Re: [PATCH] net: check skb partial checksum offset after trim
From: Vasily Averin <hidden>
Date: 2020-12-11 16:34:03
Originally it was reported on Ubuntu 4.14 kernel, then I've reproduced it on upstream 5.10-rc7. If I'm right the problem is quite old and should affect all maintained stable kernels too. It seems for me the similar problem can happen in __skb_trim_rcsum(). Also I doubt that that skb_checksum_start_offset(skb) checks in __skb_postpull_rcsum() and skb_csum_unnecessary() are correct, becasue they do not guarantee that skb have correct CHECKSUM_PARTIAL. Could somebody confirm it? Thank you, Vasily Averin On 12/11/20 6:00 PM, Vasily Averin wrote:
quoted hunk ↗ jump to hunk
syzkaller reproduces BUG_ON in skb_checksum_help(): tun creates skb with big partial checksum area and small ip packet inside, then ip_rcv() decreases skb size of below length of checksummed area, then checksum_tg() called via netfilter hook detects incorrect skb: offset = skb_checksum_start_offset(skb); BUG_ON(offset >= skb_headlen(skb)); This patch drops CHEKSUM_PARTIAL mark when skb is trimmed below size of checksummed area. Link: https://syzkaller.appspot.com/bug?id=b419a5ca95062664fe1a60b764621eb4526e2cd0 Reported-by: syzbot+7010af67ced6105e5ab6@syzkaller.appspotmail.com Signed-off-by: Vasily Averin <redacted> --- include/linux/skbuff.h | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-)diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index a828cf9..0a9545d 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h@@ -3419,9 +3419,18 @@ static inline void *skb_push_rcsum(struct sk_buff *skb, unsigned int len) static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len) { + int ret; + if (likely(len >= skb->len)) return 0; - return pskb_trim_rcsum_slow(skb, len); + ret = pskb_trim_rcsum_slow(skb, len); + if (!ret && (skb->ip_summed == CHECKSUM_PARTIAL)) { + int offset = skb_checksum_start_offset(skb) + skb->csum_offset; + + if (offset + sizeof(__sum16) > skb_headlen(skb)) + skb->ip_summed = CHECKSUM_NONE; + } + return ret; } static inline int __skb_trim_rcsum(struct sk_buff *skb, unsigned int len)