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x86/bugs, KVM: Support the combination of guest and host IBRS 

A guest may modify the SPEC_CTRL MSR from the value used by the
kernel. Since the kernel doesn't use IBRS, this means a value of zero is
what is needed in the host.

But the 336996-Speculative-Execution-Side-Channel-Mitigations.pdf refers to
the other bits as reserved so the kernel should respect the boot time
SPEC_CTRL value and use that.

This allows to deal with future extensions to the SPEC_CTRL interface if
any at all.

Note: This uses wrmsrl() instead of native_wrmsl(). I does not make any
difference as paravirt will over-write the callq *0xfff.. with the wrmsrl
assembler code.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Konrad Rzeszutek Wilk 2018-04-25 22:04:19 -04:00 committed by Thomas Gleixner
parent 1b86883ccb
commit 5cf6875487
4 changed files with 32 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
arch/x86/include/asm/nospec-branch.h
View File

@ -228,6 +228,16 @@ enum spectre_v2_mitigation {
extern void x86_spec_ctrl_set(u64);
extern u64 x86_spec_ctrl_get_default(void);
/*
* On VMENTER we must preserve whatever view of the SPEC_CTRL MSR
* the guest has, while on VMEXIT we restore the host view. This
* would be easier if SPEC_CTRL were architecturally maskable or
* shadowable for guests but this is not (currently) the case.
* Takes the guest view of SPEC_CTRL MSR as a parameter.
*/
extern void x86_spec_ctrl_set_guest(u64);
extern void x86_spec_ctrl_restore_host(u64);
extern char __indirect_thunk_start[];
extern char __indirect_thunk_end[];

18
arch/x86/kernel/cpu/bugs.c
View File

@ -123,6 +123,24 @@ u64 x86_spec_ctrl_get_default(void)
}
EXPORT_SYMBOL_GPL(x86_spec_ctrl_get_default);
void x86_spec_ctrl_set_guest(u64 guest_spec_ctrl)
{
if (!boot_cpu_has(X86_FEATURE_IBRS))
return;
if (x86_spec_ctrl_base != guest_spec_ctrl)
wrmsrl(MSR_IA32_SPEC_CTRL, guest_spec_ctrl);
}
EXPORT_SYMBOL_GPL(x86_spec_ctrl_set_guest);
void x86_spec_ctrl_restore_host(u64 guest_spec_ctrl)
{
if (!boot_cpu_has(X86_FEATURE_IBRS))
return;
if (x86_spec_ctrl_base != guest_spec_ctrl)
wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
}
EXPORT_SYMBOL_GPL(x86_spec_ctrl_restore_host);
#ifdef RETPOLINE
static bool spectre_v2_bad_module;

6
arch/x86/kvm/svm.c
View File

@ -5557,8 +5557,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
* is no need to worry about the conditional branch over the wrmsr
* being speculatively taken.
*/
if (svm->spec_ctrl)
native_wrmsrl(MSR_IA32_SPEC_CTRL, svm->spec_ctrl);
x86_spec_ctrl_set_guest(svm->spec_ctrl);
asm volatile (
"push %%" _ASM_BP "; \n\t"
@ -5670,8 +5669,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
if (svm->spec_ctrl)
native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
x86_spec_ctrl_restore_host(svm->spec_ctrl);
/* Eliminate branch target predictions from guest mode */
vmexit_fill_RSB();

6
arch/x86/kvm/vmx.c
View File

@ -9720,8 +9720,7 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
* is no need to worry about the conditional branch over the wrmsr
* being speculatively taken.
*/
if (vmx->spec_ctrl)
native_wrmsrl(MSR_IA32_SPEC_CTRL, vmx->spec_ctrl);
x86_spec_ctrl_set_guest(vmx->spec_ctrl);
vmx->__launched = vmx->loaded_vmcs->launched;
@ -9869,8 +9868,7 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
vmx->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
if (vmx->spec_ctrl)
native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
x86_spec_ctrl_restore_host(vmx->spec_ctrl);
/* Eliminate branch target predictions from guest mode */
vmexit_fill_RSB();