bcachefs: suspicious mm pointer in struct dio_write

[Jann Horn <jannh@google.com>]

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.