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Subject[PATCH 3.2 140/152] x86, cpu, amd: Add workaround for family 16h, erratum 793
3.2.67-rc1 review patch.  If anyone has any objections, please let me know.


From: Borislav Petkov <>

commit 3b56496865f9f7d9bcb2f93b44c63f274f08e3b6 upstream.

This adds the workaround for erratum 793 as a precaution in case not
every BIOS implements it. This addresses CVE-2013-6885.

Erratum text:

[Revision Guide for AMD Family 16h Models 00h-0Fh Processors,
document 51810 Rev. 3.04 November 2013]

793 Specific Combination of Writes to Write Combined Memory Types and
Locked Instructions May Cause Core Hang


Under a highly specific and detailed set of internal timing
conditions, a locked instruction may trigger a timing sequence whereby
the write to a write combined memory type is not flushed, causing the
locked instruction to stall indefinitely.

Potential Effect on System

Processor core hang.

Suggested Workaround

BIOS should set MSR
C001_1020[15] = 1b.

Fix Planned

No fix planned

[ hpa: updated description, fixed typo in MSR name ]

Signed-off-by: Borislav Petkov <>
Tested-by: Aravind Gopalakrishnan <>
Signed-off-by: H. Peter Anvin <>
[bwh: Backported to 3.2:
- Adjust filename
- Venkatesh Srinivas pointed out we should use {rd,wr}msrl_safe() to
avoid crashing on KVM. This was fixed upstream by commit 8f86a7373a1c
("x86, AMD: Convert to the new bit access MSR accessors") but that's too
much trouble to backport. Here we must use {rd,wr}msrl_amd_safe().]
Signed-off-by: Ben Hutchings <>
Cc: Moritz Muehlenhoff <>
Cc: Venkatesh Srinivas <>
arch/x86/include/asm/msr-index.h | 1 +
arch/x86/kernel/cpu/amd.c | 10 ++++++++++
2 files changed, 11 insertions(+)

--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -123,6 +123,7 @@
#define MSR_AMD64_PATCH_LOADER 0xc0010020
#define MSR_AMD64_OSVW_ID_LENGTH 0xc0010140
#define MSR_AMD64_OSVW_STATUS 0xc0010141
+#define MSR_AMD64_LS_CFG 0xc0011020
#define MSR_AMD64_DC_CFG 0xc0011022
#define MSR_AMD64_IBSFETCHCTL 0xc0011030
#define MSR_AMD64_IBSFETCHLINAD 0xc0011031
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -408,6 +408,16 @@ static void __cpuinit early_init_amd_mc(

c->x86_coreid_bits = bits;
+ /* F16h erratum 793, CVE-2013-6885 */
+ if (c->x86 == 0x16 && c->x86_model <= 0xf) {
+ u64 val;
+ if (!rdmsrl_amd_safe(MSR_AMD64_LS_CFG, &val) &&
+ !(val & BIT(15)))
+ wrmsrl_amd_safe(MSR_AMD64_LS_CFG, val | BIT(15));
+ }

static void __cpuinit bsp_init_amd(struct cpuinfo_x86 *c)

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