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From: Eric Dumazet <edumazet@google.com>
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Date: 2016-08-17 12:56:26
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Subject: [PATCH net] tcp: fix use after free in tcp_xmit_retransmit_queue()
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When tcp_sendmsg() allocates a fresh and empty skb, it puts it at the
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tail of the write queue using tcp_add_write_queue_tail()
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Then it attempts to copy user data into this fresh skb.
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If the copy fails, we undo the work and remove the fresh skb.
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Unfortunately, this undo lacks the change done to tp->highest_sack and
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we can leave a dangling pointer (to a freed skb)
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Later, tcp_xmit_retransmit_queue() can dereference this pointer and
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access freed memory. For regular kernels where memory is not unmapped,
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this might cause SACK bugs because tcp_highest_sack_seq() is buggy,
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returning garbage instead of tp->snd_nxt, but with various debug
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features like CONFIG_DEBUG_PAGEALLOC, this can crash the kernel.
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This bug was found by Marco Grassi thanks to syzkaller.
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Fixes: 6859d49475d4 ("[TCP]: Abstract tp->highest_sack accessing & point to next skb")
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Reported-by: Marco Grassi <marco.gra@gmail.com>
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Signed-off-by: Eric Dumazet <edumazet@google.com>
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Cc: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
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Cc: Yuchung Cheng <ycheng@google.com>
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Cc: Neal Cardwell <ncardwell@google.com>
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---
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include/net/tcp.h | 2 ++
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1 file changed, 2 insertions(+)
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diff --git a/include/net/tcp.h b/include/net/tcp.h
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index c00e7d51bb18..7717302cab91 100644
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--- a/include/net/tcp.h
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+++ b/include/net/tcp.h
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@@ -1523,6 +1523,8 @@ static inline void tcp_check_send_head(struct sock *sk, struct sk_buff *skb_unli
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{
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if (sk->sk_send_head == skb_unlinked)
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sk->sk_send_head = NULL;
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+ if (tcp_sk(sk)->highest_sack == skb_unlinked)
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+ tcp_sk(sk)->highest_sack = NULL;
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}
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static inline void tcp_init_send_head(struct sock *sk)
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