Re: Random shadow stack pointer corruption

From: Andy Lutomirski
Date: Sat Jul 18 2020 - 14:27:53 EST


On Sat, Jul 18, 2020 at 11:25 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
> On Sat, 2020-07-18 at 11:00 -0700, Andy Lutomirski wrote:
> > On Sat, Jul 18, 2020 at 10:58 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > > Hi,
> > >
> > > My shadow stack tests start to have random shadow stack pointer corruption after
> > > v5.7 (excluding). The symptom looks like some locking issue or the kernel is
> > > confused about which CPU a task is on. In later tip/master, this can be
> > > triggered by creating two tasks and each does continuous
> > > pthread_create()/pthread_join(). If the kernel has max_cpus=1, the issue goes
> > > away. I also checked XSAVES/XRSTORS, but this does not seem to be an issue
> > > coming from there.
> >
> > What do you mean "shadow stack pointer corruption"? Is SSP itself
> > corrupt while running in the kernel? Is one of the MSRs getting
> > corrupted? Is the memory to which the shadow stack points getting
> > corrupted? Is the CPU rejecting an attempt to change SSP?
>
> What I see is, a new thread after ret_from_fork() and iret back to ring-3,
> its shadow stack pointer (MSR_IA32_PL3_SSP) is corrupted.
>
>
>

This is going to be impossible to diagnose, given that the upstream
kernel doesn't know about these MSRs at all. If you point to a git
tree, maybe I can spot the issue.