cachefiles: fix the race between cachefiles_bury_object() and rmdir(2)

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.

Cc: stable@vger.kernel.org
Fixes: 9ae326a69004 (CacheFiles: A cache that backs onto a mounted filesystem)
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index af2b17b..95983c7 100644 (file)
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -343,7 +343,7 @@ 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);