linux-kernel - [PATCH 5.0 226/238] KVM: VMX: Zero out *all* general purpose registers after VM-Exit

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Message-Id: <20190322111311.688384234@linuxfoundation.org>
Date:   Fri, 22 Mar 2019 12:17:25 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 5.0 226/238] KVM: VMX: Zero out *all* general purpose registers after VM-Exit

5.0-stable review patch.  If anyone has any objections, please let me know.

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

From: Sean Christopherson <sean.j.christopherson@...el.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@...r.kernel.org>
Cc: Jim Mattson <jmattson@...gle.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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),