x86/bugs: Use IBPB for retbleed if used by SRSO

If spec_rstack_overflow=ibpb then this mitigates retbleed as well.  This
is relevant for AMD Zen1 and Zen2 CPUs which are vulnerable to both bugs.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: H . Peter Anvin <hpa@zytor.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250625155805.600376-3-david.kaplan@amd.com

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 6c991afb7b78e3b8a6017ed47988b14b7d516c70..b2634198323040a9a6dea7c79341de440b59b2a1 100644 (file)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1171,6 +1171,21 @@ static enum retbleed_mitigation retbleed_mitigation __ro_after_init =
 
 static int __ro_after_init retbleed_nosmt = false;
 
+enum srso_mitigation {
+       SRSO_MITIGATION_NONE,
+       SRSO_MITIGATION_AUTO,
+       SRSO_MITIGATION_UCODE_NEEDED,
+       SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED,
+       SRSO_MITIGATION_MICROCODE,
+       SRSO_MITIGATION_NOSMT,
+       SRSO_MITIGATION_SAFE_RET,
+       SRSO_MITIGATION_IBPB,
+       SRSO_MITIGATION_IBPB_ON_VMEXIT,
+       SRSO_MITIGATION_BP_SPEC_REDUCE,
+};
+
+static enum srso_mitigation srso_mitigation __ro_after_init = SRSO_MITIGATION_AUTO;
+
 static int __init retbleed_parse_cmdline(char *str)
 {
        if (!str)
@@ -1280,6 +1295,10 @@ static void __init retbleed_update_mitigation(void)
        if (its_mitigation == ITS_MITIGATION_RETPOLINE_STUFF)
                retbleed_mitigation = RETBLEED_MITIGATION_STUFF;
 
+       /* If SRSO is using IBPB, that works for retbleed too */
+       if (srso_mitigation == SRSO_MITIGATION_IBPB)
+               retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
+
        if (retbleed_mitigation == RETBLEED_MITIGATION_STUFF &&
            !cdt_possible(spectre_v2_enabled)) {
                pr_err("WARNING: retbleed=stuff depends on retpoline\n");
@@ -2845,19 +2864,6 @@ early_param("l1tf", l1tf_cmdline);
 #undef pr_fmt
 #define pr_fmt(fmt)    "Speculative Return Stack Overflow: " fmt
 
-enum srso_mitigation {
-       SRSO_MITIGATION_NONE,
-       SRSO_MITIGATION_AUTO,
-       SRSO_MITIGATION_UCODE_NEEDED,
-       SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED,
-       SRSO_MITIGATION_MICROCODE,
-       SRSO_MITIGATION_NOSMT,
-       SRSO_MITIGATION_SAFE_RET,
-       SRSO_MITIGATION_IBPB,
-       SRSO_MITIGATION_IBPB_ON_VMEXIT,
-       SRSO_MITIGATION_BP_SPEC_REDUCE,
-};
-
 static const char * const srso_strings[] = {
        [SRSO_MITIGATION_NONE]                  = "Vulnerable",
        [SRSO_MITIGATION_UCODE_NEEDED]          = "Vulnerable: No microcode",
@@ -2870,8 +2876,6 @@ static const char * const srso_strings[] = {
        [SRSO_MITIGATION_BP_SPEC_REDUCE]        = "Mitigation: Reduced Speculation"
 };
 
-static enum srso_mitigation srso_mitigation __ro_after_init = SRSO_MITIGATION_AUTO;
-
 static int __init srso_parse_cmdline(char *str)
 {
        if (!str)