On 7/15/19 3:30 PM, Liran Alon wrote: > 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. > The SMAP violation will occur from CPL3 so CPL==3 is a valid check. See [1] for complete discussion https://patchwork.kernel.org/patch/10808075/#22479271 However, code mistakenly checked CR4.SMAP==0, it should be CR4.SMAP==1 > 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); > Ah good catch. thank > /* > - * 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); > } > >
