[PATCH v2] powerpc: Don't clobber fr0/vs0 during fp|altivec register save

[Timothy Pearson]

----- Original Message -----
> From: "Michael Ellerman" <mpe at ellerman.id.au>
> To: "Timothy Pearson" <tpearson at raptorengineering.com>
> Cc: "Jens Axboe" <axboe at kernel.dk>, "regressions" <regressions at lists.linux.dev>, "npiggin" <npiggin at gmail.com>,
> "christophe leroy" <christophe.leroy at csgroup.eu>, "linuxppc-dev" <linuxppc-dev at lists.ozlabs.org>
> Sent: Monday, November 20, 2023 5:39:52 PM
> Subject: Re: [PATCH v2] powerpc: Don't clobber fr0/vs0 during fp|altivec register  save

> Timothy Pearson <tpearson at raptorengineering.com> writes:
>> ----- Original Message -----
>>> From: "Michael Ellerman" <mpe at ellerman.id.au>
> ...
>>> 
>>> But we now have a new path, because io-uring can call copy_process() via
>>> create_io_thread() from the signal handling path. That's OK if the signal is
>>> handled as we return from a syscall, but it's not OK if the signal is handled
>>> due to some other interrupt.
>>> 
>>> Which is:
>>> 
>>> interrupt_return_srr_user()
>>>  interrupt_exit_user_prepare()
>>>    interrupt_exit_user_prepare_main()
>>>      do_notify_resume()
>>>        get_signal()
>>>          task_work_run()
>>>            create_worker_cb()
>>>              create_io_worker()
>>>                copy_process()
>>>                  dup_task_struct()
>>>                    arch_dup_task_struct()
>>>                      flush_all_to_thread()
>>>                        save_all()
>>>                          if (tsk->thread.regs->msr & MSR_FP)
>>>                            save_fpu()
>>>                            # fr0 is clobbered and potentially live in userspace
>>> 
>>> 
>>> So tldr I think the corruption is only an issue since io-uring started doing
>>> the clone via signal, which I think matches the observed timeline of this bug
>>> appearing.
>>
>> I agree the corruption really only started showing up in earnest on
>> io_uring clone-via-signal, as this was confirmed several times in the
>> course of debugging.
> 
> Thanks.
> 
>> Note as well that I may very well have a wrong call order in the
>> commit message, since I was relying on a couple of WARN_ON() macros I
>> inserted to check for a similar (but not identical) condition and
>> didn't spend much time getting new traces after identifying the root
>> cause.
> 
> Yep no worries. I'll reword it to incorporate the full path from my mail.
> 
>> I went back and grabbed some real world system-wide stack traces, since I now
>> know what to trigger on.  A typical example is:
>>
>> interrupt_return_srr_user()
>>  interrupt_exit_user_prepare()
>>   interrupt_exit_user_prepare_main()
>>    schedule()
>>     __schedule()
>>      __switch_to()
>>       giveup_all()
>>        # tsk->thread.regs->msr MSR_FP is still set here
>>        __giveup_fpu()
>>         save_fpu()
>>         # fr0 is clobbered and potentially live in userspace
> 
> fr0 is not live there.
<snip> 
> ie. it clears the FP etc. bits from the task's MSR. That means the FP
> state will be reloaded from the thread struct before the task is run again.

So a little more detail on this, just to put it to rest properly vs. assuming hand analysis caught every possible pathway. :)

The debugging that generates this stack trace also verifies the following in __giveup_fpu():

1.) tsk->thread.fp_state.fpr doesn't contain the FPSCR contents prior to calling save_fpu()
2.) tsk->thread.fp_state.fpr contains the FPSCR contents directly after calling save_fpu()
3.) MSR_FP is set both in the task struct and in the live MSR.

Only if all three conditions are met will it generate the trace.  This is a generalization of the hack I used to find the problem in the first place.

If the state will subsequently be reloaded from the thread struct, that means we're reloading the registers from the thread struct that we just verified was corrupted by the earlier save_fpu() call.  There are only two ways I can see for that to be true -- one is if the registers were already clobbered when giveup_all() was entered, and the other is if save_fpu() went ahead and clobbered them right here inside giveup_all().

To see which scenario we were dealing with, I added a bit more instrumentation to dump the current register state if MSR_FP bit was already set in registers (i.e. not dumping data from task struct, but using the live FPU registers instead), and sure enough the registers are corrupt on entry, so something else has already called save_fpu() before we even hit giveup_all() in this call chain.

Unless I'm missing something, doesn't this effectively mean that anything interrupting a task can hit this bug?  Or, put another way, I'm seeing several processes hit this exact call chain with the corrupt register going back out to userspace without io_uring even in the mix, so there seems to be another pathway in play.  These traces are from a qemu guest, in case it matters given the kvm path is possibly susceptible.

Just a few things to think about.  The FPU patch itself definitely resolves the problems; I used a sledgehammer approach *specifically* so that there is no place for a rare call sequence we didn't consider to hit it again down the line. :)