[PATCH net] dccp: fix undefined behavior with 'cwnd' shift in ccid2_cwnd_restart()

[PATCH net] dccp: fix undefined behavior with 'cwnd' shift in ccid2_cwnd_restart()

[Alexey Kodanev]

Make sure that the value of "(now - hc->tx_lsndtime) / hc->tx_rto" is
properly limited when shifting 'u32 cwnd' with it, otherwise we can get:

[40850.963623] UBSAN: Undefined behaviour in net/dccp/ccids/ccid2.c:237:7
[40851.043858] shift exponent 67 is too large for 32-bit type 'unsigned int'
[40851.127163] CPU: 3 PID: 15940 Comm: netstress Tainted: G        W   E     4.18.0-rc7.x86_64 #1
...
[40851.377176] Call Trace:
[40851.408503]  dump_stack+0xf1/0x17b
[40851.451331]  ? show_regs_print_info+0x5/0x5
[40851.503555]  ubsan_epilogue+0x9/0x7c
[40851.548363]  __ubsan_handle_shift_out_of_bounds+0x25b/0x2b4
[40851.617109]  ? __ubsan_handle_load_invalid_value+0x18f/0x18f
[40851.686796]  ? xfrm4_output_finish+0x80/0x80
[40851.739827]  ? lock_downgrade+0x6d0/0x6d0
[40851.789744]  ? xfrm4_prepare_output+0x160/0x160
[40851.845912]  ? ip_queue_xmit+0x810/0x1db0
[40851.895845]  ? ccid2_hc_tx_packet_sent+0xd36/0x10a0 [dccp]
[40851.963530]  ccid2_hc_tx_packet_sent+0xd36/0x10a0 [dccp]
[40852.029063]  dccp_xmit_packet+0x1d3/0x720 [dccp]
[40852.086254]  dccp_write_xmit+0x116/0x1d0 [dccp]
[40852.142412]  dccp_sendmsg+0x428/0xb20 [dccp]
[40852.195454]  ? inet_dccp_listen+0x200/0x200 [dccp]
[40852.254833]  ? sched_clock+0x5/0x10
[40852.298508]  ? sched_clock+0x5/0x10
[40852.342194]  ? inet_create+0xdf0/0xdf0
[40852.388988]  sock_sendmsg+0xd9/0x160
...

Fixes: 113ced1f52e5 ("dccp ccid-2: Perform congestion-window validation")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
---
 net/dccp/ccids/ccid2.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/dccp/ccids/ccid2.c b/net/dccp/ccids/ccid2.c
index 2b75df4..2821180 100644
--- a/net/dccp/ccids/ccid2.c
+++ b/net/dccp/ccids/ccid2.c
@@ -234,7 +234,7 @@ static void ccid2_cwnd_restart(struct sock *sk, const u32 now)
 
 	/* don't reduce cwnd below the initial window (IW) */
 	restart_cwnd = min(cwnd, iwnd);
-	cwnd >>= (now - hc->tx_lsndtime) / hc->tx_rto;
+	cwnd >>= min((now - hc->tx_lsndtime) / hc->tx_rto, 31U);
 	hc->tx_cwnd = max(cwnd, restart_cwnd);
 
 	hc->tx_cwnd_stamp = now;
-- 
1.8.3.1

Re: [PATCH net] dccp: fix undefined behavior with 'cwnd' shift in ccid2_cwnd_restart()

[David Miller]

From: Alexey Kodanev <alexey.kodanev@oracle.com>
Date: Thu,  2 Aug 2018 19:22:05 +0300

> Make sure that the value of "(now - hc->tx_lsndtime) / hc->tx_rto" is
> properly limited when shifting 'u32 cwnd' with it, otherwise we can get:
 ...
> Fixes: 113ced1f52e5 ("dccp ccid-2: Perform congestion-window validation")
> Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
 ...
> @@ -234,7 +234,7 @@ static void ccid2_cwnd_restart(struct sock *sk, const u32 now)
>  
>  	/* don't reduce cwnd below the initial window (IW) */
>  	restart_cwnd = min(cwnd, iwnd);
> -	cwnd >>= (now - hc->tx_lsndtime) / hc->tx_rto;
> +	cwnd >>= min((now - hc->tx_lsndtime) / hc->tx_rto, 31U);
>  	hc->tx_cwnd = max(cwnd, restart_cwnd);
>  
>  	hc->tx_cwnd_stamp = now;

Better to mimick the TCP cwnd validation code, something like:

	s32 delta = now - hc->tx_lsndtime;
	while ((delta -= hc->tx_rto) > 0 && cwnd > restart_cwnd)
		cwnd >>= 1;

Thanks.