linux-kernel - [PATCH 1/4] cachefiles: fix the race between cachefiles_bury

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Message-ID: <153978620674.8478.12752931380482175663.stgit@warthog.procyon.org.uk>
Date:   Wed, 17 Oct 2018 15:23:26 +0100
From:   David Howells <dhowells@...hat.com>
To:     gregkh@...ux-foundation.org
Cc:     stable@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        viro@...iv.linux.org.uk, sandeen@...hat.com, dhowells@...hat.com,
        linux-cachefs@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/4] cachefiles: fix the race between
 cachefiles_bury_object() and rmdir(2)

From: Al Viro <viro@...iv.linux.org.uk>

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@...r.kernel.org
Fixes: 9ae326a69004 (CacheFiles: A cache that backs onto a mounted filesystem)
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: David Howells <dhowells@...hat.com>
---

 fs/cachefiles/namei.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index af2b17b21b94..95983c744164 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -343,7 +343,7 @@ static int cachefiles_bury_object(struct cachefiles_cache *cache,
 	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);