[Part2 PATCH v4 28/29] KVM: X86: Restart the guest when insn_len is zero and SEV is enabled

[Brijesh Singh]

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 <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: "Radim KrÄmÃÅ" <rkrcmar@xxxxxxxxxx>
Cc: Joerg Roedel <joro@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: kvm@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
---
 arch/x86/kvm/mmu.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index eca30c1eb1d9..2bc0fe84aca2 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -4953,6 +4953,23 @@ 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).
+	 *
+	 * 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.
+	 */
+	if (unlikely(!insn_len) &&
+	    kvm_x86_ops->mem_enc_enabled &&
+	    kvm_x86_ops->mem_enc_enabled(vcpu))
+		return 1;
+
 	er = x86_emulate_instruction(vcpu, cr2, emulation_type, insn, insn_len);
 
 	switch (er) {
-- 
2.9.5