KVM: X86: Restart the guest when insn_len is zero and SEV is enabled

Web:        https://git.kernel.org/torvalds/c/00b10fe1046c4b2232097a7ffaa9238c7e479388
Commit:     00b10fe1046c4b2232097a7ffaa9238c7e479388
Parent:     35c6f649bbb2e3f367116307273f998f0bf3e08e
Refname:    refs/heads/master
Author:     Brijesh Singh <[email protected]>
AuthorDate: Mon Dec 4 10:57:40 2017 -0600
Committer:  Brijesh Singh <[email protected]>
CommitDate: Mon Dec 4 13:33:14 2017 -0600

    KVM: X86: Restart the guest when insn_len is zero and SEV is enabled
    
    On AMD platforms, under certain conditions insn_len may be zero on #NPF.
    This can happen if a guest gets a page-fault on data access but the HW
    table walker is not able to read the instruction page (e.g instruction
    page is not present in memory).
    
    Typically, when insn_len is zero, x86_emulate_instruction() walks the
    guest page table and fetches the instruction bytes from guest memory.
    When SEV is enabled, the guest memory is encrypted with guest-specific
    key hence hypervisor will not able to fetch the instruction bytes.
    In those cases we simply restart the guest.
    
    I have encountered this issue when running kernbench inside the guest.
    
    Cc: Thomas Gleixner <[email protected]>
    Cc: Ingo Molnar <[email protected]>
    Cc: "H. Peter Anvin" <[email protected]>
    Cc: Paolo Bonzini <[email protected]>
    Cc: "Radim Krčmář" <[email protected]>
    Cc: Joerg Roedel <[email protected]>
    Cc: Borislav Petkov <[email protected]>
    Cc: Tom Lendacky <[email protected]>
    Cc: [email protected]
    Cc: [email protected]
    Cc: [email protected]
    Signed-off-by: Brijesh Singh <[email protected]>
---
 arch/x86/kvm/mmu.c | 10 ++++++++++
 arch/x86/kvm/svm.c |  6 ++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index e5e66e5c6640..d5e5dbd0e5ad 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4950,6 +4950,16 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u64 error_code,
 	if (mmio_info_in_cache(vcpu, cr2, direct))
 		emulation_type = 0;
 emulate:
+	/*
+	 * On AMD platforms, under certain conditions insn_len may be zero on #NPF.
+	 * This can happen if a guest gets a page-fault on data access but the HW
+	 * table walker is not able to read the instruction page (e.g instruction
+	 * page is not present in memory). In those cases we simply restart the
+	 * guest.
+	 */
+	if (unlikely(insn && !insn_len))
+		return 1;
+
 	er = x86_emulate_instruction(vcpu, cr2, emulation_type, insn, insn_len);
 
 	switch (er) {
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 3e848f952b4f..ec5df5752995 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -2436,7 +2436,8 @@ static int pf_interception(struct vcpu_svm *svm)
 	u64 error_code = svm->vmcb->control.exit_info_1;
 
 	return kvm_handle_page_fault(&svm->vcpu, error_code, fault_address,
-			svm->vmcb->control.insn_bytes,
+			static_cpu_has(X86_FEATURE_DECODEASSISTS) ?
+			svm->vmcb->control.insn_bytes : NULL,
 			svm->vmcb->control.insn_len);
 }
 
@@ -2447,7 +2448,8 @@ static int npf_interception(struct vcpu_svm *svm)
 
 	trace_kvm_page_fault(fault_address, error_code);
 	return kvm_mmu_page_fault(&svm->vcpu, fault_address, error_code,
-			svm->vmcb->control.insn_bytes,
+			static_cpu_has(X86_FEATURE_DECODEASSISTS) ?
+			svm->vmcb->control.insn_bytes : NULL,
 			svm->vmcb->control.insn_len);
 }
 
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