Re: [PATCH v5 3/3] x86/bugs: Add 'spectre_bhi=vmexit' cmdline option

From: Daniel Sneddon
Date: Tue May 07 2024 - 10:58:32 EST


On 5/6/24 22:30, Josh Poimboeuf wrote:
> In cloud environments it can be useful to *only* enable the vmexit
> mitigation and leave syscalls vulnerable. Add that as an option.
>
> This is similar to the old spectre_bhi=auto option which was removed
> with the following commit:
>
> 36d4fe147c87 ("x86/bugs: Remove CONFIG_BHI_MITIGATION_AUTO and spectre_bhi=auto")
>
> with the main difference being that this has a more descriptive name and
> is disabled by default.
>
> Requested-by: Maksim Davydov <davydov-max@xxxxxxxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---

Does the KConfig option need to be updated to support this as well? Other than
that,
Reviewed-by: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>

> Documentation/admin-guide/kernel-parameters.txt | 12 +++++++++---
> arch/x86/kernel/cpu/bugs.c | 16 +++++++++++-----
> 2 files changed, 20 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 213d0719e2b7..9c1f63f04502 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6072,9 +6072,15 @@
> deployment of the HW BHI control and the SW BHB
> clearing sequence.
>
> - on - (default) Enable the HW or SW mitigation
> - as needed.
> - off - Disable the mitigation.
> + on - (default) Enable the HW or SW mitigation as
> + needed. This protects the kernel from
> + both syscalls and VMs.
> + vmexit - On systems which don't have the HW mitigation
> + available, enable the SW mitigation on vmexit
> + ONLY. On such systems, the host kernel is
> + protected from VM-originated BHI attacks, but
> + may still be vulnerable to syscall attacks.
> + off - Disable the mitigation.
>
> spectre_v2= [X86,EARLY] Control mitigation of Spectre variant 2
> (indirect branch speculation) vulnerability.
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index ab18185894df..6974c8c9792d 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1625,6 +1625,7 @@ static bool __init spec_ctrl_bhi_dis(void)
> enum bhi_mitigations {
> BHI_MITIGATION_OFF,
> BHI_MITIGATION_ON,
> + BHI_MITIGATION_VMEXIT_ONLY,
> };
>
> static enum bhi_mitigations bhi_mitigation __ro_after_init =
> @@ -1639,6 +1640,8 @@ static int __init spectre_bhi_parse_cmdline(char *str)
> bhi_mitigation = BHI_MITIGATION_OFF;
> else if (!strcmp(str, "on"))
> bhi_mitigation = BHI_MITIGATION_ON;
> + else if (!strcmp(str, "vmexit"))
> + bhi_mitigation = BHI_MITIGATION_VMEXIT_ONLY;
> else
> pr_err("Ignoring unknown spectre_bhi option (%s)", str);
>
> @@ -1659,19 +1662,22 @@ static void __init bhi_select_mitigation(void)
> return;
> }
>
> + /* Mitigate in hardware if supported */
> if (spec_ctrl_bhi_dis())
> return;
>
> if (!IS_ENABLED(CONFIG_X86_64))
> return;
>
> - /* Mitigate KVM by default */
> - setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
> - pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit\n");
> + if (bhi_mitigation == BHI_MITIGATION_VMEXIT_ONLY) {
> + pr_info("Spectre BHI mitigation: SW BHB clearing on vm exit only\n");
> + setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
> + return;
> + }
>
> - /* Mitigate syscalls when the mitigation is forced =on */
> + pr_info("Spectre BHI mitigation: SW BHB clearing on syscall and vm exit\n");
> setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);
> - pr_info("Spectre BHI mitigation: SW BHB clearing on syscall\n");
> + setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT);
> }
>
> static void __init spectre_v2_select_mitigation(void)