On Thu, Nov 14, 2024 at 09:48:36PM -0800, Josh Poimboeuf wrote:
> According to the docs, classic IBRS also needs RSB filling at context
> switch to protect against corrupt RSB entries (as opposed to RSB
> underflow).
Correct.
> Something like so...
>
>
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 47a01d4028f6..7b9c0a21e478 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1579,27 +1579,44 @@ static void __init spec_ctrl_disable_kernel_rrsba(void)
> rrsba_disabled = true;
> }
>
> -static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_mitigation mode)
> +static void __init spectre_v2_mitigate_rsb(enum spectre_v2_mitigation mode)
> {
> /*
> - * Similar to context switches, there are two types of RSB attacks
> - * after VM exit:
> + * In general there are two types of RSB attacks:
> *
> - * 1) RSB underflow
> + * 1) RSB underflow ("Intel Retbleed")
> + *
> + * Some Intel parts have "bottomless RSB". When the RSB is empty,
> + * speculated return targets may come from the branch predictor,
> + * which could have a user-poisoned BTB or BHB entry.
> + *
> + * user->user attacks are mitigated by IBPB on context switch.
> + *
> + * user->kernel attacks via context switch are mitigated by IBRS,
> + * eIBRS, or RSB filling.
> + *
> + * user->kernel attacks via kernel entry are mitigated by IBRS,
> + * eIBRS, or call depth tracking.
> + *
> + * On VMEXIT, guest->host attacks are mitigated by IBRS, eIBRS, or
> + * RSB filling.
> *
> * 2) Poisoned RSB entry
> *
> - * When retpoline is enabled, both are mitigated by filling/clearing
> - * the RSB.
> + * On a context switch, the previous task can poison RSB entries
> + * used by the next task, controlling its speculative return
> + * targets. Poisoned RSB entries can also be created by "AMD
> + * Retbleed" or SRSO.
> *
> - * When IBRS is enabled, while #1 would be mitigated by the IBRS branch
> - * prediction isolation protections, RSB still needs to be cleared
> - * because of #2. Note that SMEP provides no protection here, unlike
> - * user-space-poisoned RSB entries.
> + * user->user attacks are mitigated by IBPB on context switch.
> *
> - * eIBRS should protect against RSB poisoning, but if the EIBRS_PBRSB
> - * bug is present then a LITE version of RSB protection is required,
> - * just a single call needs to retire before a RET is executed.
> + * user->kernel attacks via context switch are prevented by
> + * SMEP+eIBRS+SRSO mitigations, or RSB clearing.
> + *
> + * guest->host attacks are mitigated by eIBRS or RSB clearing on
> + * VMEXIT. eIBRS implementations with X86_BUG_EIBRS_PBRSB still
> + * need "lite" RSB filling which retires a CALL before the first
> + * RET.
> */
> switch (mode) {
> case SPECTRE_V2_NONE:
> @@ -1608,8 +1625,8 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
> case SPECTRE_V2_EIBRS_LFENCE:
> case SPECTRE_V2_EIBRS:
> if (boot_cpu_has_bug(X86_BUG_EIBRS_PBRSB)) {
> - setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT_LITE);
> pr_info("Spectre v2 / PBRSB-eIBRS: Retire a single CALL on VMEXIT\n");
> + setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT_LITE);
> }
> return;
>
> @@ -1617,12 +1634,13 @@ static void __init spectre_v2_determine_rsb_fill_type_at_vmexit(enum spectre_v2_
> case SPECTRE_V2_RETPOLINE:
> case SPECTRE_V2_LFENCE:
> case SPECTRE_V2_IBRS:
> + pr_info("Spectre v2 / SpectreRSB : Filling RSB on context switch and VMEXIT\n");
> + setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
> setup_force_cpu_cap(X86_FEATURE_RSB_VMEXIT);
> - pr_info("Spectre v2 / SpectreRSB : Filling RSB on VMEXIT\n");
> return;
> }
>
> - pr_warn_once("Unknown Spectre v2 mode, disabling RSB mitigation at VM exit");
> + pr_warn_once("Unknown Spectre v2 mode, disabling RSB mitigation\n");
> dump_stack();
> }
>
> @@ -1817,48 +1835,7 @@ static void __init spectre_v2_select_mitigation(void)
> spectre_v2_enabled = mode;
> pr_info("%s\n", spectre_v2_strings[mode]);
>
> - /*
> - * If Spectre v2 protection has been enabled, fill the RSB during a
> - * context switch. In general there are two types of RSB attacks
> - * across context switches, for which the CALLs/RETs may be unbalanced.
> - *
> - * 1) RSB underflow
> - *
> - * Some Intel parts have "bottomless RSB". When the RSB is empty,
> - * speculated return targets may come from the branch predictor,
> - * which could have a user-poisoned BTB or BHB entry.
> - *
> - * AMD has it even worse: *all* returns are speculated from the BTB,
> - * regardless of the state of the RSB.
> - *
> - * When IBRS or eIBRS is enabled, the "user -> kernel" attack
> - * scenario is mitigated by the IBRS branch prediction isolation
> - * properties, so the RSB buffer filling wouldn't be necessary to
> - * protect against this type of attack.
> - *
> - * The "user -> user" attack scenario is mitigated by RSB filling.
> - *
> - * 2) Poisoned RSB entry
> - *
> - * If the 'next' in-kernel return stack is shorter than 'prev',
> - * 'next' could be tricked into speculating with a user-poisoned RSB
> - * entry.
> - *
> - * The "user -> kernel" attack scenario is mitigated by SMEP and
> - * eIBRS.
> - *
> - * The "user -> user" scenario, also known as SpectreBHB, requires
> - * RSB clearing.
> - *
> - * So to mitigate all cases, unconditionally fill RSB on context
> - * switches.
> - *
> - * FIXME: Is this pointless for retbleed-affected AMD?
> - */
> - setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
> - pr_info("Spectre v2 / SpectreRSB mitigation: Filling RSB on context switch\n");
> -
> - spectre_v2_determine_rsb_fill_type_at_vmexit(mode);
> + spectre_v2_mitigate_rsb(mode);
>
> /*
> * Retpoline protects the kernel, but doesn't protect firmware. IBRS
This LGTM.
I think SPECTRE_V2_EIBRS_RETPOLINE is placed in the wrong leg, it
doesn't need RSB filling on context switch, and only needs VMEXIT_LITE.
Does below change on top of your patch look okay?
---
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 7b9c0a21e478..d3b9a0d7a2b5 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1622,6 +1622,7 @@ static void __init spectre_v2_mitigate_rsb(enum spectre_v2_mitigation mode)
case SPECTRE_V2_NONE:
return;
+ case SPECTRE_V2_EIBRS_RETPOLINE:
case SPECTRE_V2_EIBRS_LFENCE:
case SPECTRE_V2_EIBRS:
if (boot_cpu_has_bug(X86_BUG_EIBRS_PBRSB)) {
@@ -1630,7 +1631,6 @@ static void __init spectre_v2_mitigate_rsb(enum spectre_v2_mitigation mode)
}
return;
- case SPECTRE_V2_EIBRS_RETPOLINE:
case SPECTRE_V2_RETPOLINE:
case SPECTRE_V2_LFENCE:
case SPECTRE_V2_IBRS:
