From: "Borislav Petkov (AMD)" <bp@alien8.de>
Date: Tue, 18 Jul 2023 11:13:40 +0200
Subject: x86/srso: Add IBPB_BRTYPE support
Git-commit: 79113e4060aba744787a81edb9014f2865193854
Patch-mainline: v6.6 or v6.5-rc4 (next release)
References: bsc#1213287, CVE-2023-20569
Add support for the synthetic CPUID flag which "if this bit is 1,
it indicates that MSR 49h (PRED_CMD) bit 0 (IBPB) flushes all branch
type predictions from the CPU branch predictor."
This flag is there so that this capability in guests can be detected
easily (otherwise one would have to track microcode revisions which is
impossible for guests).
It is also needed only for Zen3 and -4. The other two (Zen1 and -2)
always flush branch type predictions by default.
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Nikolay Borisov <nik.borisov@suse.com>
---
arch/x86/include/asm/cpufeatures.h | 2 ++
arch/x86/kernel/cpu/bugs.c | 12 +++++++++++-
2 files changed, 13 insertions(+), 1 deletion(-)
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -380,6 +380,8 @@
/* AMD-defined Extended Feature 2 EAX, CPUID level 0x80000021 (EAX), word 20 */
#define X86_FEATURE_LFENCE_RDTSC (20*32+ 2) /* "" LFENCE always serializing / synchronizes RDTSC */
+#define X86_FEATURE_IBPB_BRTYPE (20*32+28) /* "" MSR_PRED_CMD[IBPB] flushes all branch type predictions */
+
/*
* BUG word(s)
*/
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1631,10 +1631,20 @@ static void __init srso_select_mitigatio
if (!boot_cpu_has_bug(X86_BUG_SRSO) || cpu_mitigations_off())
return;
- has_microcode = cpu_has_ibpb_brtype_microcode();
+ /*
+ * The first check is for the kernel running as a guest in order
+ * for guests to verify whether IBPB is a viable mitigation.
+ */
+ has_microcode = boot_cpu_has(X86_FEATURE_IBPB_BRTYPE) || cpu_has_ibpb_brtype_microcode();
if (!has_microcode) {
pr_warn("IBPB-extending microcode not applied!\n");
pr_warn(SRSO_NOTICE);
+ } else {
+ /*
+ * Enable the synthetic (even if in a real CPUID leaf)
+ * flag for guests.
+ */
+ setup_force_cpu_cap(X86_FEATURE_IBPB_BRTYPE);
}
switch (srso_cmd) {