Re: Random shadow stack pointer corruption

[Andy Lutomirski]

> On Jul 28, 2020, at 5:36 PM, H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
> 
> ïOn Sat, Jul 18, 2020 at 4:35 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>> 
>>> 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@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.
>>> 
>>> 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.
>> 
> 
> We have identified that
> 
> ommit 91eeafea1e4b7c95cc4f38af186d7d48fceef89a
> Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Date:   Thu May 21 22:05:28 2020 +0200
> 
>    x86/entry: Switch page fault exception to IDTENTRY_RAW
> 
>    Convert page fault exceptions to IDTENTRY_RAW:
> 
>      - Implement the C entry point with DEFINE_IDTENTRY_RAW
>      - Add the CR2 read into the exception handler
>      - Add the idtentry_enter/exit_cond_rcu() invocations in
>        in the regular page fault handler and in the async PF
>        part.
>      - Emit the ASM stub with DECLARE_IDTENTRY_RAW
>      - Remove the ASM idtentry in 64-bit
>      - Remove the CR2 read from 64-bit
>      - Remove the open coded ASM entry code in 32-bit
>      - Fix up the XEN/PV code
>      - Remove the old prototypes
> 
>    No functional change.
> 
> triggered the shadow stack corruption when the process returned from syscall.
> SSP MSR somehow was changed between setting SSP MSR and IRET.    Could
> there be a page fault between setting SSP MSR and IRET?

Not upstream because thereâs no SSP MSR.

> 
> -- 
> H.J.