Re: [PATCH v4 03/36] x86/bugs: Restructure mmio mitigation
From: Borislav Petkov
Date: Mon Mar 24 2025 - 05:30:04 EST
On Thu, Mar 13, 2025 at 12:26:06PM -0700, Pawan Gupta wrote:
> Hmm, that would not be straightforward, specially for sysfs status.
See below:
- the unknown thing is done only for this vuln and not for the others
- it doesn't do anything besides reporting things differently - it doesn't
apply any mitigations - it is simply causing unnecessary complications which
don't bring anything besides maintenance overhead. Unless I'm missing an
angle...
- all the currently unaffected CPUs can also be in "unknown" status so why is
this special?
IOW, just whack the thing.
diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 46935f29805c..75d77a5c28d7 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -518,7 +518,7 @@
#define X86_BUG_ITLB_MULTIHIT X86_BUG(23) /* "itlb_multihit" CPU may incur MCE during certain page attribute changes */
#define X86_BUG_SRBDS X86_BUG(24) /* "srbds" CPU may leak RNG bits if not mitigated */
#define X86_BUG_MMIO_STALE_DATA X86_BUG(25) /* "mmio_stale_data" CPU is affected by Processor MMIO Stale Data vulnerabilities */
-#define X86_BUG_MMIO_UNKNOWN X86_BUG(26) /* "mmio_unknown" CPU is too old and its MMIO Stale Data status is unknown */
+/* unused, was #define X86_BUG_MMIO_UNKNOWN X86_BUG(26) "mmio_unknown" CPU is too old and its MMIO Stale Data status is unknown */
#define X86_BUG_RETBLEED X86_BUG(27) /* "retbleed" CPU is affected by RETBleed */
#define X86_BUG_EIBRS_PBRSB X86_BUG(28) /* "eibrs_pbrsb" EIBRS is vulnerable to Post Barrier RSB Predictions */
#define X86_BUG_SMT_RSB X86_BUG(29) /* "smt_rsb" CPU is vulnerable to Cross-Thread Return Address Predictions */
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 4386aa6c69e1..a91a1cac6183 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -428,7 +428,6 @@ static const char * const mmio_strings[] = {
static void __init mmio_select_mitigation(void)
{
if (!boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA) ||
- boot_cpu_has_bug(X86_BUG_MMIO_UNKNOWN) ||
cpu_mitigations_off()) {
mmio_mitigation = MMIO_MITIGATION_OFF;
return;
@@ -591,8 +590,6 @@ static void __init md_clear_update_mitigation(void)
pr_info("TAA: %s\n", taa_strings[taa_mitigation]);
if (boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA))
pr_info("MMIO Stale Data: %s\n", mmio_strings[mmio_mitigation]);
- else if (boot_cpu_has_bug(X86_BUG_MMIO_UNKNOWN))
- pr_info("MMIO Stale Data: Unknown: No mitigations\n");
if (boot_cpu_has_bug(X86_BUG_RFDS))
pr_info("Register File Data Sampling: %s\n", rfds_strings[rfds_mitigation]);
}
@@ -2819,9 +2816,6 @@ static ssize_t tsx_async_abort_show_state(char *buf)
static ssize_t mmio_stale_data_show_state(char *buf)
{
- if (boot_cpu_has_bug(X86_BUG_MMIO_UNKNOWN))
- return sysfs_emit(buf, "Unknown: No mitigations\n");
-
if (mmio_mitigation == MMIO_MITIGATION_OFF)
return sysfs_emit(buf, "%s\n", mmio_strings[mmio_mitigation]);
@@ -3006,7 +3000,6 @@ static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr
return srbds_show_state(buf);
case X86_BUG_MMIO_STALE_DATA:
- case X86_BUG_MMIO_UNKNOWN:
return mmio_stale_data_show_state(buf);
case X86_BUG_RETBLEED:
@@ -3075,10 +3068,7 @@ ssize_t cpu_show_srbds(struct device *dev, struct device_attribute *attr, char *
ssize_t cpu_show_mmio_stale_data(struct device *dev, struct device_attribute *attr, char *buf)
{
- if (boot_cpu_has_bug(X86_BUG_MMIO_UNKNOWN))
- return cpu_show_common(dev, attr, buf, X86_BUG_MMIO_UNKNOWN);
- else
- return cpu_show_common(dev, attr, buf, X86_BUG_MMIO_STALE_DATA);
+ return cpu_show_common(dev, attr, buf, X86_BUG_MMIO_STALE_DATA);
}
ssize_t cpu_show_retbleed(struct device *dev, struct device_attribute *attr, char *buf)
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 12126adbc3a9..4ada55f126ae 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1402,15 +1402,10 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
* Affected CPU list is generally enough to enumerate the vulnerability,
* but for virtualization case check for ARCH_CAP MSR bits also, VMM may
* not want the guest to enumerate the bug.
- *
- * Set X86_BUG_MMIO_UNKNOWN for CPUs that are neither in the blacklist,
- * nor in the whitelist and also don't enumerate MSR ARCH_CAP MMIO bits.
*/
if (!arch_cap_mmio_immune(x86_arch_cap_msr)) {
if (cpu_matches(cpu_vuln_blacklist, MMIO))
setup_force_cpu_bug(X86_BUG_MMIO_STALE_DATA);
- else if (!cpu_matches(cpu_vuln_whitelist, NO_MMIO))
- setup_force_cpu_bug(X86_BUG_MMIO_UNKNOWN);
}
if (!cpu_has(c, X86_FEATURE_BTC_NO)) {
diff --git a/tools/arch/x86/include/asm/cpufeatures.h b/tools/arch/x86/include/asm/cpufeatures.h
index 9e3fa7942e7d..e88500d90309 100644
--- a/tools/arch/x86/include/asm/cpufeatures.h
+++ b/tools/arch/x86/include/asm/cpufeatures.h
@@ -508,7 +508,7 @@
#define X86_BUG_ITLB_MULTIHIT X86_BUG(23) /* "itlb_multihit" CPU may incur MCE during certain page attribute changes */
#define X86_BUG_SRBDS X86_BUG(24) /* "srbds" CPU may leak RNG bits if not mitigated */
#define X86_BUG_MMIO_STALE_DATA X86_BUG(25) /* "mmio_stale_data" CPU is affected by Processor MMIO Stale Data vulnerabilities */
-#define X86_BUG_MMIO_UNKNOWN X86_BUG(26) /* "mmio_unknown" CPU is too old and its MMIO Stale Data status is unknown */
+/* unused, was #define X86_BUG_MMIO_UNKNOWN X86_BUG(26) * "mmio_unknown" CPU is too old and its MMIO Stale Data status is unknown */
#define X86_BUG_RETBLEED X86_BUG(27) /* "retbleed" CPU is affected by RETBleed */
#define X86_BUG_EIBRS_PBRSB X86_BUG(28) /* "eibrs_pbrsb" EIBRS is vulnerable to Post Barrier RSB Predictions */
#define X86_BUG_SMT_RSB X86_BUG(29) /* "smt_rsb" CPU is vulnerable to Cross-Thread Return Address Predictions */
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette