[tip: x86/bugs] x86/bugs: Restructure SRSO mitigation

tip-bot2 for David Kaplan
2025-05-02T10:33:34Z

The following commit has been merged into the x86/bugs branch of tip:

Commit-ID: 1f4bb068b498a544ae913764a797449463ef620c
Gitweb: https://git.kernel.org/tip/1f4bb068b498a544ae913764a797449463ef620c
Author: David Kaplan <david.kaplan@amd.com>
AuthorDate: Fri, 18 Apr 2025 11:17:21 -05:00
Committer: Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Wed, 30 Apr 2025 10:25:45 +02:00

x86/bugs: Restructure SRSO mitigation

Restructure SRSO to use select/update/apply functions to create
consistent vulnerability handling. Like with retbleed, the command line
options directly select mitigations which can later be modified.

While at it, remove a comment which doesn't apply anymore due to the
changed mitigation detection flow.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/20250418161721.1855190-17-david.kaplan@amd.com
---
arch/x86/kernel/cpu/bugs.c | 215 ++++++++++++++++--------------------
1 file changed, 101 insertions(+), 114 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 25d84e2..a4f3b1d 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -84,6 +84,8 @@ static void __init srbds_select_mitigation(void);
static void __init srbds_apply_mitigation(void);
static void __init l1d_flush_select_mitigation(void);
static void __init srso_select_mitigation(void);
+static void __init srso_update_mitigation(void);
+static void __init srso_apply_mitigation(void);
static void __init gds_select_mitigation(void);
static void __init gds_apply_mitigation(void);
static void __init bhi_select_mitigation(void);
@@ -208,11 +210,6 @@ void __init cpu_select_mitigations(void)
rfds_select_mitigation();
srbds_select_mitigation();
l1d_flush_select_mitigation();
-
- /*
- * srso_select_mitigation() depends and must run after
- * retbleed_select_mitigation().
- */
srso_select_mitigation();
gds_select_mitigation();
bhi_select_mitigation();
@@ -240,6 +237,8 @@ void __init cpu_select_mitigations(void)
mmio_update_mitigation();
rfds_update_mitigation();
bhi_update_mitigation();
+ /* srso_update_mitigation() depends on retbleed_update_mitigation(). */
+ srso_update_mitigation();

spectre_v1_apply_mitigation();
spectre_v2_apply_mitigation();
@@ -252,6 +251,7 @@ void __init cpu_select_mitigations(void)
mmio_apply_mitigation();
rfds_apply_mitigation();
srbds_apply_mitigation();
+ srso_apply_mitigation();
gds_apply_mitigation();
bhi_apply_mitigation();
}
@@ -2674,6 +2674,7 @@ early_param("l1tf", l1tf_cmdline);

enum srso_mitigation {
SRSO_MITIGATION_NONE,
+ SRSO_MITIGATION_AUTO,
SRSO_MITIGATION_UCODE_NEEDED,
SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED,
SRSO_MITIGATION_MICROCODE,
@@ -2683,14 +2684,6 @@ enum srso_mitigation {
SRSO_MITIGATION_BP_SPEC_REDUCE,
};

-enum srso_mitigation_cmd {
- SRSO_CMD_OFF,
- SRSO_CMD_MICROCODE,
- SRSO_CMD_SAFE_RET,
- SRSO_CMD_IBPB,
- SRSO_CMD_IBPB_ON_VMEXIT,
-};
-
static const char * const srso_strings[] = {
[SRSO_MITIGATION_NONE] = "Vulnerable",
[SRSO_MITIGATION_UCODE_NEEDED] = "Vulnerable: No microcode",
@@ -2702,8 +2695,7 @@ 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_NONE;
-static enum srso_mitigation_cmd srso_cmd __ro_after_init = SRSO_CMD_SAFE_RET;
+static enum srso_mitigation srso_mitigation __ro_after_init = SRSO_MITIGATION_AUTO;

static int __init srso_parse_cmdline(char *str)
{
@@ -2711,15 +2703,15 @@ static int __init srso_parse_cmdline(char *str)
return -EINVAL;

if (!strcmp(str, "off"))
- srso_cmd = SRSO_CMD_OFF;
+ srso_mitigation = SRSO_MITIGATION_NONE;
else if (!strcmp(str, "microcode"))
- srso_cmd = SRSO_CMD_MICROCODE;
+ srso_mitigation = SRSO_MITIGATION_MICROCODE;
else if (!strcmp(str, "safe-ret"))
- srso_cmd = SRSO_CMD_SAFE_RET;
+ srso_mitigation = SRSO_MITIGATION_SAFE_RET;
else if (!strcmp(str, "ibpb"))
- srso_cmd = SRSO_CMD_IBPB;
+ srso_mitigation = SRSO_MITIGATION_IBPB;
else if (!strcmp(str, "ibpb-vmexit"))
- srso_cmd = SRSO_CMD_IBPB_ON_VMEXIT;
+ srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
else
pr_err("Ignoring unknown SRSO option (%s).", str);

