[PATCH net] sctp: use the correct skb for security_sctp_assoc_request

[Ondrej Mosnacek]

Adding LSM and SELinux lists to CC for awareness; the original patch
is available at:
https://lore.kernel.org/netdev/a77a584b3ce9761eb5dda5828192e1cab94571f0.1649037151.git.lucien.xin@gmail.com/T/
https://patchwork.kernel.org/project/netdevbpf/patch/a77a584b3ce9761eb5dda5828192e1cab94571f0.1649037151.git.lucien.xin@gmail.com/

On Mon, Apr 4, 2022 at 3:53 AM Xin Long <lucien.xin at gmail.com> wrote:
>
> Yi Chen reported an unexpected sctp connection abort, and it occurred when
> COOKIE_ECHO is bundled with DATA Fragment by SCTP HW GSO. As the IP header
> is included in chunk->head_skb instead of chunk->skb, it failed to check
> IP header version in security_sctp_assoc_request().
>
> According to Ondrej, SELinux only looks at IP header (address and IPsec
> options) and XFRM state data, and these are all included in head_skb for
> SCTP HW GSO packets. So fix it by using head_skb when calling
> security_sctp_assoc_request() in processing COOKIE_ECHO.

The logic looks good to me, but I still have one unanswered concern.
The head_skb member of struct sctp_chunk is defined inside a union:

struct sctp_chunk {
        [...]
        union {
                /* In case of GSO packets, this will store the head one */
                struct sk_buff *head_skb;
                /* In case of auth enabled, this will point to the shkey */
                struct sctp_shared_key *shkey;
        };
        [...]
};

What guarantees that this chunk doesn't have "auth enabled" and the
head_skb pointer isn't actually a non-NULL shkey pointer? Maybe it's
obvious to a Linux SCTP expert, but at least for me as an outsider it
isn't - that's usually a good hint that there should be a code comment
explaining it.

>
> Fixes: e215dab1c490 ("security: call security_sctp_assoc_request in sctp_sf_do_5_1D_ce")
> Reported-by: Yi Chen <yiche at redhat.com>
> Signed-off-by: Xin Long <lucien.xin at gmail.com>
> ---
>  net/sctp/sm_statefuns.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/sctp/sm_statefuns.c b/net/sctp/sm_statefuns.c
> index 7f342bc12735..883f9b849ee5 100644
> --- a/net/sctp/sm_statefuns.c
> +++ b/net/sctp/sm_statefuns.c
> @@ -781,7 +781,7 @@ enum sctp_disposition sctp_sf_do_5_1D_ce(struct net *net,
>                 }
>         }
>
> -       if (security_sctp_assoc_request(new_asoc, chunk->skb)) {
> +       if (security_sctp_assoc_request(new_asoc, chunk->head_skb ?: chunk->skb)) {
>                 sctp_association_free(new_asoc);
>                 return sctp_sf_pdiscard(net, ep, asoc, type, arg, commands);
>         }
> @@ -2262,7 +2262,7 @@ enum sctp_disposition sctp_sf_do_5_2_4_dupcook(
>         }
>
>         /* Update socket peer label if first association. */
> -       if (security_sctp_assoc_request(new_asoc, chunk->skb)) {
> +       if (security_sctp_assoc_request(new_asoc, chunk->head_skb ?: chunk->skb)) {
>                 sctp_association_free(new_asoc);
>                 return sctp_sf_pdiscard(net, ep, asoc, type, arg, commands);
>         }
> --
> 2.31.1
>

--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.