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tcp: fix use after free in tcp_xmit_retransmit_queue()
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. Change-Id: I264f97d30d0a623011d9ee811c63fa0e0c2149a2 Fixes: 6859d49 ("[TCP]: Abstract tp->highest_sack accessing & point to next skb") Reported-by: Marco Grassi <email@example.com> Signed-off-by: Eric Dumazet <firstname.lastname@example.org> Cc: Ilpo Järvinen <email@example.com> Cc: Yuchung Cheng <firstname.lastname@example.org> Cc: Neal Cardwell <email@example.com> Acked-by: Neal Cardwell <firstname.lastname@example.org> Reviewed-by: Cong Wang <email@example.com> Signed-off-by: David S. Miller <firstname.lastname@example.org>
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