[PATCH] xfs: dquot shrinker doesn't check for XFS_DQFLAG_FREEING

[Dave Chinner]

On Mon, Dec 05, 2022 at 02:35:39AM -0800, syzbot wrote:
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    0ba09b173387 Revert "mm: align larger anonymous mappings o..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15550c47880000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=2325e409a9a893e1
> dashboard link: https://syzkaller.appspot.com/bug?extid=912776840162c13db1a3
> compiler:       Debian clang version 13.0.1-++20220126092033+75e33f71c2da-1~exp1~20220126212112.63, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=128c9e23880000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/9758ec2c06f4/disk-0ba09b17.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/06781dbfd581/vmlinux-0ba09b17.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/3d44a22d15fa/bzImage-0ba09b17.xz
> mounted in repro: https://storage.googleapis.com/syzbot-assets/335889b2d730/mount_0.gz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+912776840162c13db1a3@xxxxxxxxxxxxxxxxxxxxxxxxx
> 
> XFS (loop1): Quotacheck: Done.
> syz-executor.1 (4657): drop_caches: 2
> ==================================================================
> BUG: KASAN: use-after-free in xfs_dquot_type fs/xfs/xfs_dquot.h:136 [inline]
> BUG: KASAN: use-after-free in xfs_qm_dqfree_one+0x12f/0x170 fs/xfs/xfs_qm.c:1604
> Read of size 1 at addr ffff888079a6aa58 by task syz-executor.1/4657

Looks like we've missed a XFS_DQUOT_FREEING check in
xfs_qm_shrink_scan(), and the dquot purge run by unmount has raced
with the shrinker. Patch below should fix it.

-Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

xfs: dquot shrinker doesn't check for XFS_DQFLAG_FREEING

From: Dave Chinner <dchinner@xxxxxxxxxx>

Resulting in a UAF if the shrinker races with some other dquot
freeing mechanism that sets XFS_DQFLAG_FREEING before the dquot is
removed from the LRU. This can occur if a dquot purge races with
drop_caches.

Reported-by: syzbot+912776840162c13db1a3@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 fs/xfs/xfs_qm.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
index 18bb4ec4d7c9..ff53d40a2dae 100644
--- a/fs/xfs/xfs_qm.c
+++ b/fs/xfs/xfs_qm.c
@@ -422,6 +422,14 @@ xfs_qm_dquot_isolate(
 	if (!xfs_dqlock_nowait(dqp))
 		goto out_miss_busy;
 
+	/*
+	 * If something else is freeing this dquot and hasn't yet removed it
+	 * from the LRU, leave it for the freeing task to complete the freeing
+	 * process rather than risk it being free from under us here.
+	 */
+	if (dqp->q_flags & XFS_DQFLAG_FREEING)
+		goto out_miss_unlock;
+
 	/*
 	 * This dquot has acquired a reference in the meantime remove it from
 	 * the freelist and try again.
@@ -441,10 +449,8 @@ xfs_qm_dquot_isolate(
 	 * skip it so there is time for the IO to complete before we try to
 	 * reclaim it again on the next LRU pass.
 	 */
-	if (!xfs_dqflock_nowait(dqp)) {
-		xfs_dqunlock(dqp);
-		goto out_miss_busy;
-	}
+	if (!xfs_dqflock_nowait(dqp))
+		goto out_miss_unlock;
 
 	if (XFS_DQ_IS_DIRTY(dqp)) {
 		struct xfs_buf	*bp = NULL;
@@ -478,6 +484,8 @@ xfs_qm_dquot_isolate(
 	XFS_STATS_INC(dqp->q_mount, xs_qm_dqreclaims);
 	return LRU_REMOVED;
 
+out_miss_unlock:
+	xfs_dqunlock(dqp);
 out_miss_busy:
 	trace_xfs_dqreclaim_busy(dqp);
 	XFS_STATS_INC(dqp->q_mount, xs_qm_dqreclaim_misses);