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x86/bhi: Mitigate KVM by default

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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@oracle.com>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Daniel Sneddon <daniel.sneddon@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
This commit is contained in:
Pawan Gupta 2024-03-11 08:57:09 -07:00 committed by Thomas Gleixner
parent ec9404e40e
commit 95a6ccbdc7
6 changed files with 23 additions and 6 deletions
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7
Documentation/admin-guide/hw-vuln/spectre.rst
View file

@ -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

5
Documentation/admin-guide/kernel-parameters.txt
View file

@ -6071,8 +6071,9 @@
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,EARLY] Control mitigation of Spectre variant 2
(indirect branch speculation) vulnerability.

1
arch/x86/include/asm/cpufeatures.h
View file

@ -469,6 +469,7 @@
#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)

5
arch/x86/include/asm/nospec-branch.h
View file

@ -330,8 +330,13 @@
.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__ */

9
arch/x86/kernel/cpu/bugs.c
View file

@ -1668,9 +1668,14 @@ static void __init bhi_select_mitigation(void)
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");
}
@ -2811,10 +2816,12 @@ static const char * const spectre_bhi_state(void)
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)";
}

2
arch/x86/kvm/vmx/vmenter.S
View file

@ -275,7 +275,7 @@ SYM_INNER_LABEL_ALIGN(vmx_vmexit, SYM_L_GLOBAL)
call vmx_spec_ctrl_restore_host
CLEAR_BRANCH_HISTORY
CLEAR_BRANCH_HISTORY_VMEXIT
/* Put return value in AX */
mov %_ASM_BX, %_ASM_AX