[PATCH RESEND v2 07/18] fs: Check for invalid i_uid in may_follow_link()

Seth Forshee
2016-01-04T18:04:54Z

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@canonical.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.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 288e8a74bf88..4ccafd391697 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -902,7 +902,7 @@ 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))
+ if (uid_valid_eq(parent->i_uid, inode->i_uid))
return 0;

if (nd->flags & LOOKUP_RCU)
diff --git a/include/linux/uidgid.h b/include/linux/uidgid.h
index 03835522dfcb..e09529fe2668 100644
--- a/include/linux/uidgid.h
+++ b/include/linux/uidgid.h
@@ -117,6 +117,16 @@ static inline bool gid_valid(kgid_t gid)
return __kgid_val(gid) != (gid_t) -1;
}

+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);
--
1.9.1