Re: [PATCH net] mptcp: fix TCP options overflow.

[Matthieu Baerts <matttbe@kernel.org>]

Hi Paolo,

On 21/12/2024 09:51, Paolo Abeni wrote:
> Syzbot reported the following splat:

(...)

> Eric noted a probable shinfo->nr_frags corruption, which indeed
> occurs.
> 
> The root cause is a buggy MPTCP option len computation in some
> circumstances: the ADD_ADDR option should be mutually exclusive
> with DSS since the blamed commit.
> 
> Still, mptcp_established_options_add_addr() tries to set the
> relevant info in mptcp_out_options, if the remaining space is
> large enough even when DSS is present.
> 
> Since the ADD_ADDR infos and the DSS share the same union
> fields, adding first corrupts the latter. In the worst-case
> scenario, such corruption increases the DSS binary layout,
> exceeding the computed length and possibly overwriting the
> skb shared info.
> 
> Address the issue by enforcing mutual exclusion in
> mptcp_established_options_add_addr(), too.

Thank you for the investigation and the fix, it looks good to me:

Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>

> Reported-by: syzbot+38a095a81f30d82884c1@syzkaller.appspotmail.com

If you don't mind, can you please add these two tags when applying the
patches to help to track the backports?

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/538
Cc: stable@vger.kernel.org

> Fixes: 1bff1e43a30e ("mptcp: optimize out option generation")
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.