Re: [PATCH v2] RDMA/siw: Fix the sendmsg byte count in siw_tcp_sendpages

[Jason Gunthorpe <jgg@ziepe.ca>]

On Tue, Jul 29, 2025 at 01:03:48PM +0100, Pedro Falcato wrote:
> Ever since commit c2ff29e99a76 ("siw: Inline do_tcp_sendpages()"),
> we have been doing this:
> 
> static int siw_tcp_sendpages(struct socket *s, struct page **page, int offset,
>                              size_t size)
> [...]
>         /* Calculate the number of bytes we need to push, for this page
>          * specifically */
>         size_t bytes = min_t(size_t, PAGE_SIZE - offset, size);
>         /* If we can't splice it, then copy it in, as normal */
>         if (!sendpage_ok(page[i]))
>                 msg.msg_flags &= ~MSG_SPLICE_PAGES;
>         /* Set the bvec pointing to the page, with len $bytes */
>         bvec_set_page(&bvec, page[i], bytes, offset);
>         /* Set the iter to $size, aka the size of the whole sendpages (!!!) */
>         iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, &bvec, 1, size);
> try_page_again:
>         lock_sock(sk);
>         /* Sendmsg with $size size (!!!) */
>         rv = tcp_sendmsg_locked(sk, &msg, size);
> 
> This means we've been sending oversized iov_iters and tcp_sendmsg calls
> for a while. This has a been a benign bug because sendpage_ok() always
> returned true. With the recent slab allocator changes being slowly
> introduced into next (that disallow sendpage on large kmalloc
> allocations), we have recently hit out-of-bounds crashes, due to slight
> differences in iov_iter behavior between the MSG_SPLICE_PAGES and
> "regular" copy paths:
> 
> (MSG_SPLICE_PAGES)
> skb_splice_from_iter
>   iov_iter_extract_pages
>     iov_iter_extract_bvec_pages
>       uses i->nr_segs to correctly stop in its tracks before OoB'ing everywhere
>   skb_splice_from_iter gets a "short" read
> 
> (!MSG_SPLICE_PAGES)
> skb_copy_to_page_nocache copy=iov_iter_count
>  [...]
>    copy_from_iter
>         /* this doesn't help */
>         if (unlikely(iter->count < len))
>                 len = iter->count;
>           iterate_bvec
>             ... and we run off the bvecs
> 
> Fix this by properly setting the iov_iter's byte count, plus sending the
> correct byte count to tcp_sendmsg_locked.
> 
> Cc: stable@vger.kernel.org
> Fixes: c2ff29e99a76 ("siw: Inline do_tcp_sendpages()")
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202507220801.50a7210-lkp@intel.com
> Reviewed-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Pedro Falcato <pfalcato@suse.de>

Applied thanks,

Jason