This is a note to let you know that I've just added the patch titled
x86/bugs: Reset speculation control settings on init
to the 4.19-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-bugs-reset-speculation-control-settings-on-init.patch
and it can be found in the queue-4.19 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 93f2c3eb405b41eefadde2d04e94bdfc383bd4d4
Author: Breno Leitao <leitao@xxxxxxxxxx>
Date: Mon Nov 28 07:31:48 2022 -0800
x86/bugs: Reset speculation control settings on init
[ Upstream commit 0125acda7d76b943ca55811df40ed6ec0ecf670f ]
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@xxxxxxxxxxxxxxx>
Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Link: https://lore.kernel.org/r/20221128153148.1129350-1-leitao@xxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 0bd07699dba38..847f3f5820d21 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -50,6 +50,10 @@
#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 */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index e298ec9d5d536..54f42ae1a61d9 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -135,9 +135,17 @@ 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();
