[PATCH net-next v2 6/8] net: skbuff: don't BUG() on leftover length in skb_copy_and_csum_bits()

[Josef Bacik <josef@toxicpanda.com>]

skb_copy_and_csum_bits() has the same check as skb_checksum(): it BUG()s
if it runs out of skb before it runs out of @len.  This one gets hit:
commit 7d63b6712538 ("icmp: guard against too small mtu") and commit
f99cd56230f5 ("net: Remove acked SYN flag from packet in the transmit
queue correctly") each fixed a crash here from icmp_glue_bits().

It can't just return, though.  Its callers copy into a buffer that is
about to go out on the wire, an ICMP error quoting the offending packet
for example, so bailing out early would send whatever was left in the
rest of that buffer.

Zero the part of the buffer we didn't fill and return 0, with a
DEBUG_NET_WARN_ON_ONCE() for debug kernels.  As with skb_checksum(), the
checksum usually won't match the data, so the receiver will usually drop
the packet, but either way it carries nothing it shouldn't.  @len is an
int, so only zero when it is positive; a negative @len from a broken
caller must not turn into a huge memset().

Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 net/core/skbuff.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c896770203f7..7fd2f8142cc4 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3713,7 +3713,13 @@ __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset,
 		}
 		start = end;
 	}
-	BUG_ON(len);
+	if (unlikely(len)) {
+		DEBUG_NET_WARN_ON_ONCE(1);
+		/* Don't hand the caller a buffer with stale bytes in it. */
+		if (len > 0)
+			memset(to, 0, len);
+		return 0;
+	}
 	return csum;
 }
 EXPORT_SYMBOL(skb_copy_and_csum_bits);

-- 
2.55.0