Re: [PATCH net v2] openvswitch: advance by the real header length in get_ipv6_ext_hdrs()

[Simon Horman <horms@kernel.org>]

On Mon, Nov 02, 2026 at 01:45:35AM +0000, Joas Antonio dos Santos wrote:
> get_ipv6_ext_hdrs() walks the IPv6 extension header chain to fill
> key->ipv6.exthdrs, but advances to the next header with ipv6_optlen()
> whatever the current header is.  That is only right for Hop-by-Hop,
> Destination Options and Routing headers:
> 
>  - an Authentication Header is ((hdrlen + 2) << 2) bytes long
>    (ipv6_authlen()), so with hdrlen = 4 the walk skips 40 bytes instead
>    of 24 and reads the next "header" from the ICV and payload;
> 
>  - a Fragment header is always 8 bytes, and the byte read as hdrlen is
>    its Reserved field, which the sender controls and receivers ignore,
>    so a non-zero value makes the walk skip up to 2048 bytes and miss
>    the headers that follow;
> 
>  - in a non-first fragment nothing after the Fragment header is a
>    header at all, it is the middle of the fragmentable part, yet the
>    walk keeps parsing it as an extension header chain.
> 
> In all cases the OFPIEH12_* bits stored in key->ipv6.exthdrs depend on
> bytes chosen by the sender rather than on the headers that are actually
> present: payload bytes that look like another AH or a Hop-by-Hop header
> add OFPIEH12_UNREP/OFPIEH12_UNSEQ, and real headers after a Fragment
> header can go unreported.  Flows that match on OVS_KEY_ATTR_IPV6_EXTHDRS
> then classify such packets wrongly.  There is no memory safety issue,
> skb_header_pointer() bounds every read.
> 
> Advance by sizeof(struct frag_hdr) for Fragment headers and by
> ipv6_authlen() for AH, and stop at the Fragment header if its offset is
> non-zero, as ipv6_find_hdr() does.
> 
> Fixes: 28a3f0601727 ("net: openvswitch: IPv6: Add IPv6 extension header support")
> Link: https://lore.kernel.org/all/178905555085.219967.6617611783437228045@kernel.org/
> Cc: stable@vger.kernel.org
> Signed-off-by: Joas Antonio dos Santos <joasantonio108@gmail.com>
> ---
> Changes in v2:
> - Stop the walk at a non-first fragment, nothing after its Fragment
>   header is a header (Ilya).
> - Drop the Assisted-by tag (Ilya).

Reviewed-by: Simon Horman <horms@kernel.org>