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 <cascardo@canonical.com>.

  [ bp: Massage commit message. ]

Suggested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220401074517.1848264-1-ricardo.canuelo@collabora.com

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6296e1e..d879a6c 100644 (file)
--- 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) {