Re: [PATCH] aio: prevent the final fput() in the middle of vfs_poll() (Re: KASAN: use-after-free Read in unix_dgram_poll)

[Dmitry Vyukov]

On Sun, Mar 3, 2019 at 4:19 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Sun, Mar 03, 2019 at 01:55:02PM +0000, Al Viro wrote:
>
> > Maybe unrelated to this bug, but...  What's to prevent a wakeup
> > that happens just after we'd been added to a waitqueue by ->poll()
> > triggering aio_poll_wake(), which gets to aio_poll_complete()
> > with its fput() *before* we'd reached the end of ->poll() instance?
> >
> > I wonder if adding
> >       get_file(req->file);  // make sure that early completion is safe
> > right after
> >         req->file = fget(iocb->aio_fildes);
> >         if (unlikely(!req->file))
> >                 return -EBADF;
> > paired with
> >       fput(req->file);
> > right after out: in aio_poll() is needed...  Am I missing something
> > obvious here?  Christoph?
>
> In more details - normally IOCB_CMD_POLL handling looks so:
>
> 1) io_submit(2) allocates aio_kiocb instance and passes it to aio_poll()
> 2) aio_poll() resolves the descriptor to struct file by
>         req->file = fget(iocb->aio_fildes)
> 3) aio_poll() sets ->woken to false and raises ->ki_refcnt of that
> aio_kiocb to 2 (bumps by 1, that is).
> 4) aio_poll() calls vfs_poll().  After sanity checks (basically, "poll_wait()
> had been called and only once") it locks the queue.  That's what the extra
> reference to iocb had been for - we know we can safely access it.
> 5) With queue locked, we check if ->woken has already been set to true
> (by aio_poll_wake()) and, if it had been, we unlock the queue, drop
> a reference to aio_kiocb and bugger off - at that point it's
> a responsibility to aio_poll_wake() and the stuff called/scheduled by
> it.  That code will drop the reference to file in req->file, along
> with the other reference to our aio_kiocb.
> 6) otherwise, we see whether we need to wait.  If we do, we unlock
> the queue, drop one reference to aio_kiocb and go away - eventual
> wakeup (or cancel) will deal with the reference to file and with the
> other reference to aio_kiocb
> 7) otherwise we remove ourselves from waitqueue (still under the queue
> lock), so that wakeup won't get us.  No async activity will be happening,
> so we can safely drop req->file and iocb ourselves.
>
> If wakeup happens while we are in vfs_poll(), we are fine - aio_kiocb
> won't get freed under us, so we can do all the checks and locking safely.
> And we don't touch ->file if we detect that case.
>
> However, vfs_poll() most certainly *does* touch the file it had been
> given.  So wakeup coming while we are still in ->poll() might end up
> doing fput() on that file.  That case is not too rare, and usually
> we are saved by the still present reference from descriptor table -
> that fput() is not the final one.  But if another thread closes that
> descriptor right after our fget() and wakeup does happen before
> ->poll() returns, we are in trouble - final fput() done while we
> are in the middle of a method.
>
> What we need is to hold on to the file reference the same way we do with
> aio_kiocb.  No need to store the reference to what we'd got in a separate
> variable - req->file is never reassigned and we'd already made sure that
> req won't be freed under us, so dropping the extra reference with
> fput(req->file) is fine in all cases.
>
> Fixes: bfe4037e722ec
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

Please add the Reported-by tag from the report.
If you don't add it, then either:
1. somebody (you) will need to remember to later go and associate fix
the the report with "#syz fix" command
2. or the bug will stay open and syzbot will never report
use-after-frees in unix_dgram_poll again (it's still the same already
reported bug)
3. or worse somebody will spend time re-debugging this bug just to
find that you already fixed it but did not include the tag

Thanks

> ---
> diff --git a/fs/aio.c b/fs/aio.c
> index 3083180a54c8..7e88bfabdac2 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1767,6 +1767,7 @@ static ssize_t aio_poll(struct aio_kiocb *aiocb, const struct iocb *iocb)
>
>         /* one for removal from waitqueue, one for this function */
>         refcount_set(&aiocb->ki_refcnt, 2);
> +       get_file(req->file);
>
>         mask = vfs_poll(req->file, &apt.pt) & req->events;
>         if (unlikely(!req->head)) {
> @@ -1793,6 +1794,7 @@ static ssize_t aio_poll(struct aio_kiocb *aiocb, const struct iocb *iocb)
>         spin_unlock_irq(&ctx->ctx_lock);
>
>  out:
> +       fput(req->file);
>         if (unlikely(apt.error)) {
>                 fput(req->file);
>                 return apt.error;
>
> --
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