[PATCH 3.16 317/328] cachefiles: fix the race between cachefiles_bury_object() and rmdir(2)

From: Ben Hutchings
Date: Sun Dec 09 2018 - 17:03:49 EST

3.16.62-rc1 review patch. If anyone has any objections, please let me know.


From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

commit 169b803397499be85bdd1e3d07d6f5e3d4bd669e upstream.

the victim might've been rmdir'ed just before the lock_rename();
unlike the normal callers, we do not look the source up after the
parents are locked - we know it beforehand and just recheck that it's
still the child of what used to be its parent. Unfortunately,
the check is too weak - we don't spot a dead directory since its
->d_parent is unchanged, dentry is positive, etc. So we sail all
the way to ->rename(), with hosting filesystems _not_ expecting
to be asked renaming an rmdir'ed subdirectory.

The fix is easy, fortunately - the lock on parent is sufficient for
making IS_DEADDIR() on child safe.

Fixes: 9ae326a69004 (CacheFiles: A cache that backs onto a mounted filesystem)
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
fs/cachefiles/namei.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -317,7 +317,7 @@ try_again:
trap = lock_rename(cache->graveyard, dir);

/* do some checks before getting the grave dentry */
- if (rep->d_parent != dir) {
+ if (rep->d_parent != dir || IS_DEADDIR(d_inode(rep))) {
/* the entry was probably culled when we dropped the parent dir
* lock */
unlock_rename(cache->graveyard, dir);