This is a note to let you know that I've just added the patch titled
x86/bhi: Mitigate KVM by default
to the 6.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-bhi-mitigate-kvm-by-default.patch
and it can be found in the queue-6.8 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From d592d3e579b2d9964b1bbf4fce310d3de9d9caf8 Mon Sep 17 00:00:00 2001
From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Date: Mon, 11 Mar 2024 08:57:09 -0700
Subject: x86/bhi: Mitigate KVM by default
From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
commit 95a6ccbdc7199a14b71ad8901cb788ba7fb5167b upstream.
BHI mitigation mode spectre_bhi=auto does not deploy the software
mitigation by default. In a cloud environment, it is a likely scenario
where userspace is trusted but the guests are not trusted. Deploying
system wide mitigation in such cases is not desirable.
Update the auto mode to unconditionally mitigate against malicious
guests. Deploy the software sequence at VMexit in auto mode also, when
hardware mitigation is not available. Unlike the force =on mode,
software sequence is not deployed at syscalls in auto mode.
Suggested-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Signed-off-by: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Alexandre Chartre <alexandre.chartre@xxxxxxxxxx>
Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/hw-vuln/spectre.rst | 7 +++++--
Documentation/admin-guide/kernel-parameters.txt | 5 +++--
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/nospec-branch.h | 5 +++++
arch/x86/kernel/cpu/bugs.c | 9 ++++++++-
arch/x86/kvm/vmx/vmenter.S | 2 +-
6 files changed, 23 insertions(+), 6 deletions(-)
--- a/Documentation/admin-guide/hw-vuln/spectre.rst
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -439,10 +439,12 @@ The possible values in this file are:
- System is protected by retpoline
* - BHI: BHI_DIS_S
- System is protected by BHI_DIS_S
- * - BHI: SW loop
+ * - BHI: SW loop; KVM SW loop
- System is protected by software clearing sequence
* - BHI: Syscall hardening
- Syscalls are hardened against BHI
+ * - BHI: Syscall hardening; KVM: SW loop
+ - System is protected from userspace attacks by syscall hardening; KVM is protected by software clearing sequence
Full mitigation might require a microcode update from the CPU
vendor. When the necessary microcode is not available, the kernel will
@@ -669,7 +671,8 @@ kernel command line.
unconditionally disable.
auto
enable if hardware mitigation
- control(BHI_DIS_S) is available.
+ control(BHI_DIS_S) is available, otherwise
+ enable alternate mitigation in KVM.
For spectre_v2_user see Documentation/admin-guide/kernel-parameters.txt
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6039,8 +6039,9 @@
on - unconditionally enable.
off - unconditionally disable.
- auto - (default) enable only if hardware mitigation
- control(BHI_DIS_S) is available.
+ auto - (default) enable hardware mitigation
+ (BHI_DIS_S) if available, otherwise enable
+ alternate mitigation in KVM.
spectre_v2= [X86] Control mitigation of Spectre variant 2
(indirect branch speculation) vulnerability.
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -465,6 +465,7 @@
#define X86_FEATURE_CLEAR_BHB_LOOP (21*32+ 1) /* "" Clear branch history at syscall entry using SW loop */
#define X86_FEATURE_BHI_CTRL (21*32+ 2) /* "" BHI_DIS_S HW control available */
#define X86_FEATURE_CLEAR_BHB_HW (21*32+ 3) /* "" BHI_DIS_S HW control enabled */
+#define X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT (21*32+ 4) /* "" Clear branch history at vmexit using SW loop */
/*
* BUG word(s)
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -339,8 +339,13 @@
.macro CLEAR_BRANCH_HISTORY
ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP
.endm
+
+.macro CLEAR_BRANCH_HISTORY_VMEXIT
+ ALTERNATIVE "", "call clear_bhb_loop", X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT
+.endm
#else
#define CLEAR_BRANCH_HISTORY
+#define CLEAR_BRANCH_HISTORY_VMEXIT
#endif
#else /* __ASSEMBLY__ */
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1667,9 +1667,14 @@ static void __init bhi_select_mitigation
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_AUTO)
return;
+ /* Mitigate syscalls when the mitigation is forced =on */
setup_force_cpu_cap(X86_FEATURE_CLEAR_BHB_LOOP);
pr_info("Spectre BHI mitigation: SW BHB clearing on syscall\n");
}
@@ -2810,10 +2815,12 @@ static const char * const spectre_bhi_st
else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_HW))
return "; BHI: BHI_DIS_S";
else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP))
- return "; BHI: SW loop";
+ return "; BHI: SW loop, KVM: SW loop";
else if (boot_cpu_has(X86_FEATURE_RETPOLINE) &&
!(x86_read_arch_cap_msr() & ARCH_CAP_RRSBA))
return "; BHI: Retpoline";
+ else if (boot_cpu_has(X86_FEATURE_CLEAR_BHB_LOOP_ON_VMEXIT))
+ return "; BHI: Syscall hardening, KVM: SW loop";
return "; BHI: Vulnerable (Syscall hardening enabled)";
}
--- a/arch/x86/kvm/vmx/vmenter.S
+++ b/arch/x86/kvm/vmx/vmenter.S
@@ -275,7 +275,7 @@ SYM_INNER_LABEL_ALIGN(vmx_vmexit, SYM_L_
call vmx_spec_ctrl_restore_host
- CLEAR_BRANCH_HISTORY
+ CLEAR_BRANCH_HISTORY_VMEXIT
/* Put return value in AX */
mov %_ASM_BX, %_ASM_AX
Patches currently in stable-queue which might be from pawan.kumar.gupta@xxxxxxxxxxxxxxx are
queue-6.8/x86-bhi-define-spec_ctrl_bhi_dis_s.patch
queue-6.8/x86-bhi-add-support-for-clearing-branch-history-at-syscall-entry.patch
queue-6.8/x86-bhi-mitigate-kvm-by-default.patch
queue-6.8/x86-bhi-enumerate-branch-history-injection-bhi-bug.patch
queue-6.8/x86-bhi-add-bhi-mitigation-knob.patch
queue-6.8/kvm-x86-add-bhi_no.patch
