x86/speculation/mds: Add BUG_MSBDS_ONLY

commit e261f209c3666e842fd645a1e31f001c3a26def9 upstream

This bug bit is set on CPUs which are only affected by Microarchitectural
Store Buffer Data Sampling (MSBDS) and not by any other MDS variant.

This is important because the Store Buffers are partitioned between
Hyper-Threads so cross thread forwarding is not possible. But if a thread
enters or exits a sleep state the store buffer is repartitioned which can
expose data from one thread to the other. This transition can be mitigated.

That means that for CPUs which are only affected by MSBDS SMT can be
enabled, if the CPU is not affected by other SMT sensitive vulnerabilities,
e.g. L1TF. The XEON PHI variants fall into that category. Also the
Silvermont/Airmont ATOMs, but for them it's not really relevant as they do
not support SMT, but mark them for completeness sake.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Jon Masters <jcm@redhat.com>
Tested-by: Jon Masters <jcm@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 1dc7b81..69037da 100644 (file)
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -380,5 +380,6 @@
 #define X86_BUG_SPEC_STORE_BYPASS      X86_BUG(17) /* CPU is affected by speculative store bypass attack */
 #define X86_BUG_L1TF                   X86_BUG(18) /* CPU is affected by L1 Terminal Fault */
 #define X86_BUG_MDS                    X86_BUG(19) /* CPU is affected by Microarchitectural data sampling */
+#define X86_BUG_MSBDS_ONLY             X86_BUG(20) /* CPU is only affected by the  MSDBS variant of BUG_MDS */
 
 #endif /* _ASM_X86_CPUFEATURES_H */

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 0ea1e4b..1073118 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -953,6 +953,7 @@ static void identify_cpu_without_cpuid(struct cpuinfo_x86 *c)
 #define NO_SSB         BIT(2)
 #define NO_L1TF                BIT(3)
 #define NO_MDS         BIT(4)
+#define MSBDS_ONLY     BIT(5)
 
 #define VULNWL(_vendor, _family, _model, _whitelist)   \
        { X86_VENDOR_##_vendor, _family, _model, X86_FEATURE_ANY, _whitelist }
@@ -976,16 +977,16 @@ static const __initconst struct x86_cpu_id cpu_vuln_whitelist[] = {
        VULNWL_INTEL(ATOM_BONNELL,              NO_SPECULATION),
        VULNWL_INTEL(ATOM_BONNELL_MID,          NO_SPECULATION),
 
-       VULNWL_INTEL(ATOM_SILVERMONT,           NO_SSB | NO_L1TF),
-       VULNWL_INTEL(ATOM_SILVERMONT_X,         NO_SSB | NO_L1TF),
-       VULNWL_INTEL(ATOM_SILVERMONT_MID,       NO_SSB | NO_L1TF),
-       VULNWL_INTEL(ATOM_AIRMONT,              NO_SSB | NO_L1TF),
-       VULNWL_INTEL(XEON_PHI_KNL,              NO_SSB | NO_L1TF),
-       VULNWL_INTEL(XEON_PHI_KNM,              NO_SSB | NO_L1TF),
+       VULNWL_INTEL(ATOM_SILVERMONT,           NO_SSB | NO_L1TF | MSBDS_ONLY),
+       VULNWL_INTEL(ATOM_SILVERMONT_X,         NO_SSB | NO_L1TF | MSBDS_ONLY),
+       VULNWL_INTEL(ATOM_SILVERMONT_MID,       NO_SSB | NO_L1TF | MSBDS_ONLY),
+       VULNWL_INTEL(ATOM_AIRMONT,              NO_SSB | NO_L1TF | MSBDS_ONLY),
+       VULNWL_INTEL(XEON_PHI_KNL,              NO_SSB | NO_L1TF | MSBDS_ONLY),
+       VULNWL_INTEL(XEON_PHI_KNM,              NO_SSB | NO_L1TF | MSBDS_ONLY),
 
        VULNWL_INTEL(CORE_YONAH,                NO_SSB),
 
-       VULNWL_INTEL(ATOM_AIRMONT_MID,          NO_L1TF),
+       VULNWL_INTEL(ATOM_AIRMONT_MID,          NO_L1TF | MSBDS_ONLY),
 
        VULNWL_INTEL(ATOM_GOLDMONT,             NO_MDS | NO_L1TF),
        VULNWL_INTEL(ATOM_GOLDMONT_X,           NO_MDS | NO_L1TF),
@@ -1029,8 +1030,11 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
        if (ia32_cap & ARCH_CAP_IBRS_ALL)
                setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
 
-       if (!cpu_matches(NO_MDS) && !(ia32_cap & ARCH_CAP_MDS_NO))
+       if (!cpu_matches(NO_MDS) && !(ia32_cap & ARCH_CAP_MDS_NO)) {
                setup_force_cpu_bug(X86_BUG_MDS);
+               if (cpu_matches(MSBDS_ONLY))
+                       setup_force_cpu_bug(X86_BUG_MSBDS_ONLY);
+       }
 
        if (cpu_matches(NO_MELTDOWN))
                return;