Re: [REGRESSION 7.2, BISECTED] KVM: x86: Starting Windows guest triggers UBSAN: array-index-out-of-bounds

[SimonP <simonp.git@mailbox.org>]

On Wed, Oct 07, 2026 at 01:18:03PM +0200, Paolo Bonzini wrote:
> On Wed, Oct 7, 2026 at 1:16 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
> >
> > On Tue, Oct 6, 2026 at 7:37 PM SimonP <simonp.git@mailbox.org> wrote:
> > > Hello,
> > >
> > > See subject. Happens in permission_fault() on line:
> > >
> > >         fault = (mmu->permissions[index] >> pte_access) & 1;
> > >
> > > Some output from a printk I added, not spammed but keeps happening:
> > >
> > > kvm: permission_fault(): pfec = 70, index = 35, not_smap = 0
> >
> > Can you confirm you are using nested virtualization, and if so what is
> > the L1 hypervisor and L2 guest combination? Also what is the processor
> > model?
> >
> > The issue is weird; it is caused by the SS bit which is apparently set
> > in the #NPF exitcode; but it shouldn't be unless bit 4 is set in the
> > misc_ctl field, and KVM never sets it. Can you check if the attached
> > patch fixes it (modulo the fact that it shouldn't happen in the first
> > place)?
> 
> Attachment added now.

Patch is good. UBSAN doesn't complain anymore, no WARN is hit, and the
guest works.

Regards,
Simon

> 
> Paolo