x86/bugs: Add SRSO_USER_KERNEL_NO support

commit 877818802c3e970f67ccb53012facc78bef5f97a upstream.

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
[ Harshit: Conflicts resolved as this commit: 7c62c442b6eb ("x86/vmscape:
  Enumerate VMSCAPE bug") has been applied already to 6.12.y ]
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.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 90f1f2f9d31400ada426413edcaf20cdab2196c9..3fc47f25cafcd7810a89ec3a8709b30f679eb7bf 100644 (file)
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -464,6 +464,7 @@
 #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 06bbc297c26c0371c2f5a0c906751d5b20a4e4bc..c3ea29efe26fd7b63d5449f618cbfee00269cbff 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2810,6 +2810,9 @@ 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
@@ -2861,6 +2864,7 @@ static void __init srso_select_mitigation(void)
                }
                break;
 
+ibpb_on_vmexit:
        case SRSO_CMD_IBPB_ON_VMEXIT:
                if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
                        if (has_microcode) {