From: Andi Kleen <a...@linux.intel.com>
On Skylake we want additional protections against spectre_v2
over the normal RETPOLINE against underflowing return buffers.
On return buffer underflow the CPU could fall back to
the poisoned indirect branch predictor.
This patch adds a feature bit, only enabled on Skylake,
to enable additional return buffer underflow protections
This will be used in the next patches
Signed-off-by: Andi Kleen <a...@linux.intel.com>
---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/kernel/cpu/bugs.c | 5 +++++
2 files changed, 6 insertions(+)
diff --git a/arch/x86/include/asm/cpufeatures.h
b/arch/x86/include/asm/cpufeatures.h
index 25b9375c1484..85108eb1876b 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -211,6 +211,7 @@
#define X86_FEATURE_MBA ( 7*32+18) /* Memory Bandwidth
Allocation */
#define X86_FEATURE_RSB_CTXSW ( 7*32+19) /* Fill RSB on context
switches */
+#define X86_FEATURE_RSB_UNDERFLOW ( 7*32+20) /* Avoid return stack
underflows */
/* Virtualization flags: Linux defined, word 8 */
#define X86_FEATURE_TPR_SHADOW ( 8*32+ 0) /* Intel TPR Shadow */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 390b3dc3d438..68d449d51673 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -249,6 +249,11 @@ static void __init spectre_v2_select_mitigation(void)
setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
pr_info("Filling RSB on context switch\n");
}
+
+ if (is_skylake_era()) {
+ setup_force_cpu_cap(X86_FEATURE_RSB_UNDERFLOW);
+ pr_info("Enabling return buffer underflow protections");
+ }
}
#undef pr_fmt
--
2.14.3
