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x86/bugs: Reset speculation control settings on init

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[ Upstream commit 0125acda7d ]

Currently, x86_spec_ctrl_base is read at boot time and speculative bits
are set if Kconfig items are enabled. For example, IBRS is enabled if
CONFIG_CPU_IBRS_ENTRY is configured, etc. These MSR bits are not cleared
if the mitigations are disabled.

This is a problem when kexec-ing a kernel that has the mitigation
disabled from a kernel that has the mitigation enabled. In this case,
the MSR bits are not cleared during the new kernel boot. As a result,
this might have some performance degradation that is hard to pinpoint.

This problem does not happen if the machine is (hard) rebooted because
the bit will be cleared by default.

  [ bp: Massage. ]

Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20221128153148.1129350-1-leitao@debian.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Breno Leitao
2022-11-28 07:31:48 -08:00
committed by Greg Kroah-Hartman
parent 3a43a366ec
commit 2135dd1be6
2 changed files with 13 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

4
arch/x86/include/asm/msr-index.h
View File

@@ -54,6 +54,10 @@
#define SPEC_CTRL_RRSBA_DIS_S_SHIFT 6 /* Disable RRSBA behavior */
#define SPEC_CTRL_RRSBA_DIS_S BIT(SPEC_CTRL_RRSBA_DIS_S_SHIFT)
/* A mask for bits which the kernel toggles when controlling mitigations */
#define SPEC_CTRL_MITIGATIONS_MASK (SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD \
| SPEC_CTRL_RRSBA_DIS_S)
#define MSR_IA32_PRED_CMD 0x00000049 /* Prediction Command */
#define PRED_CMD_IBPB BIT(0) /* Indirect Branch Prediction Barrier */

10
arch/x86/kernel/cpu/bugs.c
View File

@@ -144,9 +144,17 @@ void __init check_bugs(void)
* have unknown values. AMD64_LS_CFG MSR is cached in the early AMD
* init code as it is not enumerated and depends on the family.
*/
if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
if (cpu_feature_enabled(X86_FEATURE_MSR_SPEC_CTRL)) {
rdmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
/*
* Previously running kernel (kexec), may have some controls
* turned ON. Clear them and let the mitigations setup below
* rediscover them based on configuration.
*/
x86_spec_ctrl_base &= ~SPEC_CTRL_MITIGATIONS_MASK;
}
/* Select the proper CPU mitigations before patching alternatives: */
spectre_v1_select_mitigation();
spectre_v2_select_mitigation();