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

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Message-ID: <CAFqZXNss=7DMb=75ZBDwL9HrrubkxJK=xu7-kqxX-Mw1FtRuuA@mail.gmail.com>
Date:   Wed, 6 Apr 2022 15:33:57 +0200
From:   Ondrej Mosnacek <omosnace@...hat.com>
To:     Xin Long <lucien.xin@...il.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 Tue, Apr 5, 2022 at 1:58 PM Xin Long <lucien.xin@...il.com> wrote:
> 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.

That makes sense, thanks. I would still be happier if this was
documented, but the comment would best fit in the struct sctp_chunk
definition and that wouldn't fit in this patch...

Actually I have one more question - what about the
security_sctp_assoc_established() call in sctp_sf_do_5_1E_ca()? Is
COOKIE ACK guaranteed to be never bundled?

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