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SubjectRe: Random shadow stack pointer corruption
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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?

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