linux-kernel - [PATCH 2/4] kvm, vmx: move register clearing out of assembly path

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Message-Id: <558fea0b4df498eefcaea5ae07a089ad9706c1a2.1540369608.git.jsteckli@amazon.de>
Date:   Wed, 24 Oct 2018 10:28:57 +0200
From:   Julian Stecklina <jsteckli@...zon.de>
To:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Cc:     Julian Stecklina <jsteckli@...zon.de>, js@...en8.de,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/4] kvm, vmx: move register clearing out of assembly path

Split the security related register clearing out of the large inline
assembly VM entry path. This results in two slightly less complicated
inline assembly statements, where it is clearer what each one does.

Signed-off-by: Julian Stecklina <jsteckli@...zon.de>
Reviewed-by: Jan H. Schönherr <jschoenh@...zon.de>
Reviewed-by: Konrad Jan Miller <kjm@...zon.de>
---
 arch/x86/kvm/vmx.c | 33 ++++++++++++++++++++-------------
 1 file changed, 20 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 93562d5..9225099 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -10797,20 +10797,7 @@ 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"
-		"xor %%r8d,  %%r8d \n\t"
-		"xor %%r9d,  %%r9d \n\t"
-		"xor %%r10d, %%r10d \n\t"
-		"xor %%r11d, %%r11d \n\t"
-		"xor %%r12d, %%r12d \n\t"
-		"xor %%r13d, %%r13d \n\t"
-		"xor %%r14d, %%r14d \n\t"
-		"xor %%r15d, %%r15d \n\t"
 #endif
-
-		"xor %%eax, %%eax \n\t"
-		"xor %%ebx, %%ebx \n\t"
-		"xor %%esi, %%esi \n\t"
-		"xor %%edi, %%edi \n\t"
 		"pop  %%" _ASM_BP "; pop  %%" _ASM_DX " \n\t"
 		".pushsection .rodata \n\t"
 		".global vmx_return \n\t"
@@ -10847,6 +10834,26 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 #endif
 	      );
 
+	/* Don't let guest register values survive. */
+	asm volatile (
+		""
+#ifdef CONFIG_X86_64
+		"xor %%r8d,  %%r8d \n\t"
+		"xor %%r9d,  %%r9d \n\t"
+		"xor %%r10d, %%r10d \n\t"
+		"xor %%r11d, %%r11d \n\t"
+		"xor %%r12d, %%r12d \n\t"
+		"xor %%r13d, %%r13d \n\t"
+		"xor %%r14d, %%r14d \n\t"
+		"xor %%r15d, %%r15d \n\t"
+#endif
+		:: "a" (0), "b" (0), "S" (0), "D" (0)
+		: "cc"
+#ifdef CONFIG_X86_64
+		  , "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15"
+#endif
+		);
+
 	/*
 	 * We do not use IBRS in the kernel. If this vCPU has used the
 	 * SPEC_CTRL MSR it may have left it on; save the value and
-- 
2.7.4