linux-kernel - [PATCH 3.16 224/305] btrfs: relocation: set trans to be NULL after ending transaction

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Message-ID: <lsq.1549201508.260461916@decadent.org.uk>
Date:   Sun, 03 Feb 2019 14:45:08 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Pan Bian" <bianpan2016@....com>, "Qu Wenruo" <wqu@...e.com>,
        "David Sterba" <dsterba@...e.com>
Subject: [PATCH 3.16 224/305] btrfs: relocation: set trans to be NULL
 after ending transaction

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

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

From: Pan Bian <bianpan2016@....com>

commit 42a657f57628402c73237547f0134e083e2f6764 upstream.

The function relocate_block_group calls btrfs_end_transaction to release
trans when update_backref_cache returns 1, and then continues the loop
body. If btrfs_block_rsv_refill fails this time, it will jump out the
loop and the freed trans will be accessed. This may result in a
use-after-free bug. The patch assigns NULL to trans after trans is
released so that it will not be accessed.

Fixes: 0647bf564f1 ("Btrfs: improve forever loop when doing balance relocation")
Reviewed-by: Qu Wenruo <wqu@...e.com>
Signed-off-by: Pan Bian <bianpan2016@....com>
Reviewed-by: David Sterba <dsterba@...e.com>
Signed-off-by: David Sterba <dsterba@...e.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 fs/btrfs/relocation.c | 1 +
 1 file changed, 1 insertion(+)

--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -3955,6 +3955,7 @@ static noinline_for_stack int relocate_b
 restart:
 		if (update_backref_cache(trans, &rc->backref_cache)) {
 			btrfs_end_transaction(trans, rc->extent_root);
+			trans = NULL;
 			continue;
 		}