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 usually won't match the data any more, so the receiver will usually drop the packet, but either way it no longer carries 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") Cc: stable@vger.kernel.org Assisted-by: LLM Signed-off-by: Josef Bacik --- This was patch 1 of the skbuff BUG_ON() series; Willem asked for it to go to net on its own: https://lore.kernel.org/r/willemdebruijn.kernel.235bf1cecf85f@gmail.com Tested on net with a module that marks a nonlinear skb unreadable: the part of the buffer past the linear data is zeroed, and a negative @len leaves the buffer alone. Thanks, Josef --- 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 4aea06d5167d..41beaf625421 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; --- base-commit: 23609bce9e1de525d1d0e73fc68c6e7971d0b49e change-id: 20261007-b4-skb-copy-csum-stale-bytes-4bf7b713258c