[PATCH v3 1/4] vfs: Check for invalid i_uid in may_follow_link()

[Seth Forshee <seth.forshee@canonical.com>]

Filesystem uids which don't map into a user namespace may result
in inode->i_uid being INVALID_UID. A symlink and its parent
could have different owners in the filesystem can both get
mapped to INVALID_UID, which may result in following a symlink
when this would not have otherwise been permitted. Prevent this
by adding a check that the uid is valid before the comparison.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
---
 fs/namei.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index a996bb48dfab..193da09e903e 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -741,7 +741,7 @@ static inline int may_follow_link(struct path *link, struct nameidata *nd)
 		return 0;
 
 	/* Allowed if parent directory and link owner match. */
-	if (uid_eq(parent->i_uid, inode->i_uid))
+	if (uid_valid(inode->i_uid) && uid_eq(parent->i_uid, inode->i_uid))
 		return 0;
 
 	audit_log_link_denied("follow_link", link);
-- 
1.9.1