On Mon, Mar 04, 2024 at 11:31:53AM -0800, Eric Biggers wrote:
> On Mon, Mar 04, 2024 at 10:29:44AM -0800, Bart Van Assche wrote:
> > Patch "fs/aio: Make io_cancel() generate completions again" is based on the
> > assumption that calling kiocb->ki_cancel() does not complete R/W requests.
> > This is incorrect: the two drivers that call kiocb_set_cancel_fn() callers
> > set a cancellation function that calls usb_ep_dequeue(). According to its
> > documentation, usb_ep_dequeue() calls the completion routine with status
> > -ECONNRESET. Hence this revert.
> >
> > Cc: Benjamin LaHaise <ben@xxxxxxxxxxxxxxxxx>
> > Cc: Eric Biggers <ebiggers@xxxxxxxxxx>
> > Cc: Christoph Hellwig <hch@xxxxxx>
> > Cc: Avi Kivity <avi@xxxxxxxxxxxx>
> > Cc: Sandeep Dhavale <dhavale@xxxxxxxxxx>
> > Cc: Jens Axboe <axboe@xxxxxxxxx>
> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reported-by: syzbot+b91eb2ed18f599dd3c31@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: 54cbc058d86b ("fs/aio: Make io_cancel() generate completions again")
> > Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
> > ---
> > fs/aio.c | 27 ++++++++++++++++-----------
> > 1 file changed, 16 insertions(+), 11 deletions(-)
> >
> > diff --git a/fs/aio.c b/fs/aio.c
> > index 28223f511931..da18dbcfcb22 100644
> > --- a/fs/aio.c
> > +++ b/fs/aio.c
> > @@ -2165,11 +2165,14 @@ COMPAT_SYSCALL_DEFINE3(io_submit, compat_aio_context_t, ctx_id,
> > #endif
> >
> > /* sys_io_cancel:
> > - * Attempts to cancel an iocb previously passed to io_submit(). If the
> > - * operation is successfully cancelled 0 is returned. May fail with
> > - * -EFAULT if any of the data structures pointed to are invalid. May
> > - * fail with -EINVAL if aio_context specified by ctx_id is invalid. Will
> > - * fail with -ENOSYS if not implemented.
> > + * Attempts to cancel an iocb previously passed to io_submit. If
> > + * the operation is successfully cancelled, the resulting event is
> > + * copied into the memory pointed to by result without being placed
> > + * into the completion queue and 0 is returned. May fail with
> > + * -EFAULT if any of the data structures pointed to are invalid.
> > + * May fail with -EINVAL if aio_context specified by ctx_id is
> > + * invalid. May fail with -EAGAIN if the iocb specified was not
> > + * cancelled. Will fail with -ENOSYS if not implemented.
> > */
> > SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, struct iocb __user *, iocb,
> > struct io_event __user *, result)
> > @@ -2200,12 +2203,14 @@ SYSCALL_DEFINE3(io_cancel, aio_context_t, ctx_id, struct iocb __user *, iocb,
> > }
> > spin_unlock_irq(&ctx->ctx_lock);
> >
> > - /*
> > - * The result argument is no longer used - the io_event is always
> > - * delivered via the ring buffer.
> > - */
> > - if (ret == 0 && kiocb->rw.ki_flags & IOCB_AIO_RW)
> > - aio_complete_rw(&kiocb->rw, -EINTR);
> > + if (!ret) {
> > + /*
> > + * The result argument is no longer used - the io_event is
> > + * always delivered via the ring buffer. -EINPROGRESS indicates
> > + * cancellation is progress:
> > + */
> > + ret = -EINPROGRESS;
> > + }
>
> Acked-by: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> It does look like all the ->ki_cancel functions complete the request already, so
> this patch was unnecessary and just introduced a bug.
>
> Note that IOCB_CMD_POLL installs a ->ki_cancel function too, and that's how
> syzbot hit the use-after-free so easily.
>
> I assume that the patch just wasn't tested? Or did you find that it actually
> fixed something (how)?
We've been wrestling aio cancellations for a while now and aimed to
actually remove it but apparently it's used in the wild. I still very
much prefer if we could finally nuke this code.
>
> By the way, libaio (https://pagure.io/libaio) has a test suite for these system
> calls. How about adding a test case that cancels an IOCB_CMD_POLL request and
> verifies that the completion event is received?
Yes, please.
