2022-04-02 07:31:41

by Ricardo Cañuelo

Subject: [PATCH v3] x86/speculation/srbds: do not try to turn mitigation off when not supported

When SRBDS is mitigated by TSX OFF, update_srbds_msr will still read and
write to MSR_IA32_MCU_OPT_CTRL even when that is not supported by the
microcode.

Checking for X86_FEATURE_SRBDS_CTRL as a CPU feature available makes
more sense than checking for SRBDS_MITIGATION_UCODE_NEEDED as the found
"mitigation".

Signed-off-by: Thadeu Lima de Souza Cascardo <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Signed-off-by: Ricardo Cañuelo <[email protected]>
Tested-by: Ricardo Cañuelo <[email protected]>
---

Changes in v3:
- Simplified and more explicit solution proposed by Boris

arch/x86/kernel/cpu/bugs.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6296e1ebed1d..d879a6c93609 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -446,6 +446,13 @@ void update_srbds_msr(void)
if (srbds_mitigation == SRBDS_MITIGATION_UCODE_NEEDED)
return;

+ /*
+ * A MDS_NO CPU for which SRBDS mitigation is not needed due to TSX
+ * being disabled and it hasn't received the SRBDS MSR microcode.
+ */
+ if (!boot_cpu_has(X86_FEATURE_SRBDS_CTRL))
+ return;
+
rdmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_ctrl);

switch (srbds_mitigation) {
--
2.25.1

2022-04-06 13:18:26

by tip-bot2 for Jacob Pan

Subject: [tip: x86/cpu] x86/speculation/srbds: Do not try to turn mitigation off when not supported

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID: 0205f8a738ab9e62d849e88e543cfa6ce4c13163
Gitweb: https://git.kernel.org/tip/0205f8a738ab9e62d849e88e543cfa6ce4c13163
Author: Ricardo Cañuelo <[email protected]>
AuthorDate: Fri, 01 Apr 2022 09:45:17 +02:00
Committer: Borislav Petkov <[email protected]>
CommitterDate: Tue, 05 Apr 2022 21:55:57 +02:00

x86/speculation/srbds: Do not try to turn mitigation off when not supported

When SRBDS is mitigated by TSX OFF, update_srbds_msr() will still read
and write to MSR_IA32_MCU_OPT_CTRL even when that MSR is not supported
due to not having loaded the appropriate microcode.

Check for X86_FEATURE_SRBDS_CTRL which is set only when the respective
microcode which adds MSR_IA32_MCU_OPT_CTRL is loaded.

Based on a patch by Thadeu Lima de Souza Cascardo <[email protected]>.

[ bp: Massage commit message. ]

Suggested-by: Pawan Gupta <[email protected]>
Signed-off-by: Ricardo Cañuelo <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
---
arch/x86/kernel/cpu/bugs.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6296e1e..d879a6c 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -446,6 +446,13 @@ void update_srbds_msr(void)
if (srbds_mitigation == SRBDS_MITIGATION_UCODE_NEEDED)
return;

+ /*
+ * A MDS_NO CPU for which SRBDS mitigation is not needed due to TSX
+ * being disabled and it hasn't received the SRBDS MSR microcode.
+ */
+ if (!boot_cpu_has(X86_FEATURE_SRBDS_CTRL))
+ return;
+
rdmsrl(MSR_IA32_MCU_OPT_CTRL, mcu_ctrl);

switch (srbds_mitigation) {