[PATCH net-next 1/9] net: skbuff: don't leave stale bytes in skb_copy_and_csum_bits()

[Josef Bacik]

When skb_copy_and_csum_bits() reaches unreadable frags it returns 0
after copying only the linear part, and the rest of the caller's buffer
is left as it was.  The callers copy into a buffer that is about to go
out on the wire: an ICMP error quoting the offending packet, or a
driver's TX bounce buffer in skb_copy_and_csum_dev().  Neither buffer
is zeroed beforehand, so whatever was in memory there gets sent.

Zero the part of the buffer we didn't fill.  The checksum is already
wrong in this case, so the packet still gets dropped by the receiver,
it just doesn't carry anything it shouldn't.  Only zero for a positive
@len, a negative one from a broken caller must not turn into a huge
memset().

Fixes: 65249feb6b3d ("net: add support for skbs with unreadable frags")
Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 net/core/skbuff.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 5c4024a03e10..512ff9cfa269 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3633,8 +3633,12 @@ __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset,
 		pos	= copy;
 	}
 
-	if (!skb_frags_readable(skb))
+	if (!skb_frags_readable(skb)) {
+		/* Don't hand the caller a buffer with stale bytes in it. */
+		if (len > 0)
+			memset(to, 0, len);
 		return 0;
+	}
 
 	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
 		int end;

-- 
2.55.0