fs: Check for invalid i_uid in may_follow_link()

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 when protected
symlinks are enabled.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

diff --git a/fs/namei.c b/fs/namei.c
index 757a32725d92e6556951988d5d33141d305f2016..8701bd9a5270329eeb043c7142f6bc37dd6195dd 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -901,6 +901,7 @@ static inline int may_follow_link(struct nameidata *nd)
 {
        const struct inode *inode;
        const struct inode *parent;
+       kuid_t puid;
 
        if (!sysctl_protected_symlinks)
                return 0;
@@ -916,7 +917,8 @@ static inline int may_follow_link(struct nameidata *nd)
                return 0;
 
        /* Allowed if parent directory and link owner match. */
-       if (uid_eq(parent->i_uid, inode->i_uid))
+       puid = parent->i_uid;
+       if (uid_valid(puid) && uid_eq(puid, inode->i_uid))
                return 0;
 
        if (nd->flags & LOOKUP_RCU)