Messages in this thread Patch in this message | | | Date | Tue, 15 Aug 2023 12:58:31 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH] x86/srso: Correct the mitigation status when SMT is disabled |
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On Tue, Aug 15, 2023 at 11:57:24AM +0200, Borislav Petkov wrote: > --- a/arch/x86/kernel/cpu/bugs.c > +++ b/arch/x86/kernel/cpu/bugs.c > @@ -2417,8 +2417,7 @@ static void __init srso_select_mitigation(void) > * Zen1/2 with SMT off aren't vulnerable after the right > * IBPB microcode has been applied. > */ > - if ((boot_cpu_data.x86 < 0x19) && > - (!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED))) { > + if (boot_cpu_data.x86 < 0x19 && !cpu_smt_possible()) { > setup_force_cpu_cap(X86_FEATURE_SRSO_NO); > return; > } > @@ -2698,8 +2697,12 @@ static ssize_t retbleed_show_state(char *buf) > > static ssize_t srso_show_state(char *buf) > { > - if (boot_cpu_has(X86_FEATURE_SRSO_NO)) > - return sysfs_emit(buf, "Not affected\n"); > + if (boot_cpu_has(X86_FEATURE_SRSO_NO)) { > + if (sched_smt_active()) > + return sysfs_emit(buf, "Not affected\n"); > + else > + return sysfs_emit(buf, "Mitigation: SMT disabled\n"); > + }
AFAICT, nowhere in the spec does it say the SRSO_NO bit won't get set by future (fixed) HW. In fact I'd expect it will, similar to other *_NO flags.
Regardless, here SRSO_NO seems to mean two different things: "reported safe by host (or HW)" and "not reported safe on Zen1/2 with SMT not possible".
Also, in this code, the SRSO_NO+SMT combo doesn't seem logically possible, as srso_show_state() only gets called if X86_BUG_SRSO is set, which only happens if SRSO_NO is not set by the HW/host in the first place. So here, if boot_cpu_has(X86_FEATURE_SRSO_NO), it means SRSO_NO was manually set by srso_select_mitigation(), and SMT can't possibly be enabled.
Instead of piggybacking on SRSO_NO, which is confusing, why not just add a new mitigation type, like:
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 6c04aef4b63b..c925b98f5a15 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -2343,6 +2343,7 @@ early_param("l1tf", l1tf_cmdline); enum srso_mitigation { SRSO_MITIGATION_NONE, SRSO_MITIGATION_MICROCODE, + SRSO_MITIGATION_SMT, SRSO_MITIGATION_SAFE_RET, SRSO_MITIGATION_IBPB, SRSO_MITIGATION_IBPB_ON_VMEXIT, @@ -2359,6 +2360,7 @@ enum srso_mitigation_cmd { static const char * const srso_strings[] = { [SRSO_MITIGATION_NONE] = "Vulnerable", [SRSO_MITIGATION_MICROCODE] = "Mitigation: microcode", + [SRSO_MITIGATION_SMT] = "Mitigation: SMT disabled", [SRSO_MITIGATION_SAFE_RET] = "Mitigation: safe RET", [SRSO_MITIGATION_IBPB] = "Mitigation: IBPB", [SRSO_MITIGATION_IBPB_ON_VMEXIT] = "Mitigation: IBPB on VMEXIT only" @@ -2407,19 +2409,15 @@ static void __init srso_select_mitigation(void) pr_warn("IBPB-extending microcode not applied!\n"); pr_warn(SRSO_NOTICE); } else { - /* - * Enable the synthetic (even if in a real CPUID leaf) - * flags for guests. - */ + /* Enable the synthetic flag, as HW doesn't set it. */ setup_force_cpu_cap(X86_FEATURE_IBPB_BRTYPE); /* * Zen1/2 with SMT off aren't vulnerable after the right * IBPB microcode has been applied. */ - if ((boot_cpu_data.x86 < 0x19) && - (!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED))) { - setup_force_cpu_cap(X86_FEATURE_SRSO_NO); + if ((boot_cpu_data.x86 < 0x19) && !cpu_smt_possible()) { + srso_mitigation = SRSO_MITIGATION_SMT; return; } } @@ -2698,9 +2696,6 @@ static ssize_t retbleed_show_state(char *buf) static ssize_t srso_show_state(char *buf) { - if (boot_cpu_has(X86_FEATURE_SRSO_NO)) - return sysfs_emit(buf, "Not affected\n"); - return sysfs_emit(buf, "%s%s\n", srso_strings[srso_mitigation], (cpu_has_ibpb_brtype_microcode() ? "" : ", no microcode"));
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