tcp: md5: fix potential overestimation of TCP option space

Back in 2008, Adam Langley fixed the corner case of packets for flows having all of the following options : MD5 TS SACK Since MD5 needs 20 bytes, and TS needs 12 bytes, no sack block can be cooked from the remaining 8 bytes. tcp_established_options() correctly sets opts->num_sack_blocks to zero, but returns 36 instead of 32. This means TCP cooks packets with 4 extra bytes at the end of options, containing unitialized bytes. Fixes: 33ad798 ("tcp: options clean up") Signed-off-by: Eric Dumazet <[email protected]> Reported-by: syzbot <[email protected]> Acked-by: Neal Cardwell <[email protected]> Acked-by: Soheil Hassas Yeganeh <[email protected]> Signed-off-by: David S. Miller <[email protected]>
josf · Dec 7, 2019 · 9424e2e · 9424e2e
1 parent 9a74542
commit 9424e2e
Showing 1 changed file with 3 additions and 2 deletions.
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
@@ -755,8 +755,9 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
 			min_t(unsigned int, eff_sacks,
 			      (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
 			      TCPOLEN_SACK_PERBLOCK);
-		size += TCPOLEN_SACK_BASE_ALIGNED +
-			opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
+		if (likely(opts->num_sack_blocks))
+			size += TCPOLEN_SACK_BASE_ALIGNED +
+				opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
 	}
 
 	return size;