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x86/bugs: Add 'spectre_bhi=vmexit' cmdline option

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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:

  36d4fe147c ("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.

Mitigation switch requested by Maksim Davydov <davydov-max@yandex-team.ru>.

  [ bp: Massage. ]

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/r/2cbad706a6d5e1da2829e5e123d8d5c80330148c.1719381528.git.jpoimboe@kernel.org
This commit is contained in:
Josh Poimboeuf 2024-06-25 23:02:02 -07:00 committed by Borislav Petkov (AMD)
parent 4586c93ebf
commit 42c141fbb6
2 changed files with 20 additions and 8 deletions
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12
Documentation/admin-guide/kernel-parameters.txt
View File

@ -6136,9 +6136,15 @@
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.

16
arch/x86/kernel/cpu/bugs.c
View File

@ -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)