According to AMD Errata 1096:
"On a nested data page fault when CR4.SMAP = 1 and the guest data read generates a SMAP violation, the
GuestInstrBytes field of the VMCB on a VMEXIT will incorrectly return 0h instead the correct guest instruction
bytes."
As stated above, errata is encountered when guest read generates a SMAP violation. i.e. vCPU runs
with CPL<3 and CR4.SMAP=1. However, code have mistakenly checked if CPL==3 and CR4.SMAP==0.
To avoid future confusion and improve code readbility, comment errata details in code and not
just in commit message.
Fixes: 05d5a4863525 ("KVM: SVM: Workaround errata#1096 (insn_len maybe zero on SMAP violation)")
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Signed-off-by: Liran Alon <liran.alon@xxxxxxxxxx>
---
arch/x86/kvm/svm.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 735b8c01895e..79023a41f7a7 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -7123,10 +7123,19 @@ 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);
+ smap = kvm_read_cr4_bits(vcpu, X86_CR4_SMAP);
/*
- * Detect and workaround Errata 1096 Fam_17h_00_0Fh
+ * Detect and workaround Errata 1096 Fam_17h_00_0Fh.
+ *
+ * Errata:
+ * On a nested page fault when CR4.SMAP=1 and the guest data read generates
+ * a SMAP violation, GuestIntrBytes field of the VMCB on a VMEXIT will
+ * incorrectly return 0 instead the correct guest instruction bytes.
+ *
+ * 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
@@ -7137,11 +7146,11 @@ 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 (!is_user && smap) {
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);
}
--
2.20.1
