Re: Random shadow stack pointer corruption

[Yu-cheng Yu <yu-cheng.yu@intel.com>]

On Sat, 2020-07-18 at 15:41 -0700, Dave Hansen wrote:
> On 7/18/20 11:24 AM, Yu-cheng Yu 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@intel.com> 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.
> 
> Does corrupt mean random?  Or is it a valid stack address, just not for
> _this_ thread?  Or NULL?  Or is it a kernel address?  Have you tried
> tracing *ALL* the WRMSR's and XRSTOR's that write to the MSR?

When a shadow stack address is changed, the address appears to be other task's. 
I traced all WRMSR's and XRSTOR's.  I also verified there have not been any
XRSTORS from a wrong buffer.  When rc6 is tagged, I will re-base, test, and
share current patches.