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[PATCH RFC] xfs/051: test buffer use after free race on I/O failure in X

To: xfs@xxxxxxxxxxx
Subject: [PATCH RFC] xfs/051: test buffer use after free race on I/O failure in XFS log recovery
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Tue, 19 Aug 2014 13:26:12 -0400
Delivered-to: xfs@xxxxxxxxxxx
A buffer use after free race was discovered in the XFS log recovery
codepath if I/O failures occur during recovery. The I/O submission path
can proceed to abort the mount and release the only reference held on
some buffers before I/O completion processing (e.g., async workqueue
processing) might have completed. Badness ensues if the I/O completion
path subsequently attempts to access said buffers.

The test manufactures the race by forcing all writes to fail (via
dm-flakey) after a fixed period of time. A delay is inserted into the
mount codepath to synchronize write failures with log recovery.

Credit for discovery of the race and definition of the reproducible test
case goes to Alex Lyakas.

[NOTE: This still depends on kernel side instrumentation. Insert a 10s
 delay immediately prior to log recovery to reproduce.]

Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
Reported-by: Alex Lyakas <alex@xxxxxxxxxxxxxxxxx>

Hi guys,

This is obviously incomplete as there is no mechanism to synchronize
write failures with log recovery. I was hoping we could get around that,
but apparently we unconditionally reset the inactive range of the log
before we get into log recovery.

Anyways, I just wanted to throw this over the wall in case it's useful
for testing in intermediate form. This reproduces the problem for me
with the 10s delay on the kernel side. The mount fails, I see a series
of BUG()s and the vm becomes generally unusable. I'll send a new version
when I have some kind of synchronization mechanism worked out.


 tests/xfs/051     | 84 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 tests/xfs/051.out |  2 ++
 tests/xfs/group   |  1 +
 3 files changed, 87 insertions(+)
 create mode 100755 tests/xfs/051
 create mode 100644 tests/xfs/051.out

diff --git a/tests/xfs/051 b/tests/xfs/051
new file mode 100755
index 0000000..25acb28
--- /dev/null
+++ b/tests/xfs/051
@@ -0,0 +1,84 @@
+#! /bin/bash
+# FS QA Test No. 051
+# Simulate a buffer use after free race in XFS log recovery. The race triggers
+# on I/O failures during log recovery. Note that this test is dangerous as it
+# causes BUG() errors or a panic.
+# Copyright (c) 2013 Oracle, Inc.  All Rights Reserved.
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License as
+# published by the Free Software Foundation.
+# This program is distributed in the hope that it would be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# GNU General Public License for more details.
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write the Free Software Foundation,
+# Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+seq=`basename $0`
+echo "QA output created by $seq"
+status=1       # failure is the default!
+trap "_cleanup; exit \$status" 0 1 2 3 15
+       cd /
+       rm -f $tmp.*
+       _scratch_unmount > /dev/null 2>&1
+# get standard environment, filters and checks
+. ./common/rc
+. ./common/dmflakey
+# Modify as appropriate.
+_supported_fs xfs
+_supported_os Linux
+echo "Silence is golden."
+_scratch_mkfs_xfs >/dev/null 2>&1
+# Start a workload and shutdown the fs. The subsequent mount will require log
+# recovery.
+$FSSTRESS_PROG -n 9999 -p 2 -w -d $SCRATCH_MNT > /dev/null 2>&1 &
+sleep 5
+src/godown -f $SCRATCH_MNT
+killall -q $FSSTRESS_PROG
+# TODO: Add a mechanism to take advantage of the 5s error delay. This currently
+# depends on a >5s delay inserted into the mount codepath prior to start of log
+# recovery.
+BLK_DEV_SIZE=`blockdev --getsz $SCRATCH_DEV`
+_load_flakey_table $FLAKEY_ALLOW_WRITES
+_mount_flakey > /dev/null 2>&1 # should fail!
+# replay the log
+# success, all done
diff --git a/tests/xfs/051.out b/tests/xfs/051.out
new file mode 100644
index 0000000..5180bc4
--- /dev/null
+++ b/tests/xfs/051.out
@@ -0,0 +1,2 @@
+QA output created by 051
+Silence is golden.
diff --git a/tests/xfs/group b/tests/xfs/group
index 4d35df5..9784dea 100644
--- a/tests/xfs/group
+++ b/tests/xfs/group
@@ -47,6 +47,7 @@
 048 other auto quick
 049 rw auto quick
 050 quota auto quick
+051 dangerous
 052 quota db auto quick
 054 quota auto quick
 055 dump ioctl remote tape

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