linux-kernel - [PATCH 3.19.y-ckt 091/146] powerpc/mce: fix off by one errors in mce event handling

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Date:	Wed, 17 Jun 2015 15:23:16 -0700
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Daniel Axtens <dja@...ens.net>,
	Michael Ellerman <mpe@...erman.id.au>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 3.19.y-ckt 091/146] powerpc/mce: fix off by one errors in mce event handling

3.19.8-ckt2 -stable review patch.  If anyone has any objections, please let me know.

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

From: Daniel Axtens <dja@...ens.net>

commit ffb2d78eca08a1451137583d4e435aecfd6af809 upstream.

Before 69111bac42f5 ("powerpc: Replace __get_cpu_var uses"), in
save_mce_event, index got the value of mce_nest_count, and
mce_nest_count was incremented *after* index was set.

However, that patch changed the behaviour so that mce_nest count was
incremented *before* setting index.

This causes an off-by-one error, as get_mce_event sets index as
mce_nest_count - 1 before reading mce_event.  Thus get_mce_event reads
bogus data, causing warnings like
"Machine Check Exception, Unknown event version 0 !"
and breaking MCEs handling.

Restore the old behaviour and unbreak MCE handling by subtracting one
from the newly incremented value.

The same broken change occured in machine_check_queue_event (which set
a queue read by machine_check_process_queued_event).  Fix that too,
unbreaking printing of MCE information.

Fixes: 69111bac42f5 ("powerpc: Replace __get_cpu_var uses")
CC: Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>
CC: Christoph Lameter <cl@...ux.com>
Signed-off-by: Daniel Axtens <dja@...ens.net>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Signed-off-by: Kamal Mostafa <kamal@...onical.com>
---
 arch/powerpc/kernel/mce.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/mce.c b/arch/powerpc/kernel/mce.c
index 15c99b6..b2eb468 100644
--- a/arch/powerpc/kernel/mce.c
+++ b/arch/powerpc/kernel/mce.c
@@ -73,7 +73,7 @@ void save_mce_event(struct pt_regs *regs, long handled,
 		    uint64_t nip, uint64_t addr)
 {
 	uint64_t srr1;
-	int index = __this_cpu_inc_return(mce_nest_count);
+	int index = __this_cpu_inc_return(mce_nest_count) - 1;
 	struct machine_check_event *mce = this_cpu_ptr(&mce_event[index]);

 	/*
@@ -184,7 +184,7 @@ void machine_check_queue_event(void)
 	if (!get_mce_event(&evt, MCE_EVENT_RELEASE))
 		return;

-	index = __this_cpu_inc_return(mce_queue_count);
+	index = __this_cpu_inc_return(mce_queue_count) - 1;
 	/* If queue is full, just return for now. */
 	if (index >= MAX_MC_EVT) {
 		__this_cpu_dec(mce_queue_count);
-- 
1.9.1

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