Bug 2394647 (CVE-2025-39758)

In the Linux kernel, the following vulnerability has been resolved:

RDMA/siw: Fix the sendmsg byte count in siw_tcp_sendpages

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.