linux-kernel - Re: Random shadow stack pointer corruption

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Date:   Sat, 18 Jul 2020 15:41:19 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Ingo Molnar <mingo@...hat.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: Random shadow stack pointer corruption

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@...el.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?