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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sat, 18 Jul 2020 11:27:37 -0700
Message-ID: <CALCETrWkvVKeoapzy6=6NNM9sg7zYV2x3dGivuX5ij8U_KYLzQ@mail.gmail.com>
Subject: Re: Random shadow stack pointer corruption
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Ingo Molnar <mingo@redhat.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Tony Luck <tony.luck@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Weijiang Yang <weijiang.yang@intel.com>
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On Sat, Jul 18, 2020 at 11:25 AM Yu-cheng Yu <yu-cheng.yu@intel.com> 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.
>
>
>

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.