
was done for posix_acl_fix_xattr_from_user?

The rest of my comments I am going to express as an incremental diff.
Using "security.capability" directly looks like a typo waiting to
happen.

You have an unnecessary include of linux/uidgid.h

Missing version checks on the magic_etc field.
And the wrong error code when the code deliberately refuses to return a
capability.

Eric

diff --git a/fs/xattr.c b/fs/xattr.c
index 75cc65ac7ea9..f6d5ce3e1030 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c

@@ -179,7 +179,7 @@ int __vfs_setxattr_noperm(struct dentry *dentry, const char *name,
 	if (issec) {
 		inode->i_flags &= ~S_NOSEC;
 
-		if (!strcmp(name, "security.capability")) {
+		if (!strcmp(name, XATTR_NAME_CAPS)) {
 			error = cap_setxattr_convert_nscap(dentry, value, size,
 					&wvalue, &wsize);
 			if (error < 0)

diff --git a/include/linux/capability.h b/include/linux/capability.h
index b97343353a11..c47febf8448b 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h

@@ -13,7 +13,6 @@
 #define _LINUX_CAPABILITY_H
 
 #include <uapi/linux/capability.h>
-#include <linux/uidgid.h>
 
 #define _KERNEL_CAPABILITY_VERSION _LINUX_CAPABILITY_VERSION_3
 #define _KERNEL_CAPABILITY_U32S    _LINUX_CAPABILITY_U32S_3

diff --git a/security/commoncap.c b/security/commoncap.c
index 8abb9bf4ec17..32e32f437ef5 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c

@@ -367,6 +367,7 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
 	kuid_t kroot;
 	uid_t root, mappedroot;
 	char *tmpbuf = NULL;
+	struct vfs_cap_data *cap;
 	struct vfs_ns_cap_data *nscap;
 	struct dentry *dentry;
 	struct user_namespace *fs_ns;

@@ -379,14 +380,16 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
 		return -EINVAL;
 
 	size = sizeof(struct vfs_ns_cap_data);
-	ret = vfs_getxattr_alloc(dentry, "security.capability",
+	ret = vfs_getxattr_alloc(dentry, XATTR_NAME_CAPS,
 				 &tmpbuf, size, GFP_NOFS);
 
 	if (ret < 0)
 		return ret;
 
 	fs_ns = inode->i_sb->s_user_ns;
-	if (ret == sizeof(struct vfs_cap_data)) {
+	cap = (struct vfs_cap_data *) tmpbuf;
+	if ((ret == sizeof(struct vfs_cap_data)) &&
+	    (cap->magic_etc == cpu_to_le32(VFS_CAP_REVISION_2))) {
 		/* If this is sizeof(vfs_cap_data) then we're ok with the
 		 * on-disk value, so return that.  */
 		if (alloc)

@@ -394,7 +397,8 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
 		else
 			kfree(tmpbuf);
 		return ret;
-	} else if (ret != sizeof(struct vfs_ns_cap_data)) {
+	} else if ((ret != sizeof(struct vfs_ns_cap_data)) ||
+		    (cap->magic_etc != cpu_to_le32(VFS_CAP_REVISION_3))) {
 		kfree(tmpbuf);
 		return -EINVAL;
 	}

@@ -417,7 +421,7 @@ int cap_inode_getsecurity(struct inode *inode, const char *name, void **buffer,
 
 	if (!rootid_owns_currentns(kroot)) {
 		kfree(tmpbuf);
-		return -EOPNOTSUPP;
+		return -ENODATA;
 	}
 
 	/* This comes from a parent namespace.  Return as a v2 capability */

@@ -474,11 +478,17 @@ int cap_setxattr_convert_nscap(struct dentry *dentry, const void *value, size_t
 		return -EINVAL;
 	if (size != XATTR_CAPS_SZ_2 && size != XATTR_CAPS_SZ_3)
 		return -EINVAL;
+	if ((size == XATTR_CAPS_SZ_2) &&
+	    (cap->magic_etc != cpu_to_le32(VFS_CAP_REVISION_2)))
+		return -EINVAL;
+	if ((size == XATTR_CAPS_SZ_3) &&
+	    (cap->magic_etc != cpu_to_le32(VFS_CAP_REVISION_3)))
+		return -EINVAL;
 	if (!capable_wrt_inode_uidgid(inode, CAP_SETFCAP))
 		return -EPERM;
 	if (size == XATTR_CAPS_SZ_2)
 		if (ns_capable(inode->i_sb->s_user_ns, CAP_SETFCAP))
-			// user is privileged, just write the v2
+			/* user is privileged, just write the v2 */
 			return 0;
 
 	rootid = rootid_from_xattr(value, size, task_ns);

@@ -855,7 +865,10 @@ int cap_inode_setxattr(struct dentry *dentry, const char *name,
 			sizeof(XATTR_SECURITY_PREFIX) - 1) != 0)
 		return 0;
 
-	// For XATTR_NAME_CAPS the check will be done in __vfs_setxattr_noperm()
+	/*
+	 * For XATTR_NAME_CAPS the check will be done in
+	 * __vfs_setxattr_noperm()
+	 */
 	if (strcmp(name, XATTR_NAME_CAPS) == 0)
 		return 0;