[PATCH net-next v2 4/8] net: skbuff: don't BUG() on leftover length in skb_checksum() and friends

[Josef Bacik <josef@toxicpanda.com>]

skb_checksum(), skb_crc32c() and __skb_to_sgvec() walk the head, frags
and frag_list and BUG() if they run out of skb before they run out of
@len.  By then nothing has been read past the end of the skb.  The walk
stopped at the end of the data, and the check only tells us the caller
asked for a range the skb doesn't have.  commit 06a0afcfe2f5 ("xfrm: do
pskb_pull properly in __xfrm_transport_prep") fixed one such caller that
crashed in __skb_to_sgvec().

Return what each function already returns when it can't do the work,
with a DEBUG_NET_WARN_ON_ONCE() for debug kernels:

 - __skb_to_sgvec() returns -EINVAL.  Every skb_to_sgvec() caller has
   checked for a negative return since it learned to return -EMSGSIZE.
 - skb_checksum() and skb_crc32c() return 0, as they already do for
   unreadable frags.  The resulting checksum is wrong, so the packet
   fails verification on receive or goes out with a bad checksum on
   transmit, rather than taking the machine down.

Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 net/core/skbuff.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index ffc78b1a8ab8..32d7f5ed25eb 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3619,7 +3619,10 @@ __wsum skb_checksum(const struct sk_buff *skb, int offset, int len, __wsum csum)
 		}
 		start = end;
 	}
-	BUG_ON(len);
+	if (unlikely(len)) {
+		DEBUG_NET_WARN_ON_ONCE(1);
+		return 0;
+	}
 
 	return csum;
 }
@@ -3780,7 +3783,10 @@ u32 skb_crc32c(const struct sk_buff *skb, int offset, int len, u32 crc)
 		}
 		start = end;
 	}
-	BUG_ON(len);
+	if (unlikely(len)) {
+		DEBUG_NET_WARN_ON_ONCE(1);
+		return 0;
+	}
 
 	return crc;
 }
@@ -5339,7 +5345,10 @@ __skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len,
 		}
 		start = end;
 	}
-	BUG_ON(len);
+	if (unlikely(len)) {
+		DEBUG_NET_WARN_ON_ONCE(1);
+		return -EINVAL;
+	}
 	return elt;
 }
 

-- 
2.55.0