Re: Random shadow stack pointer corruption

[Yu-cheng Yu]

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.