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 cant 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 weve 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 OoBing 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 doesnt help */ if (unlikely(iter->count < len)) len = iter->count; iterate_bvec … and we run off the bvecs

Fix this by properly setting the iov_iters byte count, plus sending the correct byte count to tcp_sendmsg_locked.

Affected Software

References

• https://git.kernel.org/stable/c/42ebc16d9d2563f1a1ce0f05b643ee68d54fabf8
• https://git.kernel.org/stable/c/5661fdd218c2799001b88c17acd19f4395e4488e
• https://git.kernel.org/stable/c/673cf582fd788af12cdacfb62a6a593083542481
• https://git.kernel.org/stable/c/c18646248fed07683d4cee8a8af933fc4fe83c0d
• https://git.kernel.org/stable/c/edf82bc8150570167a33a7d54627d66614cbf841