From febec6a7076499d16b37f556724a942633a270a6 Mon Sep 17 00:00:00 2001 From: Alireza Asgari Date: Thu, 24 Sep 2026 13:00:24 +0000 Subject: [PATCH] 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 Link: https://patch.msgid.link/20260924-fix-udp-splice-checksum-v1-1-fe61d65447a8@asgari.net Signed-off-by: Jakub Kicinski --- net/core/skbuff.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/net/core/skbuff.c b/net/core/skbuff.c index c3042d822afa..4aea06d5167d 100644 --- a/net/core/skbuff.c +++ b/net/core/skbuff.c @@ -7409,7 +7409,8 @@ nodefer: kfree_skb_napi_cache(skb); } static void skb_splice_csum_page(struct sk_buff *skb, struct page *page, - size_t offset, size_t len) + size_t offset, size_t len, + unsigned int csum_offset) { const char *kaddr; __wsum csum; @@ -7417,7 +7418,7 @@ static void skb_splice_csum_page(struct sk_buff *skb, struct page *page, kaddr = kmap_local_page(page); csum = csum_partial(kaddr + offset, len, 0); kunmap_local(kaddr); - skb->csum = csum_block_add(skb->csum, csum, skb->len); + skb->csum = csum_block_add(skb->csum, csum, csum_offset); } /** @@ -7477,7 +7478,8 @@ ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter, } if (skb->ip_summed == CHECKSUM_NONE) - skb_splice_csum_page(skb, page, off, part); + skb_splice_csum_page(skb, page, off, part, + skb->len + spliced); off = 0; spliced += part;