netdev - Re: [PATCH net] sctp: use the correct skb for security_sctp_assoc

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Date:   Tue, 5 Apr 2022 07:58:20 -0400
From:   Xin Long <lucien.xin@...il.com>
To:     Ondrej Mosnacek <omosnace@...hat.com>
Cc:     network dev <netdev@...r.kernel.org>,
        "linux-sctp @ vger . kernel . org" <linux-sctp@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        Linux Security Module list 
        <linux-security-module@...r.kernel.org>,
        SElinux list <selinux@...r.kernel.org>
Subject: Re: [PATCH net] sctp: use the correct skb for security_sctp_assoc_request

On Mon, Apr 4, 2022 at 6:15 AM Ondrej Mosnacek <omosnace@...hat.com> wrote:
>
> 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@...il.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.
Hi Ondrej,

shkey is for tx skbs only, while head_skb is for skbs on rx path.

Thanks.

>
> >
> > Fixes: e215dab1c490 ("security: call security_sctp_assoc_request in sctp_sf_do_5_1D_ce")
> > Reported-by: Yi Chen <yiche@...hat.com>
> > Signed-off-by: Xin Long <lucien.xin@...il.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.
>