linux-kernel - [PATCH] KVM: SVM: move more vmentry code to assembly

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Message-Id: <20200413075032.5546-2-pbonzini@redhat.com>
Date:   Mon, 13 Apr 2020 03:50:32 -0400
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     ubizjak@...il.com
Subject: [PATCH] KVM: SVM: move more vmentry code to assembly

Manipulate IF around vmload/vmsave to remove the confusing usage of
local_irq_enable where interrupts are actually disabled via GIF.
And stuff the RSB immediately without waiting for a RET to avoid
Spectre-v2 attacks.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/include/asm/nospec-branch.h | 21 ---------------------
 arch/x86/kvm/svm/svm.c               |  7 -------
 arch/x86/kvm/svm/vmenter.S           |  9 +++++++++
 3 files changed, 9 insertions(+), 28 deletions(-)

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 07e95dcb40ad..7e9a281e2660 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -237,27 +237,6 @@ enum ssb_mitigation {
 extern char __indirect_thunk_start[];
 extern char __indirect_thunk_end[];
 
-/*
- * On VMEXIT we must ensure that no RSB predictions learned in the guest
- * can be followed in the host, by overwriting the RSB completely. Both
- * retpoline and IBRS mitigations for Spectre v2 need this; only on future
- * CPUs with IBRS_ALL *might* it be avoided.
- */
-static inline void vmexit_fill_RSB(void)
-{
-#ifdef CONFIG_RETPOLINE
-	unsigned long loops;
-
-	asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
-		      ALTERNATIVE("jmp 910f",
-				  __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
-				  X86_FEATURE_RETPOLINE)
-		      "910:"
-		      : "=r" (loops), ASM_CALL_CONSTRAINT
-		      : : "memory" );
-#endif
-}
-
 static __always_inline
 void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
 {
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 2be5bbae3a40..117bb0b28535 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3330,13 +3330,8 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
 	 */
 	x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
 
-	local_irq_enable();
-
 	__svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs);
 
-	/* Eliminate branch target predictions from guest mode */
-	vmexit_fill_RSB();
-
 #ifdef CONFIG_X86_64
 	wrmsrl(MSR_GS_BASE, svm->host.gs_base);
 #else
@@ -3366,8 +3361,6 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
 
 	reload_tss(vcpu);
 
-	local_irq_disable();
-
 	x86_spec_ctrl_restore_host(svm->spec_ctrl, svm->virt_spec_ctrl);
 
 	vcpu->arch.cr2 = svm->vmcb->save.cr2;
diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
index fa1af90067e9..723887e35e95 100644
--- a/arch/x86/kvm/svm/vmenter.S
+++ b/arch/x86/kvm/svm/vmenter.S
@@ -3,6 +3,7 @@
 #include <asm/asm.h>
 #include <asm/bitsperlong.h>
 #include <asm/kvm_vcpu_regs.h>
+#include <asm/nospec-branch.h>
 
 #define WORD_SIZE (BITS_PER_LONG / 8)
 
@@ -78,6 +79,7 @@ SYM_FUNC_START(__svm_vcpu_run)
 	pop %_ASM_AX
 
 	/* Enter guest mode */
+	sti
 1:	vmload %_ASM_AX
 	jmp 3f
 2:	cmpb $0, kvm_rebooting
@@ -99,6 +101,13 @@ SYM_FUNC_START(__svm_vcpu_run)
 	ud2
 	_ASM_EXTABLE(5b, 6b)
 7:
+	cli
+
+#ifdef CONFIG_RETPOLINE
+	/* IMPORTANT: Stuff the RSB immediately after VM-Exit, before RET! */
+	FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS, X86_FEATURE_RETPOLINE
+#endif
+
 	/* "POP" @regs to RAX. */
 	pop %_ASM_AX
 
-- 
2.18.2