@@ -2731,132 +2723,83 @@ early_param("spec_rstack_overflow", srso_parse_cmdline);

static void __init srso_select_mitigation(void)
{
- bool has_microcode = boot_cpu_has(X86_FEATURE_IBPB_BRTYPE);
+ bool has_microcode;

- if (!boot_cpu_has_bug(X86_BUG_SRSO) ||
- cpu_mitigations_off() ||
- srso_cmd == SRSO_CMD_OFF) {
- if (boot_cpu_has(X86_FEATURE_SBPB))
- x86_pred_cmd = PRED_CMD_SBPB;
- goto out;
- }
+ if (!boot_cpu_has_bug(X86_BUG_SRSO) || cpu_mitigations_off())
+ srso_mitigation = SRSO_MITIGATION_NONE;

+ if (srso_mitigation == SRSO_MITIGATION_NONE)
+ return;
+
+ if (srso_mitigation == SRSO_MITIGATION_AUTO)
+ srso_mitigation = SRSO_MITIGATION_SAFE_RET;
+
+ has_microcode = boot_cpu_has(X86_FEATURE_IBPB_BRTYPE);
if (has_microcode) {
/*
* Zen1/2 with SMT off aren't vulnerable after the right
* IBPB microcode has been applied.
- *
- * Zen1/2 don't have SBPB, no need to try to enable it here.
*/
if (boot_cpu_data.x86 < 0x19 && !cpu_smt_possible()) {
setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
- goto out;
- }
-
- if (retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
- srso_mitigation = SRSO_MITIGATION_IBPB;
- goto out;
+ srso_mitigation = SRSO_MITIGATION_NONE;
+ return;
}
} else {
pr_warn("IBPB-extending microcode not applied!\n");
pr_warn(SRSO_NOTICE);
-
- /* may be overwritten by SRSO_CMD_SAFE_RET below */
- srso_mitigation = SRSO_MITIGATION_UCODE_NEEDED;
}

- switch (srso_cmd) {
- case SRSO_CMD_MICROCODE:
- if (has_microcode) {
- srso_mitigation = SRSO_MITIGATION_MICROCODE;
- pr_warn(SRSO_NOTICE);
- }
- break;
-
- case SRSO_CMD_SAFE_RET:
- if (boot_cpu_has(X86_FEATURE_SRSO_USER_KERNEL_NO))
+ switch (srso_mitigation) {
+ case SRSO_MITIGATION_SAFE_RET:
+ if (boot_cpu_has(X86_FEATURE_SRSO_USER_KERNEL_NO)) {
+ srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
goto ibpb_on_vmexit;
+ }

- if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
- /*
- * Enable the return thunk for generated code
- * like ftrace, static_call, etc.
- */
- setup_force_cpu_cap(X86_FEATURE_RETHUNK);
- setup_force_cpu_cap(X86_FEATURE_UNRET);
-
- if (boot_cpu_data.x86 == 0x19) {
- setup_force_cpu_cap(X86_FEATURE_SRSO_ALIAS);
- x86_return_thunk = srso_alias_return_thunk;
- } else {
- setup_force_cpu_cap(X86_FEATURE_SRSO);
- x86_return_thunk = srso_return_thunk;
- }
- if (has_microcode)
- srso_mitigation = SRSO_MITIGATION_SAFE_RET;
- else
- srso_mitigation = SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED;
- } else {
+ if (!IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
pr_err("WARNING: kernel not compiled with MITIGATION_SRSO.\n");
+ srso_mitigation = SRSO_MITIGATION_NONE;
}
- break;

