linux-kernel - [PATCH] fs/block_dev: fix potential NULL ptr deref in freeze

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Date:   Tue, 23 Aug 2016 18:55:31 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Jens Axboe <axboe@...com>, Alexander Viro <viro@...iv.linux.org.uk>
CC:     <linux-fsdevel@...r.kernel.org>,
        Maxim Patlasov <mpatlasov@...tuozzo.com>,
        Christoph Hellwig <hch@....de>, <linux-kernel@...r.kernel.org>,
        "Andrey Ryabinin" <aryabinin@...tuozzo.com>
Subject: [PATCH] fs/block_dev: fix potential NULL ptr deref in freeze_bdev()

Calling freeze_bdev() twice on the same block device without mounted
filesystem get_super() will return NULL, which will lead to NULL-ptr
dereference later in drop_super().

Check get_super() result to fix that.

Note, that this is a purely theoretical issue. We have only 3
freeze_bdev() callers. 2 of them are in filesystem code and used on a
device with mounted fs. The third one in lock_fs() has protection in
upper-layer code against freezing block device the second time without
thawing it first.

Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
---
 fs/block_dev.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index c3cdde8..c18b083 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -249,7 +249,8 @@ struct super_block *freeze_bdev(struct block_device *bdev)
 		 * thaw_bdev drops it.
 		 */
 		sb = get_super(bdev);
-		drop_super(sb);
+		if (sb)
+			drop_super(sb);
 		mutex_unlock(&bdev->bd_fsfreeze_mutex);
 		return sb;
 	}
-- 
2.7.3