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-rw-r--r--arch/x86/kvm/svm.c42
1 files changed, 35 insertions, 7 deletions
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 583b9fa656f3..19f69df96758 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -7128,13 +7128,41 @@ static int nested_enable_evmcs(struct kvm_vcpu *vcpu,
static bool svm_need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
{
- bool is_user, smap;
-
- is_user = svm_get_cpl(vcpu) == 3;
- smap = !kvm_read_cr4_bits(vcpu, X86_CR4_SMAP);
+ unsigned long cr4 = kvm_read_cr4(vcpu);
+ bool smep = cr4 & X86_CR4_SMEP;
+ bool smap = cr4 & X86_CR4_SMAP;
+ bool is_user = svm_get_cpl(vcpu) == 3;
/*
- * Detect and workaround Errata 1096 Fam_17h_00_0Fh
+ * Detect and workaround Errata 1096 Fam_17h_00_0Fh.
+ *
+ * Errata:
+ * When CPU raise #NPF on guest data access and vCPU CR4.SMAP=1, it is
+ * possible that CPU microcode implementing DecodeAssist will fail
+ * to read bytes of instruction which caused #NPF. In this case,
+ * GuestIntrBytes field of the VMCB on a VMEXIT will incorrectly
+ * return 0 instead of the correct guest instruction bytes.
+ *
+ * This happens because CPU microcode reading instruction bytes
+ * uses a special opcode which attempts to read data using CPL=0
+ * priviledges. The microcode reads CS:RIP and if it hits a SMAP
+ * fault, it gives up and returns no instruction bytes.
+ *
+ * Detection:
+ * We reach here in case CPU supports DecodeAssist, raised #NPF and
+ * returned 0 in GuestIntrBytes field of the VMCB.
+ * First, errata can only be triggered in case vCPU CR4.SMAP=1.
+ * Second, if vCPU CR4.SMEP=1, errata could only be triggered
+ * in case vCPU CPL==3 (Because otherwise guest would have triggered
+ * a SMEP fault instead of #NPF).
+ * Otherwise, vCPU CR4.SMEP=0, errata could be triggered by any vCPU CPL.
+ * As most guests enable SMAP if they have also enabled SMEP, use above
+ * logic in order to attempt minimize false-positive of detecting errata
+ * while still preserving all cases semantic correctness.
+ *
+ * Workaround:
+ * To determine what instruction the guest was executing, the hypervisor
+ * will have to decode the instruction at the instruction pointer.
*
* In non SEV guest, hypervisor will be able to read the guest
* memory to decode the instruction pointer when insn_len is zero
@@ -7145,11 +7173,11 @@ static bool svm_need_emulation_on_page_fault(struct kvm_vcpu *vcpu)
* instruction pointer so we will not able to workaround it. Lets
* print the error and request to kill the guest.
*/
- if (is_user && smap) {
+ if (smap && (!smep || is_user)) {
if (!sev_guest(vcpu->kvm))
return true;
- pr_err_ratelimited("KVM: Guest triggered AMD Erratum 1096\n");
+ pr_err_ratelimited("KVM: SEV Guest triggered AMD Erratum 1096\n");
kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
}