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

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

On 30.07.2025 11:26, Pedro Falcato wrote:
> On Tue, Jul 29, 2025 at 08:53:02PM +0200, Bernard Metzler wrote:
>> 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>
>
> Thanks!
>
> Do you want to take the fix through your tree? Otherwise I suspect Vlastimil
> could simply take it (and possibly resubmit the SLAB PR, which hasn't been
> merged yet).
>
Thanks Pedro. Having Vlastimil taking care sounds good to me.

I am currently without development infrastructure (small village

in the mountains thing). And fixing the SLAB PR in the

first place would be even better.


Best,

Bernard.