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KVM: VMX: Zero out *all* general purpose registers after VM-Exit

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...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: 0cb5b30698 ("kvm: vmx: Scrub hardware GPRs at VM-exit")
Cc: <stable@vger.kernel.org>
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2019-01-25 07:40:50 -08:00 committed by Paolo Bonzini
parent 1ce072cbfd
commit 0e0ab73c9a
1 changed files with 11 additions and 3 deletions
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12
arch/x86/kvm/vmx/vmx.c
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@ -6452,9 +6452,14 @@ 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"
@ -6470,8 +6475,11 @@ 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),