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

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Message-Id: <20160818.232447.1977166448213196018.davem@davemloft.net>
Date:   Thu, 18 Aug 2016 23:24:47 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     netdev@...r.kernel.org, marco.gra@...il.com, ycheng@...gle.com,
        ncardwell@...gle.com, gregkh@...uxfoundation.org,
        ilpo.jarvinen@...sinki.fi
Subject: Re: [PATCH net] tcp: fix use after free in
 tcp_xmit_retransmit_queue()

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 17 Aug 2016 05:56:26 -0700

> 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>

Applied and queued up for -stable.