- case SRSO_CMD_IBPB:
- if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
- if (has_microcode) {
- setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
- setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
- srso_mitigation = SRSO_MITIGATION_IBPB;
-
- /*
- * IBPB on entry already obviates the need for
- * software-based untraining so clear those in case some
- * other mitigation like Retbleed has selected them.
- */
- setup_clear_cpu_cap(X86_FEATURE_UNRET);
- setup_clear_cpu_cap(X86_FEATURE_RETHUNK);
-
- /*
- * There is no need for RSB filling: write_ibpb() ensures
- * all predictions, including the RSB, are invalidated,
- * regardless of IBPB implementation.
- */
- setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
- }
- } else {
- pr_err("WARNING: kernel not compiled with MITIGATION_IBPB_ENTRY.\n");
- }
+ if (!has_microcode)
+ srso_mitigation = SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED;
break;
-
ibpb_on_vmexit:
- case SRSO_CMD_IBPB_ON_VMEXIT:
+ case SRSO_MITIGATION_IBPB_ON_VMEXIT:
if (boot_cpu_has(X86_FEATURE_SRSO_BP_SPEC_REDUCE)) {
pr_notice("Reducing speculation to address VM/HV SRSO attack vector.\n");
srso_mitigation = SRSO_MITIGATION_BP_SPEC_REDUCE;
break;
}
-
- if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
- if (has_microcode) {
- setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
- srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
-
- /*
- * There is no need for RSB filling: write_ibpb() ensures
- * all predictions, including the RSB, are invalidated,
- * regardless of IBPB implementation.
- */
- setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
- }
- } else {
+ fallthrough;
+ case SRSO_MITIGATION_IBPB:
+ if (!IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
pr_err("WARNING: kernel not compiled with MITIGATION_IBPB_ENTRY.\n");
+ srso_mitigation = SRSO_MITIGATION_NONE;
}
+
+ if (!has_microcode)
+ srso_mitigation = SRSO_MITIGATION_UCODE_NEEDED;
break;
default:
break;
}
+}

-out:
+static void __init srso_update_mitigation(void)
+{
+ /* If retbleed is using IBPB, that works for SRSO as well */
+ if (retbleed_mitigation == RETBLEED_MITIGATION_IBPB &&
+ boot_cpu_has(X86_FEATURE_IBPB_BRTYPE))
+ srso_mitigation = SRSO_MITIGATION_IBPB;
+
+ if (boot_cpu_has_bug(X86_BUG_SRSO) && !cpu_mitigations_off())
+ pr_info("%s\n", srso_strings[srso_mitigation]);
+}
+
+static void __init srso_apply_mitigation(void)
+{
/*
* Clear the feature flag if this mitigation is not selected as that
* feature flag controls the BpSpecReduce MSR bit toggling in KVM.
@@ -2864,8 +2807,52 @@ out:
if (srso_mitigation != SRSO_MITIGATION_BP_SPEC_REDUCE)
setup_clear_cpu_cap(X86_FEATURE_SRSO_BP_SPEC_REDUCE);

- if (srso_mitigation != SRSO_MITIGATION_NONE)
- pr_info("%s\n", srso_strings[srso_mitigation]);
+ if (srso_mitigation == SRSO_MITIGATION_NONE) {
+ if (boot_cpu_has(X86_FEATURE_SBPB))
+ x86_pred_cmd = PRED_CMD_SBPB;
+ return;
+ }
+
+ switch (srso_mitigation) {
+ case SRSO_MITIGATION_SAFE_RET:
+ case SRSO_MITIGATION_SAFE_RET_UCODE_NEEDED:
+ /*
+ * Enable the return thunk for generated code
+ * like ftrace, static_call, etc.
+ */
+ setup_force_cpu_cap(X86_FEATURE_RETHUNK);
+ setup_force_cpu_cap(X86_FEATURE_UNRET);
+
+ if (boot_cpu_data.x86 == 0x19) {
+ setup_force_cpu_cap(X86_FEATURE_SRSO_ALIAS);
+ x86_return_thunk = srso_alias_return_thunk;
+ } else {
+ setup_force_cpu_cap(X86_FEATURE_SRSO);
+ x86_return_thunk = srso_return_thunk;
+ }
+ break;
+ case SRSO_MITIGATION_IBPB:
+ setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
+ /*
+ * IBPB on entry already obviates the need for
+ * software-based untraining so clear those in case some
+ * other mitigation like Retbleed has selected them.
+ */
+ setup_clear_cpu_cap(X86_FEATURE_UNRET);
+ setup_clear_cpu_cap(X86_FEATURE_RETHUNK);
+ fallthrough;
+ case SRSO_MITIGATION_IBPB_ON_VMEXIT:
+ setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
+ /*
+ * There is no need for RSB filling: entry_ibpb() ensures
+ * all predictions, including the RSB, are invalidated,
+ * regardless of IBPB implementation.
+ */
+ setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
+ break;
+ default:
+ break;
+ }
}

#undef pr_fmt