linux-kernel - [PATCH] perf/x86/intel/lbr: Zero the xstate buffer on allocation

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Message-ID: <87wnr0wo2z.ffs@nanos.tec.linutronix.de>
Date:   Fri, 11 Jun 2021 15:03:16 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Kan Liang <kan.liang@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org
Subject: [PATCH] perf/x86/intel/lbr: Zero the xstate buffer on allocation

XRSTORS requires a valid xstate buffer to work correctly. XSAVES does not
guarantee to write a fully valid buffer according to the SDM:

  "XSAVES does not write to any parts of the XSAVE header other than the
   XSTATE_BV and XCOMP_BV fields."

XRSTORS triggers a #GP:

  "If bytes 63:16 of the XSAVE header are not all zero."

It's dubious at best how this can work at all when the buffer is not zeroed
before use.

Allocate the buffers with __GFP_ZERO to prevent XRSTORS failure.

Fixes: ce711ea3cab9 ("perf/x86/intel/lbr: Support XSAVES/XRSTORS for LBR context switch")
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Kan Liang <kan.liang@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org
---
 arch/x86/events/intel/lbr.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/arch/x86/events/intel/lbr.c
+++ b/arch/x86/events/intel/lbr.c
@@ -731,7 +731,8 @@ void reserve_lbr_buffers(void)
 		if (!kmem_cache || cpuc->lbr_xsave)
 			continue;
 
-		cpuc->lbr_xsave = kmem_cache_alloc_node(kmem_cache, GFP_KERNEL,
+		cpuc->lbr_xsave = kmem_cache_alloc_node(kmem_cache,
+							GFP_KERNEL | __GFP_ZERO,
 							cpu_to_node(cpu));
 	}
 }