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

[Xin Long <lucien.xin@gmail.com>]

On Wed, Apr 6, 2022 at 11:04 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
>
> On Wed, Apr 6, 2022 at 4:21 PM Xin Long <lucien.xin@gmail.com> wrote:
> > On Wed, Apr 6, 2022 at 9:34 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> > >
> > > On Tue, Apr 5, 2022 at 1:58 PM Xin Long <lucien.xin@gmail.com> wrote:
> > > > On Mon, Apr 4, 2022 at 6:15 AM Ondrej Mosnacek <omosnace@redhat.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@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.
> > > > 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?
> > COOKIE ACK could also be bundled with DATA.
> > I didn't change it as it would not break SCTP.
> > (security_inet_conn_established() returns void)
> > But I don't mind changing it if you think it's necessary.
>
> security_inet_conn_established? Are you looking at an old version of
> the code, perhaps? In mainline, sctp_sf_do_5_1E_ca() now calls the new
> security_sctp_assoc_established() hook, which may return an error. But
> even if it didn't, I believe we want to make sure that an skb with
> valid inet headers and XFRM state is passed to the hooks as SELinux
> relies on these to correctly process the SCTP association.
Sorry, I was looking at the old one.
OK, I will post v2 with the fix in sctp_sf_do_5_1E_ca().

Thanks for reviewing.

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