Patch "KVM: X86: Restart the guest when insn_len is zero and SEV is enabled" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-x86-restart-the-guest-when-insn_len-is-zero-and-sev-is-enabled.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Brijesh Singh <[email protected]>
Date: Mon, 4 Dec 2017 10:57:40 -0600
Subject: KVM: X86: Restart the guest when insn_len is zero and SEV is enabled

From: Brijesh Singh <[email protected]>

[ Upstream commit 00b10fe1046c4b2232097a7ffaa9238c7e479388 ]

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.

---
 arch/x86/kvm/mmu.c |   10 ++++++++++
 arch/x86/kvm/svm.c |    6 ++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

@@ -4951,6 +4951,16 @@ int kvm_mmu_page_fault(struct kvm_vcpu *
 	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) {

@@ -2178,7 +2178,8 @@ static int pf_interception(struct vcpu_s
 	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);
 }
 

@@ -2189,7 +2190,8 @@ static int npf_interception(struct vcpu_
 
 	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);
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/kvm-x86-restart-the-guest-when-insn_len-is-zero-and-sev-is-enabled.patch