linux-kernel - [PATCH] perf/x86/intel/bts: don't dereference ds unconditionally

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Message-ID: <20160920131220.xg5pbdjtznszuyzb@breakpoint.cc>
Date:   Tue, 20 Sep 2016 15:12:21 +0200
From:   Sebastian Andrzej Siewior <sebastian@...akpoint.cc>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        linux-kernel@...r.kernel.org, vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        tglx@...utronix.de
Subject: [PATCH] perf/x86/intel/bts: don't dereference ds unconditionally

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

Since commit 4d4c47412464 ("perf/x86/intel/bts: Fix BTS PMI detection")
my box goes boom on boot:

| .... node  #0, CPUs:      #1 #2 #3 #4 #5 #6 #7
| BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
| IP: [<ffffffff8100c463>] intel_bts_interrupt+0x43/0x130
| Call Trace:
|  <NMI> d [<ffffffff8100b341>] intel_pmu_handle_irq+0x51/0x4b0
|  [<ffffffff81004d47>] perf_event_nmi_handler+0x27/0x40

I don't know what is going on here but ds is not always dereferenced
unconditionally hence here the `ds' check to avoid the crash.

Fixes: 4d4c47412464 ("perf/x86/intel/bts: Fix BTS PMI detection")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 arch/x86/events/intel/bts.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/events/intel/bts.c b/arch/x86/events/intel/bts.c
index bdcd6510992c..6ff66efa0feb 100644
--- a/arch/x86/events/intel/bts.c
+++ b/arch/x86/events/intel/bts.c
@@ -455,7 +455,7 @@ int intel_bts_interrupt(void)
 	 * The only surefire way of knowing if this NMI is ours is by checking
 	 * the write ptr against the PMI threshold.
 	 */
-	if (ds->bts_index >= ds->bts_interrupt_threshold)
+	if (ds && (ds->bts_index >= ds->bts_interrupt_threshold))
 		handled = 1;
 
 	/*
-- 
2.9.3