[Devel] [PATCH RH7 14/22] port/fs: Check for invalid i_uid in may_follow_link()

[Pavel Tikhomirov]

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.

Add a new helper function, uid_valid_eq(), and use this to
validate that the ids in may_follow_link() are both equal and
valid. Also add an equivalent helper for gids, which is
currently unused.

Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
Acked-by: Serge Hallyn <serge.hallyn at canonical.com>
Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
 fs/namei.c             |  2 +-
 include/linux/uidgid.h | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 0f12ce7..913fa68 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -818,7 +818,7 @@ 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_eq(parent->i_uid, inode->i_uid))
 		return 0;
 
 	audit_log_link_denied("follow_link", link);
diff --git a/include/linux/uidgid.h b/include/linux/uidgid.h
index 8e522cbc..33b9273 100644
--- a/include/linux/uidgid.h
+++ b/include/linux/uidgid.h
@@ -127,6 +127,16 @@ static inline bool gid_valid(kgid_t gid)
 	return !gid_eq(gid, INVALID_GID);
 }
 
+static inline bool uid_valid_eq(kuid_t left, kuid_t right)
+{
+	return uid_eq(left, right) && uid_valid(left);
+}
+
+static inline bool gid_valid_eq(kgid_t left, kgid_t right)
+{
+	return gid_eq(left, right) && gid_valid(left);
+}
+
 #ifdef CONFIG_USER_NS
 
 extern kuid_t make_kuid(struct user_namespace *from, uid_t uid);
-- 
1.9.3