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SubjectRe: [PATCH] ipv6: check return value of ipv6_skip_exthdr
On Wed, Nov 17, 2021 at 07:16:10PM +0100, Jordy Zomer wrote:
> The offset value is used in pointer math on skb->data.
> Since ipv6_skip_exthdr may return -1 the pointer to uh and th
> may not point to the actual udp and tcp headers and potentially
> overwrite other stuff. This is why I think this should be checked.
>
> Signed-off-by: Jordy Zomer <jordy@pwning.systems>
> ---
> net/ipv6/esp6.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
> index ed2f061b8768..dc4251655df9 100644
> --- a/net/ipv6/esp6.c
> +++ b/net/ipv6/esp6.c
> @@ -808,6 +808,11 @@ int esp6_input_done2(struct sk_buff *skb, int err)
> struct tcphdr *th;
>
> offset = ipv6_skip_exthdr(skb, offset, &nexthdr, &frag_off);
> +
> + if (offset < 0)
> + err = -EINVAL;
> + goto out;
> +

Ew. Yeah, it seems like ipv6_skip_exthdr() needs to be checked in a lot
of places. If this is part of protocol decoding, I'm surprised fuzzers
haven't found this? Is this state reachable?

I assume so, as there have been similar fixes in the past:
https://git.kernel.org/linus/92c6058024e87087cf1b99b0389d67c0a886360e

> uh = (void *)(skb->data + offset);
> th = (void *)(skb->data + offset);
> hdr_len += offset;
> --
> 2.27.0
>

--
Kees Cook

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