linux-kernel - [PATCH v3 2/3] kvm, vmx: move register clearing out of assembly path

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Message-Id: <e084cefd9c0620692787b8581312ec896c96056b.1541079752.git.jsteckli@amazon.de>
Date:   Thu,  1 Nov 2018 14:42:48 +0100
From:   Julian Stecklina <jsteckli@...zon.de>
To:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Cc:     Julian Stecklina <jsteckli@...zon.de>,
        Julian Stecklina <js@...en8.de>, linux-kernel@...r.kernel.org
Subject: [PATCH v3 2/3] 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>
Reviewed-by: Jim Mattson <jmattson@...gle.com>
---
 arch/x86/kvm/vmx.c | 46 +++++++++++++++++++++++++++++-----------------
 1 file changed, 29 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index a6e5a5c..8ebd41d 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -11281,24 +11281,7 @@ 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"
-		/*
-		* Clear host registers marked as clobbered to prevent
-		* speculative use.
-		*/
-		"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"
@@ -11336,6 +11319,35 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 	      );
 
 	/*
+         * Explicitly clear (in addition to marking them as clobbered) all GPRs
+         * that have not been loaded with host state to prevent speculatively
+         * using the guest's values.
+         */
+	asm volatile (
+		"xor %%eax, %%eax \n\t"
+		"xor %%ebx, %%ebx \n\t"
+		"xor %%esi, %%esi \n\t"
+		"xor %%edi, %%edi \n\t"
+#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
+		::: "cc"
+#ifdef CONFIG_X86_64
+		 , "rax", "rbx", "rsi", "rdi"
+		 , "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15"
+#else
+		 , "eax", "ebx", "esi", "edi"
+#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
 	 * turn it off. This is much more efficient than blindly adding
-- 
2.7.4