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

This change “KVM: VMX: Zero out *all* general purpose registers after VM-Exit” (commit 0e0ab73) in Linux kernel is authored by Sean Christopherson <sean.j.christopherson [at] intel.com> on Fri Jan 25 07:40:50 2019 -0800.

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

The change “KVM: VMX: Zero out *all* general purpose registers after VM-Exit” introduces changes as follows.

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

...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>

Linux kernel releases containing commit 0e0ab73

The Linux kernel releases containing this commit are as follows.

Linux kernel code changes from "KVM: VMX: Zero out *all* general purpose registers after VM-Exit"

There are 14 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 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 fdb6305cc971..10fee67a6dcd 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6452,10 +6452,15 @@ 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"
@@ -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),

The commit for this change in Linux stable tree is 0e0ab73 (patch).

Last modified: 2020/02/09