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

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Date:	Wed, 17 Aug 2016 10:09:16 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Marco Grassi <marco.gra@...il.com>,
	Yuchung Cheng <ycheng@...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 8:56 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>

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks, Eric.

neal