[tip: x86/bugs] x86/bugs: Restructure spectre_v2_user mitigation

tip-bot2 for David Kaplan
2025-05-02T10:33:29Z

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

Commit-ID: ddfca9430a617780c8ad9691bf44660ae49e2a35
Gitweb: https://git.kernel.org/tip/ddfca9430a617780c8ad9691bf44660ae49e2a35
Author: David Kaplan <david.kaplan@amd.com>
AuthorDate: Fri, 18 Apr 2025 11:17:16 -05:00
Committer: Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Tue, 29 Apr 2025 18:51:21 +02:00

x86/bugs: Restructure spectre_v2_user mitigation

Restructure spectre_v2_user to use select/update/apply functions to
create consistent vulnerability handling.

The IBPB/STIBP choices are first decided based on the spectre_v2_user
command line but can be modified by the spectre_v2 command line option
as well.

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/20250418161721.1855190-12-david.kaplan@amd.com
---
arch/x86/kernel/cpu/bugs.c | 168 ++++++++++++++++++++----------------
1 file changed, 94 insertions(+), 74 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 207a472..dc75195 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -60,6 +60,8 @@ static void __init retbleed_select_mitigation(void);
static void __init retbleed_update_mitigation(void);
static void __init retbleed_apply_mitigation(void);
static void __init spectre_v2_user_select_mitigation(void);
+static void __init spectre_v2_user_update_mitigation(void);
+static void __init spectre_v2_user_apply_mitigation(void);
static void __init ssb_select_mitigation(void);
static void __init l1tf_select_mitigation(void);
static void __init mds_select_mitigation(void);
@@ -190,11 +192,6 @@ void __init cpu_select_mitigations(void)
spectre_v1_select_mitigation();
spectre_v2_select_mitigation();
retbleed_select_mitigation();
- /*
- * spectre_v2_user_select_mitigation() relies on the state set by
- * retbleed_select_mitigation(); specifically the STIBP selection is
- * forced for UNRET or IBPB.
- */
spectre_v2_user_select_mitigation();
ssb_select_mitigation();
l1tf_select_mitigation();
@@ -217,6 +214,13 @@ void __init cpu_select_mitigations(void)
* choices.
*/
retbleed_update_mitigation();
+
+ /*
+ * spectre_v2_user_update_mitigation() depends on
+ * retbleed_update_mitigation(), specifically the STIBP
+ * selection is forced for UNRET or IBPB.
+ */
+ spectre_v2_user_update_mitigation();
mds_update_mitigation();
taa_update_mitigation();
mmio_update_mitigation();
@@ -224,6 +228,7 @@ 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();
@@ -1379,6 +1384,8 @@ 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,
@@ -1417,31 +1424,18 @@ 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)
+static enum spectre_v2_user_cmd __init spectre_v2_parse_user_cmdline(void)
{
- 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)) {
@@ -1452,7 +1446,7 @@ 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)
@@ -1460,60 +1454,72 @@ 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_mitigation mode = SPECTRE_V2_USER_NONE;
- 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:
- 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;
break;
case SPECTRE_V2_USER_CMD_AUTO:
case SPECTRE_V2_USER_CMD_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:
- mode = SPECTRE_V2_USER_PRCTL;
+ spectre_v2_user_ibpb = SPECTRE_V2_USER_STRICT;
+ spectre_v2_user_stibp = SPECTRE_V2_USER_PRCTL;
break;
case SPECTRE_V2_USER_CMD_SECCOMP:
+ if (IS_ENABLED(CONFIG_SECCOMP))
+ spectre_v2_user_ibpb = SPECTRE_V2_USER_SECCOMP;
+ else
+ spectre_v2_user_ibpb = SPECTRE_V2_USER_PRCTL;
+ spectre_v2_user_stibp = spectre_v2_user_ibpb;
+ break;
case SPECTRE_V2_USER_CMD_SECCOMP_IBPB:
+ spectre_v2_user_ibpb = SPECTRE_V2_USER_STRICT;
if (IS_ENABLED(CONFIG_SECCOMP))
- mode = SPECTRE_V2_USER_SECCOMP;
+ spectre_v2_user_stibp = SPECTRE_V2_USER_SECCOMP;
else
- mode = SPECTRE_V2_USER_PRCTL;
+ spectre_v2_user_stibp = SPECTRE_V2_USER_PRCTL;
break;
}

- /* Initialize Indirect Branch Prediction Barrier */
- if (boot_cpu_has(X86_FEATURE_IBPB)) {
- static_branch_enable(&switch_vcpu_ibpb);
+ /*
+ * At this point, an STIBP mode other than "off" has been set.
+ * If STIBP support is not being forced, check if STIBP always-on
+ * is preferred.
+ */
+ if ((spectre_v2_user_stibp == SPECTRE_V2_USER_PRCTL ||
+ spectre_v2_user_stibp == SPECTRE_V2_USER_SECCOMP) &&
+ boot_cpu_has(X86_FEATURE_AMD_STIBP_ALWAYS_ON))
+ spectre_v2_user_stibp = SPECTRE_V2_USER_STRICT_PREFERRED;

