netdev - Re: [PATCH net] tcp: fix use after free in tcp_xmit_retransmit

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAM_iQpXf3DQ428gKCnTvbYFa8y_RLA7GMJ6aUdw85LWO4xyeXw@mail.gmail.com>
Date:	Wed, 17 Aug 2016 09:12:34 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Marco Grassi <marco.gra@...il.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Subject: Re: [PATCH net] tcp: fix use after free in tcp_xmit_retransmit_queue()

On Wed, Aug 17, 2016 at 5:56 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> When tcp_sendmsg() allocates a fresh and empty skb, it puts it at the
> tail of the write queue using tcp_add_write_queue_tail()
>
> Then it attempts to copy user data into this fresh skb.
>
> If the copy fails, we undo the work and remove the fresh skb.
>
> Unfortunately, this undo lacks the change done to tp->highest_sack and
> we can leave a dangling pointer (to a freed skb)
>
> Later, tcp_xmit_retransmit_queue() can dereference this pointer and
> access freed memory. For regular kernels where memory is not unmapped,
> this might cause SACK bugs because tcp_highest_sack_seq() is buggy,
> returning garbage instead of tp->snd_nxt, but with various debug
> features like CONFIG_DEBUG_PAGEALLOC, this can crash the kernel.
>
> This bug was found by Marco Grassi thanks to syzkaller.
>
> Fixes: 6859d49475d4 ("[TCP]: Abstract tp->highest_sack accessing & point to next skb")
> Reported-by: Marco Grassi <marco.gra@...il.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> ---
>  include/net/tcp.h |    2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index c00e7d51bb18..7717302cab91 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -1523,6 +1523,8 @@ static inline void tcp_check_send_head(struct sock *sk, struct sk_buff *skb_unli
>  {
>         if (sk->sk_send_head == skb_unlinked)
>                 sk->sk_send_head = NULL;
> +       if (tcp_sk(sk)->highest_sack == skb_unlinked)
> +               tcp_sk(sk)->highest_sack = NULL;
>  }
>

Nit: the function name probably needs to change too, since it now
checks more than just send_head. ;) But we can always do this
for net-next, don't let this be a blocker for this security fix.

Reviewed-by: Cong Wang <xiyou.wangcong@...il.com>

Thanks!