x86/speculation/srbds: Update SRBDS mitigation selection

commit 22cac9c677c95f3ac5c9244f8ca0afdc7c8afb19 upstream

Currently, Linux disables SRBDS mitigation on CPUs not affected by
MDS and have the TSX feature disabled. On such CPUs, secrets cannot
be extracted from CPU fill buffers using MDS or TAA. Without SRBDS
mitigation, Processor MMIO Stale Data vulnerabilities can be used to
extract RDRAND, RDSEED, and EGETKEY data.

Do not disable SRBDS mitigation by default when CPU is also affected by
Processor MMIO Stale Data vulnerabilities.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 9bc318390bcc3381bd0e645e3ea55f3fa83563ac..dc87aa9ad22921783dd92c4f1a7ee8844053fc37 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -595,11 +595,13 @@ static void __init srbds_select_mitigation(void)
                return;
 
        /*
-        * Check to see if this is one of the MDS_NO systems supporting
-        * TSX that are only exposed to SRBDS when TSX is enabled.
+        * Check to see if this is one of the MDS_NO systems supporting TSX that
+        * are only exposed to SRBDS when TSX is enabled or when CPU is affected
+        * by Processor MMIO Stale Data vulnerability.
         */
        ia32_cap = x86_read_arch_cap_msr();
-       if ((ia32_cap & ARCH_CAP_MDS_NO) && !boot_cpu_has(X86_FEATURE_RTM))
+       if ((ia32_cap & ARCH_CAP_MDS_NO) && !boot_cpu_has(X86_FEATURE_RTM) &&
+           !boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA))
                srbds_mitigation = SRBDS_MITIGATION_TSX_OFF;
        else if (boot_cpu_has(X86_FEATURE_HYPERVISOR))
                srbds_mitigation = SRBDS_MITIGATION_HYPERVISOR;