- spectre_v2_user_ibpb = mode;
- switch (cmd) {
- case SPECTRE_V2_USER_CMD_NONE:
- break;
- case SPECTRE_V2_USER_CMD_FORCE:
- case SPECTRE_V2_USER_CMD_PRCTL_IBPB:
- case SPECTRE_V2_USER_CMD_SECCOMP_IBPB:
- static_branch_enable(&switch_mm_always_ibpb);
- spectre_v2_user_ibpb = SPECTRE_V2_USER_STRICT;
- break;
- case SPECTRE_V2_USER_CMD_PRCTL:
- case SPECTRE_V2_USER_CMD_AUTO:
- case SPECTRE_V2_USER_CMD_SECCOMP:
- static_branch_enable(&switch_mm_cond_ibpb);
- break;
- }
+ if (!boot_cpu_has(X86_FEATURE_IBPB))
+ spectre_v2_user_ibpb = SPECTRE_V2_USER_NONE;

- pr_info("mitigation: Enabling %s Indirect Branch Prediction Barrier\n",
- static_key_enabled(&switch_mm_always_ibpb) ?
- "always-on" : "conditional");
+ if (!boot_cpu_has(X86_FEATURE_STIBP))
+ spectre_v2_user_stibp = SPECTRE_V2_USER_NONE;
+}
+
+static void __init spectre_v2_user_update_mitigation(void)
+{
+ if (!boot_cpu_has(X86_FEATURE_IBPB) && !boot_cpu_has(X86_FEATURE_STIBP))
+ return;
+
+ /* The spectre_v2 cmd line can override spectre_v2_user options */
+ if (spectre_v2_cmd == SPECTRE_V2_CMD_NONE) {
+ spectre_v2_user_ibpb = SPECTRE_V2_USER_NONE;
+ spectre_v2_user_stibp = SPECTRE_V2_USER_NONE;
+ } else if (spectre_v2_cmd == SPECTRE_V2_CMD_FORCE) {
+ spectre_v2_user_ibpb = SPECTRE_V2_USER_STRICT;
+ spectre_v2_user_stibp = SPECTRE_V2_USER_STRICT;
}

/*
@@ -1531,30 +1537,44 @@ spectre_v2_user_select_mitigation(void)
if (!boot_cpu_has(X86_FEATURE_STIBP) ||
!cpu_smt_possible() ||
(spectre_v2_in_eibrs_mode(spectre_v2_enabled) &&
- !boot_cpu_has(X86_FEATURE_AUTOIBRS)))
+ !boot_cpu_has(X86_FEATURE_AUTOIBRS))) {
+ spectre_v2_user_stibp = SPECTRE_V2_USER_NONE;
return;
+ }

- /*
- * At this point, an STIBP mode other than "off" has been set.
- * If STIBP support is not being forced, check if STIBP always-on
- * is preferred.
- */
- if (mode != SPECTRE_V2_USER_STRICT &&
- boot_cpu_has(X86_FEATURE_AMD_STIBP_ALWAYS_ON))
- mode = SPECTRE_V2_USER_STRICT_PREFERRED;
-
- if (retbleed_mitigation == RETBLEED_MITIGATION_UNRET ||
- retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
- if (mode != SPECTRE_V2_USER_STRICT &&
- mode != SPECTRE_V2_USER_STRICT_PREFERRED)
+ if (spectre_v2_user_stibp != SPECTRE_V2_USER_NONE &&
+ (retbleed_mitigation == RETBLEED_MITIGATION_UNRET ||
+ retbleed_mitigation == RETBLEED_MITIGATION_IBPB)) {
+ if (spectre_v2_user_stibp != SPECTRE_V2_USER_STRICT &&
+ spectre_v2_user_stibp != SPECTRE_V2_USER_STRICT_PREFERRED)
pr_info("Selecting STIBP always-on mode to complement retbleed mitigation\n");
- mode = SPECTRE_V2_USER_STRICT_PREFERRED;
+ spectre_v2_user_stibp = SPECTRE_V2_USER_STRICT_PREFERRED;
}
+ pr_info("%s\n", spectre_v2_user_strings[spectre_v2_user_stibp]);
+}

- spectre_v2_user_stibp = mode;
+static void __init spectre_v2_user_apply_mitigation(void)
+{
+ /* Initialize Indirect Branch Prediction Barrier */
+ if (spectre_v2_user_ibpb != SPECTRE_V2_USER_NONE) {
+ static_branch_enable(&switch_vcpu_ibpb);
+
+ switch (spectre_v2_user_ibpb) {
+ case SPECTRE_V2_USER_STRICT:
+ static_branch_enable(&switch_mm_always_ibpb);
+ break;
+ case SPECTRE_V2_USER_PRCTL:
+ case SPECTRE_V2_USER_SECCOMP:
+ static_branch_enable(&switch_mm_cond_ibpb);
+ break;
+ default:
+ break;
+ }

-set_mode:
- pr_info("%s\n", spectre_v2_user_strings[mode]);
+ pr_info("mitigation: Enabling %s Indirect Branch Prediction Barrier\n",
+ static_key_enabled(&switch_mm_always_ibpb) ?
+ "always-on" : "conditional");
+ }
}

static const char * const spectre_v2_strings[] = {