From: Sean Christopherson sean.j.christopherson@intel.com
commit 0e0ab73c9a0243736bcd779b30b717e23ba9a56d upstream.
...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: Jim Mattson jmattson@google.com Signed-off-by: Sean Christopherson sean.j.christopherson@intel.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com [bwh: Backported to 4.19: adjust filename, context] Signed-off-by: Ben Hutchings ben.hutchings@codethink.co.uk Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/kvm/vmx.c | 12 ++++++++++++ 1 file changed, 12 insertions(+)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index d37b481..e4d0ad0 100644 --- a/arch/x86/kvm/vmx.c +++ b/arch/x86/kvm/vmx.c @@ -10841,6 +10841,15 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu) "mov %%r13, %c[r13](%0) \n\t" "mov %%r14, %c[r14](%0) \n\t" "mov %%r15, %c[r15](%0) \n\t" + + /* + * 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" @@ -10855,8 +10864,11 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
"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" ".pushsection .rodata \n\t" ".global vmx_return \n\t"