Re: [PATCH 12/18] io_uring: add support for pre-mapped user IO buffers

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On Tue, Jan 29, 2019 at 11:56 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
> On 1/29/19 3:44 PM, Jann Horn wrote:
> > On Tue, Jan 29, 2019 at 8:27 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
> >> If we have fixed user buffers, we can map them into the kernel when we
> >> setup the io_context. That avoids the need to do get_user_pages() for
> >> each and every IO.
> >>
> >> To utilize this feature, the application must call io_uring_register()
> >> after having setup an io_uring context, passing in
> >> IORING_REGISTER_BUFFERS as the opcode. The argument must be a pointer
> >> to an iovec array, and the nr_args should contain how many iovecs the
> >> application wishes to map.
> >>
> >> If successful, these buffers are now mapped into the kernel, eligible
> >> for IO. To use these fixed buffers, the application must use the
> >> IORING_OP_READ_FIXED and IORING_OP_WRITE_FIXED opcodes, and then
> >> set sqe->index to the desired buffer index. sqe->addr..sqe->addr+seq->len
> >> must point to somewhere inside the indexed buffer.
> >>
> >> The application may register buffers throughout the lifetime of the
> >> io_uring context. It can call io_uring_register() with
> >> IORING_UNREGISTER_BUFFERS as the opcode to unregister the current set of
> >> buffers, and then register a new set. The application need not
> >> unregister buffers explicitly before shutting down the io_uring context.
[...]
> >> +       imu = &ctx->user_bufs[index];
> >> +       buf_addr = READ_ONCE(sqe->addr);
> >> +       if (buf_addr < imu->ubuf || buf_addr + len > imu->ubuf + imu->len)
> >
> > This can wrap around if `buf_addr` or `len` is very big, right? Then
> > you e.g. get past the first check because `buf_addr` is sufficiently
> > big, and get past the second check because `buf_addr + len` wraps
> > around and becomes small.
>
> Good point. I wonder if we have a verification helper for something like
> this?

check_add_overflow() exists, I guess that might help a bit. I don't
think I've seen a more specific helper for this situation.



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