Re: bcachefs: suspicious mm pointer in struct dio_write

[Jann Horn <jannh@google.com>]

On Wed, Nov 27, 2024 at 10:16 PM Jens Axboe <axboe@kernel.dk> wrote:
> On 11/27/24 2:08 PM, Kent Overstreet wrote:
> > On Wed, Nov 27, 2024 at 09:44:21PM +0100, Jann Horn wrote:
> >> On Wed, Nov 27, 2024 at 9:25?PM Kent Overstreet
> >> <kent.overstreet@linux.dev> wrote:
> >>> On Wed, Nov 27, 2024 at 11:09:14AM -0700, Jens Axboe 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.
> >>>
> >>> Wouldn't delivery of completions be tied to an address space (not a
> >>> process) like it is for aio?
> >>
> >> An io_uring instance is primarily exposed to userspace as a file
> >> descriptor, so AFAIK it is possible for the io_uring instance to live
> >> beyond when the last mmput() happens. io_uring initially only holds an
> >> mmgrab() reference on the MM (a comment above that explains: "This is
> >> just grabbed for accounting purposes"), which I think is not enough to
> >> make it stable enough for kthread_use_mm(); I think in io_uring, only
> >> the worker threads actually keep the MM fully alive (and AFAIK the
> >> uring worker threads can exit before the uring instance itself is torn
> >> down).
> >>
> >> To receive io_uring completions, there are multiple ways, but they
> >> don't use a pointer from the io_uring instance to the MM to access
> >> userspace memory. Instead, you can have a VMA that points to the
> >> io_uring instance, created by calling mmap() on the io_uring fd; or
> >> alternatively, with IORING_SETUP_NO_MMAP, you can have io_uring grab
> >> references to userspace-provided pages.
> >>
> >> On top of that, I think it might currently be possible to use the
> >> io_uring file descriptor from another task to submit work. (That would
> >> probably be fairly nonsensical, but I think the kernel does not
> >> currently prevent it.)
> >
> > Ok, that's a wrinkle.
>
> I'd argue the fact that you are using an mm from a different process
> without grabbing a reference is the wrinkle. I just don't think it's a
> problem right now, but it could be... aio is tied to the mm because of
> how it does completions, potentially, and hence needs this exit_aio()
> hack because of that. aio also doesn't care, because it doesn't care
> about blocking - it'll happily block during issue.
>
> > Jens, is it really FMODE_NOWAIT that controls whether we can hit this? A
> > very cursory glance leads me to suspect "no", it seems like this is a
> > bug if io_uring is allowed on bcachefs at all.
>
> I just looked at bcachefs dio writes, which look to be the only case of
> this. And yes, for writes, if FMODE_NOWAIT isn't set, then io-wq is
> always involved for the IO.

I guess it could be an issue if the iocb can outlive the io-wq thread?
Like, a userspace task creates a uring instance and starts a write;
the write will be punted to a uring worker because of missing
FMODE_NOWAIT; then the uring worker enters io_write() and starts a
write on a kiocb. Can this write initiated from the worker be async?
And could the uring worker exit (for example, because the userspace
task exited) while the kiocb is still in flight?

Anyway, I see Kent just posted a patch to make this more robust.