Re: [PATCH net] net: skbuff: don't leave stale bytes in skb_copy_and_csum_bits()

[Mina Almasry]

On Wed, Oct 7, 2026 at 10:29 AM Josef Bacik <josef@xxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>

Reviewed-by: Mina Almasry <almasrymina@xxxxxxxxxx>

We probably need some better csum handling with unreadable skbs
eventually. I took a shortcut in the initial implementation and
returned an invalid csum because there was no way to return error from
the csum functions. With LLMs now this is probably easier to fix.

-- 
Thanks,
Mina