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Message-ID: <2cbad706a6d5e1da2829e5e123d8d5c80330148c.1719381528.git.jpoimboe@kernel.org>
Date: Tue, 25 Jun 2024 23:02:02 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
Linus Torvalds <torvalds@...ux-foundation.org>,
Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Thomas Gleixner <tglx@...utronix.de>,
Alexandre Chartre <alexandre.chartre@...cle.com>,
Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
Peter Zijlstra <peterz@...radead.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Sean Christopherson <seanjc@...gle.com>,
Andrew Cooper <andrew.cooper3@...rix.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Nikolay Borisov <nik.borisov@...e.com>,
KP Singh <kpsingh@...nel.org>,
Waiman Long <longman@...hat.com>,
Borislav Petkov <bp@...en8.de>,
Ingo Molnar <mingo@...nel.org>,
Maksim Davydov <davydov-max@...dex-team.ru>
Subject: [PATCH v6 3/3] x86/bugs: Add 'spectre_bhi=vmexit' cmdline option
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@...dex-team.ru>
Reviewed-by: Daniel Sneddon <daniel.sneddon@...ux.intel.com>
Reviewed-by: Nikolay Borisov <nik.borisov@...e.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
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 11e57ba2985c..ddf4eff727ab 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -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.
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index b6f927f6c567..94bcf29df465 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)
--
2.45.1
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