x86/mm/mpx: Work around MPX erratum SKD046

[mirror_ubuntu-artful-kernel.git] / arch / x86 / kernel / cpu / intel.c

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index e4393bfc7f0d9bb39520adceeae2968652779a5c..b47df99dc5d21396e142c5551beb7c6d87d218af 100644 (file)
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -25,6 +25,41 @@
 #include <asm/apic.h>
 #endif
 
+/*
+ * Just in case our CPU detection goes bad, or you have a weird system,
+ * allow a way to override the automatic disabling of MPX.
+ */
+static int forcempx;
+
+static int __init forcempx_setup(char *__unused)
+{
+       forcempx = 1;
+
+       return 1;
+}
+__setup("intel-skd-046-workaround=disable", forcempx_setup);
+
+void check_mpx_erratum(struct cpuinfo_x86 *c)
+{
+       if (forcempx)
+               return;
+       /*
+        * Turn off the MPX feature on CPUs where SMEP is not
+        * available or disabled.
+        *
+        * Works around Intel Erratum SKD046: "Branch Instructions
+        * May Initialize MPX Bound Registers Incorrectly".
+        *
+        * This might falsely disable MPX on systems without
+        * SMEP, like Atom processors without SMEP.  But there
+        * is no such hardware known at the moment.
+        */
+       if (cpu_has(c, X86_FEATURE_MPX) && !cpu_has(c, X86_FEATURE_SMEP)) {
+               setup_clear_cpu_cap(X86_FEATURE_MPX);
+               pr_warn("x86/mpx: Disabling MPX since SMEP not present\n");
+       }
+}
+
 static void early_init_intel(struct cpuinfo_x86 *c)
 {
        u64 misc_enable;
@@ -173,6 +208,8 @@ static void early_init_intel(struct cpuinfo_x86 *c)
                if (edx & (1U << 28))
                        c->x86_coreid_bits = get_count_order((ebx >> 16) & 0xff);
        }
+
+       check_mpx_erratum(c);
 }
 
 #ifdef CONFIG_X86_32