[PATCH] tcp: Explicit bound check on options length field

[Stefano Brivio <sbrivio@redhat.com>]

For some reason, Coverity only reports this (harmless) warning after
David's series unifying IPv4 and IPv6 sockets for TCP: an untrusted
loop bound (CWE-606) in tcp_opt_get(), coming from the fact that we
use indeed the value of a TCP header field as loop bound.

Note, though, that the loop already checks we're not exceeding the
length of the option field, and this field is used as 8-bit unsigned
value, so we can't really look for options past the end of the
header.

In any case, make Coverity happy with an explicit check.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 tcp.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tcp.c b/tcp.c
index 8044617..8874789 100644
--- a/tcp.c
+++ b/tcp.c
@@ -1146,7 +1146,11 @@ static int tcp_opt_get(const char *opts, size_t len, uint8_t type_find,
 			break;
 		default:
 			type = *(opts++);
-			optlen = *(opts++) - 2;
+
+			if (*opts < 2 || *(uint8_t *)opts > len)
+				return -1;
+
+			optlen = *((uint8_t *)opts++) - 2;
 			len -= 2;
 
 			if (type != type_find)
-- 
@@ -1146,7 +1146,11 @@ static int tcp_opt_get(const char *opts, size_t len, uint8_t type_find,
 			break;
 		default:
 			type = *(opts++);
-			optlen = *(opts++) - 2;
+
+			if (*opts < 2 || *(uint8_t *)opts > len)
+				return -1;
+
+			optlen = *((uint8_t *)opts++) - 2;
 			len -= 2;
 
 			if (type != type_find)
-- 
2.35.1