x86/bugs: Add SRSO_USER_KERNEL_NO support

If the machine has:

  CPUID Fn8000_0021_EAX[30] (SRSO_USER_KERNEL_NO) -- If this bit is 1,
  it indicates the CPU is not subject to the SRSO vulnerability across
  user/kernel boundaries.

have it fall back to IBPB on VMEXIT only, in the case it is going to run
VMs:

  Speculative Return Stack Overflow: Mitigation: IBPB on VMEXIT only

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/r/20241202120416.6054-2-bp@kernel.org

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 645aa360628da78356e2aeb575ff166bbaab61e9..0e2d81763615e474c765257b7d1a0f88cd2ae854 100644 (file)
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -465,6 +465,7 @@
 #define X86_FEATURE_SBPB               (20*32+27) /* Selective Branch Prediction Barrier */
 #define X86_FEATURE_IBPB_BRTYPE                (20*32+28) /* MSR_PRED_CMD[IBPB] flushes all branch type predictions */
 #define X86_FEATURE_SRSO_NO            (20*32+29) /* CPU is not affected by SRSO */
+#define X86_FEATURE_SRSO_USER_KERNEL_NO        (20*32+30) /* CPU is not affected by SRSO across user/kernel boundaries */
 
 /*
  * Extended auxiliary flags: Linux defined - for features scattered in various

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 47a01d4028f60e012d1e625e1667842b52948038..5a505aa654899a6249c9713abc17a87dd99faacc 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2615,6 +2615,9 @@ static void __init srso_select_mitigation(void)
                break;
 
        case SRSO_CMD_SAFE_RET:
+               if (boot_cpu_has(X86_FEATURE_SRSO_USER_KERNEL_NO))
+                       goto ibpb_on_vmexit;
+
                if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
                        /*
                         * Enable the return thunk for generated code
@@ -2658,6 +2661,7 @@ static void __init srso_select_mitigation(void)
                }
                break;
 
+ibpb_on_vmexit:
        case SRSO_CMD_IBPB_ON_VMEXIT:
                if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
                        if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 3e9037690814b331b3433a4abdecc25368c2a662..7e8d811b51c83382144727c14b3918a244784b11 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1270,6 +1270,7 @@ static const struct x86_cpu_id cpu_vuln_blacklist[] __initconst = {
        VULNBL_AMD(0x17, RETBLEED | SMT_RSB | SRSO),
        VULNBL_HYGON(0x18, RETBLEED | SMT_RSB | SRSO),
        VULNBL_AMD(0x19, SRSO),
+       VULNBL_AMD(0x1a, SRSO),
        {}
 };