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Subject[PATCH 5.0 226/238] KVM: VMX: Zero out *all* general purpose registers after VM-Exit
Date
5.0-stable review patch.  If anyone has any objections, please let me know.

------------------

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: <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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kvm/vmx/vmx.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6449,10 +6449,15 @@ static void __vmx_vcpu_run(struct kvm_vc
"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"
@@ -6467,8 +6472,11 @@ static void __vmx_vcpu_run(struct kvm_vc

"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),

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