linux-kernel - [PATCH 3.16 317/328] cachefiles: fix the race between cachefiles_bury

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Message-ID: <lsq.1544392233.430055360@decadent.org.uk>
Date:   Sun, 09 Dec 2018 21:50:33 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "David Howells" <dhowells@...hat.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Al Viro" <viro@...iv.linux.org.uk>
Subject: [PATCH 3.16 317/328] cachefiles: fix the race between
 cachefiles_bury_object() and rmdir(2)

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

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

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

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@...iv.linux.org.uk>
Signed-off-by: David Howells <dhowells@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 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);