x86, cpu, amd: Add workaround for family 16h, erratum 793

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

Description

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 <bp@suse.de>
Link: http://lkml.kernel.org/r/20140114230711.GS29865@pd.tnic
Tested-by: Aravind Gopalakrishnan <aravind.gopalakrishnan@amd.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index bca023bdd6b..59bfebc8c80 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -508,6 +508,16 @@ static void early_init_amd(struct cpuinfo_x86 *c)
 			set_cpu_cap(c, X86_FEATURE_EXTD_APICID);
 	}
 #endif
+
+	/* F16h erratum 793, CVE-2013-6885 */
+	if (c->x86 == 0x16 && c->x86_model <= 0xf) {
+		u64 val;
+
+		rdmsrl(MSR_AMD64_LS_CFG, val);
+		if (!(val & BIT(15)))
+			wrmsrl(MSR_AMD64_LS_CFG, val | BIT(15));
+	}
+
 }
 
 static const int amd_erratum_383[];