Re: bcachefs: suspicious mm pointer in struct dio_write

[Jens Axboe <axboe@kernel.dk>]

On 11/27/24 12:43 PM, Jann Horn wrote:
> On Wed, Nov 27, 2024 at 7:09?PM Jens Axboe <axboe@kernel.dk> wrote:
>> On 11/27/24 9:57 AM, Jann Horn wrote:
>>> Hi!
>>>
>>> In fs/bcachefs/fs-io-direct.c, "struct dio_write" contains a pointer
>>> to an mm_struct. This pointer is grabbed in bch2_direct_write()
>>> (without any kind of refcount increment), and used in
>>> bch2_dio_write_continue() for kthread_use_mm()/kthread_unuse_mm()
>>> which are used to enable userspace memory access from kthread context.
>>> I believe kthread_use_mm()/kthread_unuse_mm() require that the caller
>>> guarantees that the MM hasn't gone through exit_mmap() yet (normally
>>> by holding an mmget() reference).
>>>
>>> If we reach this codepath via io_uring, do we have a guarantee that
>>> the mm_struct that called bch2_direct_write() is still alive and
>>> hasn't yet gone through exit_mmap() when it is accessed from
>>> bch2_dio_write_continue()?
>>>
>>> I don't know the async direct I/O codepath particularly well, so I
>>> cc'ed the uring maintainers, who probably know this better than me.
>>
>> I _think_ this is fine as-is, even if it does look dubious and bcachefs
>> arguably should grab an mm ref for this just for safety to avoid future
>> problems. The reason is that bcachefs doesn't set FMODE_NOWAIT, which
>> means that on the io_uring side it cannot do non-blocking issue of
>> requests. This is slower as it always punts to an io-wq thread, which
>> shares the same mm. Hence if the request is alive, there's always a
>> thread with the same mm alive as well.
>>
>> Now if FMODE_NOWAIT was set, then the original task could exit. I'd need
>> to dig a bit deeper to verify that would always be safe and there's not
>> a of time today with a few days off in the US looming, so I'll defer
>> that to next week. It certainly would be fine with an mm ref grabbed.
> 
> Ah, thanks for looking into it! I missed this implication of not
> setting FMODE_NOWAIT.
> 
> Anyway, what you said sounds like it would be cleaner for bcachefs to
> grab its own extra reference, maybe by initially grabbing an mm
> reference with mmgrab() in bch2_direct_write(), and then use
> mmget_not_zero() in bch2_dio_write_continue() to ensure the MM is
> stable.

Yep I think that would definitely make it more sturdy, and also less
headscratchy in terms of being able to verify it's actually safe.

> What do other file systems do for this? I think they normally grab
> page references so that they don't need the MM anymore when
> asynchronously fulfilling the request, right? Like in
> iomap_dio_bio_iter(), which uses bio_iov_iter_get_pages() to grab
> references to the pages corresponding to the userspace regions in
> dio->submit.iter?

Not aware of anything else doing it like this, where it's punted to a
kthread and then the mm used from there. The upfront page
getting/mapping is the common approach, like you described. Which does
seem like a much better choice, rather than needing to rely on the mm in
a kworker.

-- 
Jens Axboe