[PATCH 4.4 30/33] vfs: dont do RCU lookup of empty pathnames

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Linus Torvalds <torvalds@linux-foundation.org>

commit c0eb027e5aef70b71e5a38ee3e264dc0b497f343 upstream.

Normal pathname lookup doesn't allow empty pathnames, but using
AT_EMPTY_PATH (with name_to_handle_at() or fstatat(), for example) you
can trigger an empty pathname lookup.

And not only is the RCU lookup in that case entirely unnecessary
(because we'll obviously immediately finalize the end result), it is
actively wrong.

Why? An empth path is a special case that will return the original
'dirfd' dentry - and that dentry may not actually be RCU-free'd,
resulting in a potential use-after-free if we were to initialize the
path lazily under the RCU read lock and depend on complete_walk()
finalizing the dentry.

Found by syzkaller and KASAN.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Eric Biggers <ebiggers3@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/namei.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2000,6 +2000,9 @@ static const char *path_init(struct name
 	int retval = 0;
 	const char *s = nd->name->name;
 
+	if (!*s)
+		flags &= ~LOOKUP_RCU;
+
 	nd->last_type = LAST_ROOT; /* if there are only slashes... */
 	nd->flags = flags | LOOKUP_JUMPED | LOOKUP_PARENT;
 	nd->depth = 0;