Re: [PATCH RFC 2/5] x86/speculation: Add support for 'cpu_spec_mitigations=' cmdline options

Borislav Petkov
2019-04-05T13:57:12Z

On Thu, Apr 04, 2019 at 11:44:12AM -0500, Josh Poimboeuf wrote:
> Configure x86 runtime CPU speculation bug mitigations in accordance with
> the 'cpu_spec_mitigations=' cmdline options. This affects Meltdown,
> Spectre v2, Speculative Store Bypass, and L1TF.
>
> The default behavior is unchanged.
>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> ---
> .../admin-guide/kernel-parameters.txt | 15 +++++++++
> arch/x86/include/asm/processor.h | 1 +
> arch/x86/kernel/cpu/bugs.c | 32 ++++++++++++++++---
> arch/x86/kvm/vmx/vmx.c | 2 ++
> arch/x86/mm/pti.c | 4 ++-
> 5 files changed, 49 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index ac42e510bd6e..29dc03971630 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2552,6 +2552,11 @@
>
> off
> Disable all speculative CPU mitigations.
> + Equivalent to: nopti [x86]
> + nospectre_v2 [x86]
> + spectre_v2_user=off [x86]
> + spec_store_bypass_disable=off [x86]
> + l1tf=off [x86]
>
> auto (default)
> Mitigate all speculative CPU vulnerabilities,
> @@ -2560,12 +2565,22 @@
> surprised by SMT getting disabled across kernel
> upgrades, or who have other ways of avoiding
> SMT-based attacks.
> + Equivalent to: pti=auto [x86]
> + spectre_v2=auto [x86]
> + spectre_v2_user=auto [x86]
> + spec_store_bypass_disable=auto [x86]
> + l1tf=flush [x86]
>
> auto,nosmt
> Mitigate all speculative CPU vulnerabilities,
> disabling SMT if needed. This is for users who
> always want to be fully mitigated, even if it
> means losing SMT.
> + Equivalent to: pti=auto [x86]
> + spectre_v2=auto [x86]
> + spectre_v2_user=auto [x86]
> + spec_store_bypass_disable=auto [x86]
> + l1tf=flush,nosmt [x86]
>
> mminit_loglevel=
> [KNL] When CONFIG_DEBUG_MEMORY_INIT is set, this

Yap, those sets look ok.

> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 2bb3a648fc12..7e95b310f869 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -982,6 +982,7 @@ void microcode_check(void);
>
> enum l1tf_mitigations {
> L1TF_MITIGATION_OFF,
> + L1TF_MITIGATION_DEFAULT,
> L1TF_MITIGATION_FLUSH_NOWARN,
> L1TF_MITIGATION_FLUSH,
> L1TF_MITIGATION_FLUSH_NOSMT,
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 2da82eff0eb4..65b95fb95ba5 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -308,8 +308,11 @@ spectre_v2_parse_user_cmdline(enum spectre_v2_mitigation_cmd v2_cmd)
>
> ret = cmdline_find_option(boot_command_line, "spectre_v2_user",
> arg, sizeof(arg));
> - if (ret < 0)
> + if (ret < 0) {
> + if (cpu_spec_mitigations == CPU_SPEC_MITIGATIONS_OFF)
> + return SPECTRE_V2_USER_CMD_NONE;

Instead of sprinkling that test in those three functions, just do it
once above in check_bugs(), before those *_select_mitigation() functions
get to run and depending on the value, you either run them or use the
default settings, for the OFF case, for example.

> 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)) {
> @@ -444,8 +447,11 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void)
> return SPECTRE_V2_CMD_NONE;
>
> ret = cmdline_find_option(boot_command_line, "spectre_v2", arg, sizeof(arg));
> - if (ret < 0)
> + if (ret < 0) {
> + if (cpu_spec_mitigations == CPU_SPEC_MITIGATIONS_OFF)
> + return SPECTRE_V2_CMD_NONE;
> return SPECTRE_V2_CMD_AUTO;
> + }
>
> for (i = 0; i < ARRAY_SIZE(mitigation_options); i++) {
> if (!match_option(arg, ret, mitigation_options[i].option))
> @@ -677,8 +683,11 @@ static enum ssb_mitigation_cmd __init ssb_parse_cmdline(void)
> } else {
> ret = cmdline_find_option(boot_command_line, "spec_store_bypass_disable",
> arg, sizeof(arg));
> - if (ret < 0)
> + if (ret < 0) {
> + if (cpu_spec_mitigations == CPU_SPEC_MITIGATIONS_OFF)
> + return SPEC_STORE_BYPASS_CMD_NONE;
> return SPEC_STORE_BYPASS_CMD_AUTO;
> + }
>
> for (i = 0; i < ARRAY_SIZE(ssb_mitigation_options); i++) {
> if (!match_option(arg, ret, ssb_mitigation_options[i].option))
> @@ -955,7 +964,7 @@ void x86_spec_ctrl_setup_ap(void)
> #define pr_fmt(fmt) "L1TF: " fmt
>
> /* Default mitigation for L1TF-affected CPUs */
> -enum l1tf_mitigations l1tf_mitigation __ro_after_init = L1TF_MITIGATION_FLUSH;
> +enum l1tf_mitigations l1tf_mitigation __ro_after_init = L1TF_MITIGATION_DEFAULT;
> #if IS_ENABLED(CONFIG_KVM_INTEL)
> EXPORT_SYMBOL_GPL(l1tf_mitigation);
> #endif
> @@ -1010,8 +1019,23 @@ static void __init l1tf_select_mitigation(void)
>
> override_cache_bits(&boot_cpu_data);
>
> + if (l1tf_mitigation == L1TF_MITIGATION_DEFAULT) {
> + switch (cpu_spec_mitigations) {
> + case CPU_SPEC_MITIGATIONS_OFF:
> + l1tf_mitigation = L1TF_MITIGATION_OFF;
> + break;
> + case CPU_SPEC_MITIGATIONS_AUTO:
> + l1tf_mitigation = L1TF_MITIGATION_FLUSH;
> + break;
> + case CPU_SPEC_MITIGATIONS_AUTO_NOSMT:
> + l1tf_mitigation = L1TF_MITIGATION_FLUSH_NOSMT;
> + break;
> + }
> + }
> +
> switch (l1tf_mitigation) {
> case L1TF_MITIGATION_OFF:
> + case L1TF_MITIGATION_DEFAULT:
> case L1TF_MITIGATION_FLUSH_NOWARN:
> case L1TF_MITIGATION_FLUSH:
> break;
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ab432a930ae8..83b5bdc3c777 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -233,6 +233,7 @@ static int vmx_setup_l1d_flush(enum vmx_l1d_flush_state l1tf)
> case L1TF_MITIGATION_FLUSH_NOWARN:
> case L1TF_MITIGATION_FLUSH:
> case L1TF_MITIGATION_FLUSH_NOSMT:
> + case L1TF_MITIGATION_DEFAULT:
> l1tf = VMENTER_L1D_FLUSH_COND;
> break;
> case L1TF_MITIGATION_FULL:
> @@ -6686,6 +6687,7 @@ static int vmx_vm_init(struct kvm *kvm)
> case L1TF_MITIGATION_FLUSH:
> case L1TF_MITIGATION_FLUSH_NOSMT:
> case L1TF_MITIGATION_FULL:
> + case L1TF_MITIGATION_DEFAULT:
> /*
> * Warn upon starting the first VM in a potentially
> * insecure environment.

The L1TF bits need to be a separate patch.

Thx.

--
Regards/Gruss,
Boris.

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