Re: [PATCH v5 11/16] x86/bugs: Restructure spectre_v2_user mitigation

Borislav Petkov
2025-04-29T08:47:39Z

On Fri, Apr 18, 2025 at 11:17:16AM -0500, David Kaplan wrote:
> @@ -217,6 +214,11 @@ void __init cpu_select_mitigations(void)
> * choices.
> */
> retbleed_update_mitigation();
> + /*
> + * spectre_v2_user_update_mitigation() depends on
> + * retbleed_update_mitigation().
> + */

Why aren't you keeping the reason for the dependency from the above comment?

That's important when we need to touch this code again...

> + spectre_v2_user_update_mitigation();
> mds_update_mitigation();
> taa_update_mitigation();
> mmio_update_mitigation();
> @@ -224,6 +226,7 @@ void __init cpu_select_mitigations(void)
>
> spectre_v1_apply_mitigation();
> retbleed_apply_mitigation();
> + spectre_v2_user_apply_mitigation();
> mds_apply_mitigation();
> taa_apply_mitigation();
> mmio_apply_mitigation();
> @@ -1374,6 +1377,8 @@ enum spectre_v2_mitigation_cmd {
> SPECTRE_V2_CMD_IBRS,
> };
>
> +static enum spectre_v2_mitigation_cmd spectre_v2_cmd __ro_after_init = SPECTRE_V2_CMD_AUTO;
> +
> enum spectre_v2_user_cmd {
> SPECTRE_V2_USER_CMD_NONE,
> SPECTRE_V2_USER_CMD_AUTO,
> @@ -1412,31 +1417,19 @@ static void __init spec_v2_user_print_cond(const char *reason, bool secure)
> pr_info("spectre_v2_user=%s forced on command line.\n", reason);
> }
>
> -static __ro_after_init enum spectre_v2_mitigation_cmd spectre_v2_cmd;
> -
> static enum spectre_v2_user_cmd __init
> spectre_v2_parse_user_cmdline(void)

Lemme unbreak that silly thing while here...

> {
> - enum spectre_v2_user_cmd mode;
> char arg[20];
> int ret, i;
>
> - mode = IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V2) ?
> - SPECTRE_V2_USER_CMD_AUTO : SPECTRE_V2_USER_CMD_NONE;
> -
> - switch (spectre_v2_cmd) {
> - case SPECTRE_V2_CMD_NONE:
> + if (cpu_mitigations_off() || !IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V2))
> return SPECTRE_V2_USER_CMD_NONE;
> - case SPECTRE_V2_CMD_FORCE:
> - return SPECTRE_V2_USER_CMD_FORCE;
> - default:
> - break;
> - }
>
> ret = cmdline_find_option(boot_command_line, "spectre_v2_user",
> arg, sizeof(arg));
> if (ret < 0)
> - return mode;
> + return SPECTRE_V2_USER_CMD_AUTO;
>
> for (i = 0; i < ARRAY_SIZE(v2_user_options); i++) {
> if (match_option(arg, ret, v2_user_options[i].option)) {
> @@ -1447,7 +1440,7 @@ spectre_v2_parse_user_cmdline(void)
> }
>
> pr_err("Unknown user space protection option (%s). Switching to default\n", arg);
> - return mode;
> + return SPECTRE_V2_USER_CMD_AUTO;
> }
>
> static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
> @@ -1458,7 +1451,6 @@ static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
> static void __init
> spectre_v2_user_select_mitigation(void)

That too.

> {
> - enum spectre_v2_user_mitigation mode = SPECTRE_V2_USER_NONE;
> enum spectre_v2_user_cmd cmd;

Might as well get rid of that one.

> if (!boot_cpu_has(X86_FEATURE_IBPB) && !boot_cpu_has(X86_FEATURE_STIBP))
> @@ -1467,48 +1459,65 @@ spectre_v2_user_select_mitigation(void)
> cmd = spectre_v2_parse_user_cmdline();
> switch (cmd) {
> case SPECTRE_V2_USER_CMD_NONE:
> - goto set_mode;
> + return;
> case SPECTRE_V2_USER_CMD_FORCE:
> - mode = SPECTRE_V2_USER_STRICT;
> + spectre_v2_user_ibpb = SPECTRE_V2_USER_STRICT;
> + spectre_v2_user_stibp = SPECTRE_V2_USER_STRICT;

Those should be aligned at the '=' sign for better readability.

...

IOW, all the changes ontop:

---

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index afea9179acdd..dc75195760ca 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -214,9 +214,11 @@ void __init cpu_select_mitigations(void)
* choices.
*/
retbleed_update_mitigation();
+
/*
* spectre_v2_user_update_mitigation() depends on
- * retbleed_update_mitigation().
+ * retbleed_update_mitigation(), specifically the STIBP
+ * selection is forced for UNRET or IBPB.
*/
spectre_v2_user_update_mitigation();
mds_update_mitigation();
@@ -1422,8 +1424,7 @@ static void __init spec_v2_user_print_cond(const char *reason, bool secure)
pr_info("spectre_v2_user=%s forced on command line.\n", reason);
}

-static enum spectre_v2_user_cmd __init
-spectre_v2_parse_user_cmdline(void)
+static enum spectre_v2_user_cmd __init spectre_v2_parse_user_cmdline(void)
{
char arg[20];
int ret, i;
@@ -1453,29 +1454,25 @@ static inline bool spectre_v2_in_ibrs_mode(enum spectre_v2_mitigation mode)
return spectre_v2_in_eibrs_mode(mode) || mode == SPECTRE_V2_IBRS;
}

-static void __init
-spectre_v2_user_select_mitigation(void)
+static void __init spectre_v2_user_select_mitigation(void)
{
- enum spectre_v2_user_cmd cmd;
-
if (!boot_cpu_has(X86_FEATURE_IBPB) && !boot_cpu_has(X86_FEATURE_STIBP))
return;

- cmd = spectre_v2_parse_user_cmdline();
- switch (cmd) {
+ switch (spectre_v2_parse_user_cmdline()) {
case SPECTRE_V2_USER_CMD_NONE:
return;
case SPECTRE_V2_USER_CMD_FORCE:
- spectre_v2_user_ibpb = SPECTRE_V2_USER_STRICT;
+ spectre_v2_user_ibpb = SPECTRE_V2_USER_STRICT;
spectre_v2_user_stibp = SPECTRE_V2_USER_STRICT;
break;
case SPECTRE_V2_USER_CMD_AUTO:
case SPECTRE_V2_USER_CMD_PRCTL:
- spectre_v2_user_ibpb = SPECTRE_V2_USER_PRCTL;
+ spectre_v2_user_ibpb = SPECTRE_V2_USER_PRCTL;
spectre_v2_user_stibp = SPECTRE_V2_USER_PRCTL;
break;
case SPECTRE_V2_USER_CMD_PRCTL_IBPB:
- spectre_v2_user_ibpb = SPECTRE_V2_USER_STRICT;
+ spectre_v2_user_ibpb = SPECTRE_V2_USER_STRICT;
spectre_v2_user_stibp = SPECTRE_V2_USER_PRCTL;
break;
case SPECTRE_V2_USER_CMD_SECCOMP:

--
Regards/Gruss,
Boris.