Hi, I found a code to set unusable flag of segment register incorrectly. https://lkml.org/lkml/2017/5/30/459 I guess above patch and current discussion could be related. I guess following sequence could happen. 1. svm_get_segment() sets var->unusable of Stack Segment incorrectly 2. svm_set_segment() clears both of s->attrib and svm->vmcb->save.cpl. Is it possible scenario? On Tue, May 30, 2017 at 6:05 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > > On 30/05/2017 17:58, Roman Penyaev wrote: >> Indeed, what is left is eventually take it from SS.RPL. J. > > Ahah! :) But I only suggested that in specific cases. > >> But jokes aside, with your last patch you seems fixed a race problem >> when "CS.RPL is not equal to the CPL in the few instructions between >> setting CR0.PE and reloading CS". > > Yes, exactly. The symptom was a crash (triple fault) when you kept > interrupting with "info cpus" a guest that repeatedly went to protected > mode and back to real mode. > >> We will have CPL in var->dpl, and it seems ok. All we need is not >> to lose it on the way kernel->userspace->kernel. > > You're right. So what do you think of the other suggestion (svm.c > doesn't clear attributes for unusable registers, QEMU only clears P for > unusable registers)? > > Thanks, > > Paolo -- Best regards, Gi-Oh Kim TEL: 0176 2697 8962
