...except RSP, which is restored by hardware as part of VM-Exit. Paolo theorized that restoring registers from the stack after a VM-Exit in lieu of zeroing them could lead to speculative execution with the guest's values, e.g. if the stack accesses miss the L1 cache[1]. Zeroing XORs are dirt cheap, so just be ultra-paranoid. Note that the scratch register (currently RCX) used to save/restore the guest state is also zeroed as its host-defined value is loaded via the stack, just with a MOV instead of a POP. [1] https://patchwork.kernel.org/patch/10771539/#22441255 Fixes: 0cb5b30698fd ("kvm: vmx: Scrub hardware GPRs at VM-exit") Cc: <stable@xxxxxxxxxxxxxxx> Cc: Jim Mattson <jmattson@xxxxxxxxxx> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> --- arch/x86/kvm/vmx/vmx.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index 57735ef11179..a8de1d7f06e1 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -6451,10 +6451,15 @@ static void __vmx_vcpu_run(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx) "mov %%r13, %c[r13](%%" _ASM_CX ") \n\t" "mov %%r14, %c[r14](%%" _ASM_CX ") \n\t" "mov %%r15, %c[r15](%%" _ASM_CX ") \n\t" + /* - * Clear host registers marked as clobbered to prevent - * speculative use. - */ + * Clear all general purpose registers (except RSP, which is loaded by + * the CPU during VM-Exit) to prevent speculative use of the guest's + * values, even those that are saved/loaded via the stack. In theory, + * an L1 cache miss when restoring registers could lead to speculative + * execution with the guest's values. Zeroing XORs are dirt cheap, + * i.e. the extra paranoia is essentially free. + */ "xor %%r8d, %%r8d \n\t" "xor %%r9d, %%r9d \n\t" "xor %%r10d, %%r10d \n\t" @@ -6469,8 +6474,11 @@ static void __vmx_vcpu_run(struct kvm_vcpu *vcpu, struct vcpu_vmx *vmx) "xor %%eax, %%eax \n\t" "xor %%ebx, %%ebx \n\t" + "xor %%ecx, %%ecx \n\t" + "xor %%edx, %%edx \n\t" "xor %%esi, %%esi \n\t" "xor %%edi, %%edi \n\t" + "xor %%ebp, %%ebp \n\t" "pop %%" _ASM_BP "; pop %%" _ASM_DX " \n\t" : ASM_CALL_CONSTRAINT : "c"(vmx), "d"((unsigned long)HOST_RSP), "S"(evmcs_rsp), -- 2.20.1
