ALERT: md/raid6 data corruption risk.

[NeilBrown <neilb@suse.de>]

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Hi all,
 There is a risk of data loss with md/raid6 arrays running on Linux since
 2.6.32.
 If:
   - the array is doubly degraded
   - one or both failed devices are being recovered, and
   - the array is written to

 then it is possible for data on the array to be lost.  The patch below fixes
 the problem.  If you apply the patch to an older kernel which has separate
 handle_stripe5() and handle_stripe6() functions, be sure that patch changes
 handle_stripe6().

 There is no risk to an optimal array or a singly-degraded array.  There is
 also no risk on a doubly-degraded array which is not recovering a device or
 is not receiving write requests.

 If you have data on a RAID6 array, please consider how to avoid corruption,
 possibly by applying the patch, possibly by removing any hot spares so
 recovery does not automatically start.

 This patch will be sent upstream shortly and will subsequently appear in
 future "-stable" kernels.

NeilBrown

From f94e37dce722ec7b6666fd04be357f422daa02b5 Mon Sep 17 00:00:00 2001
From: NeilBrown <neilb@suse.de>
Date: Wed, 13 Aug 2014 09:57:07 +1000
Subject: [PATCH] md/raid6: avoid data corruption during recovery of
 double-degraded RAID6

During recovery of a double-degraded RAID6 it is possible for
some blocks not to be recovered properly, leading to corruption.

If a write happens to one block in a stripe that would be written to a
missing device, and at the same time that stripe is recovering data
to the other missing device, then that recovered data may not be written.

This patch skips, in the double-degraded case, an optimisation that is
only safe for single-degraded arrays.

Bug was introduced in 2.6.32 and fix is suitable for any kernel since
then.  In an older kernel with separate handle_stripe5() and
handle_stripe6() functions that patch must change handle_stripe6().

Cc: stable@vger.kernel.org (2.6.32+)
Fixes: 6c0069c0ae9659e3a91b68eaed06a5c6c37f45c8
Cc: Yuri Tikhonov <yur@emcraft.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Reported-by: "Manibalan P" <pmanibalan@amiindia.co.in>
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1090423
Signed-off-by: NeilBrown <neilb@suse.de>

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 6b2d615d1094..183588b11fc1 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3817,6 +3817,8 @@ static void handle_stripe(struct stripe_head *sh)
 				set_bit(R5_Wantwrite, &dev->flags);
 				if (prexor)
 					continue;
+				if (s.failed > 1)
+					continue;
 				if (!test_bit(R5_Insync, &dev->flags) ||
 				    ((i == sh->pd_idx || i == sh->qd_idx)  &&
 				     s.failed == 0))

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