x86/speculation/srbds: Update SRBDS mitigation selection

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 <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>

Author: pa1gupta

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;