Re: [lwip-devel] use-after-free caused by tcp_input_delayed_close

[Douglas]

Hi,

Might the following guard in lwip_netconn_do_close() protect the
shutdown() call? "if ((msg->conn->pcb.tcp != NULL)"

The close() path appears to use lwip_netconn_do_delconn() which has the
following guard "if (msg->conn->pcb.tcp != NULL)"

It looks like err_tcp() is called when the other end closes the
connection, from tcp_input_delayed_close(), and this function sets
conn->pcb.tcp to NULL, and that NULL should protect later calls to
shutdown() which would return ERR_CONN, and to close().

Would you be able to check this, see if these guards are working when
this can be reproduced? Can you get a better idea of the failure path.

Douglas

On 1/24/19 8:01 PM, Michael Zimmermann wrote:
> Hi,
> 
> I'm running a tcp server using LWIP and upon termination of the
> connection, both sides do a "shutdown(sock, SHUT_WR)", wait for recv to
> return 0, call "shutdown(sock, SHUT_RDWR)", call "close(sock)".
> 
> The bug occurs in form of a race condition:
> - the lwip server calls SHUT_WR
> - the client calls SHUT_WR, once lwip saw this, it sets TF_RXCLOSED in
> "pcb->flags"
> - the client closes the connection, lwip sees this, adds TF_CLOSED to
> recv_flags, and then deletes the pcb within "tcp_input_delayed_close".
> 
> The problem here is that "tcp_input_delayed_close" only calls the
> "pcb->errf" callback on this condition:
> "if (!(pcb->flags & TF_RXCLOSED))"
> 
> I don't really know why that was done in first place, but because of
> this, the pcb gets freed without notifying the user(which would set
> conn->pcb.tcp to NULL) in case the RX side was closed already.
> 
> On the next call to shutdown or close, this results in use-after-free.
> 
> I'm posting this to the mailing list first instead of the bug tracker to
> discuss the intention behind the condition and to come up with a proper
> solution.
> 
> Thanks
> Michael Zimmermann
> 
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