net: fix checksum offsets in skb_splice_from_iter()

skb_splice_from_iter() appends multiple page fragments before updating
skb->len. However, skb_splice_csum_page() uses that unchanged length as the
offset for every fragment checksum. If bytes already appended during the
call have an odd total length, a later fragment's checksum is combined
with the wrong parity.

For example, prime a UDP socket with sendto(MSG_MORE) and splice two
distinct pipe buffers containing "abc" and "DEFGH". Uncorking reports
success, but the receiver discards the packet for a bad checksum.
This also affects IPv6 and fragments beginning near a page boundary.

Pass the initial skb length plus the bytes already spliced to the checksum
helper. Keep the existing final length update and partial-progress error
handling unchanged. CHECKSUM_PARTIAL does not use this helper and remains
unaffected.

Fixes: 2e910b95329c ("net: Add a function to splice pages into an skbuff for MSG_SPLICE_PAGES")
Cc: stable@vger.kernel.org
Signed-off-by: Alireza Asgari <alireza@asgari.net>
Link: https://patch.msgid.link/20260924-fix-udp-splice-checksum-v1-1-fe61d65447a8@asgari.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions