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

[Bernard Metzler <bernard.metzler@linux.dev>]

On 29.07.2025 14:03, 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>
> ---
>
> v2:
>   - Add David Howells's Rb on the original patch
>   - Remove the offset increment, since it's dead code
>
>   drivers/infiniband/sw/siw/siw_qp_tx.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/infiniband/sw/siw/siw_qp_tx.c b/drivers/infiniband/sw/siw/siw_qp_tx.c
> index 3a08f57d2211..f7dd32c6e5ba 100644
> --- a/drivers/infiniband/sw/siw/siw_qp_tx.c
> +++ b/drivers/infiniband/sw/siw/siw_qp_tx.c
> @@ -340,18 +340,17 @@ static int siw_tcp_sendpages(struct socket *s, struct page **page, int offset,
>   		if (!sendpage_ok(page[i]))
>   			msg.msg_flags &= ~MSG_SPLICE_PAGES;
>   		bvec_set_page(&bvec, page[i], bytes, offset);
> -		iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, &bvec, 1, size);
> +		iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, &bvec, 1, bytes);
>   
>   try_page_again:
>   		lock_sock(sk);
> -		rv = tcp_sendmsg_locked(sk, &msg, size);
> +		rv = tcp_sendmsg_locked(sk, &msg, bytes)
>   		release_sock(sk);
>   
>   		if (rv > 0) {
>   			size -= rv;
>   			sent += rv;
>   			if (rv != bytes) {
> -				offset += rv;
>   				bytes -= rv;
>   				goto try_page_again;
>   			}

Acked-by: Bernard Metzler <bernard.metzler@linux.dev>