Re: bcachefs: suspicious mm pointer in struct dio_write

[Kent Overstreet <kent.overstreet@linux.dev>]

On Wed, Nov 27, 2024 at 05:57:03PM +0100, 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 don't know about io_uring but aio guarantees that outstanding kiocbs
are completed before exiting the mm_struct - I would expect some sort of
similar guarantee to hold, because otherwise where are we going to
deliver the completion?