[PATCH net 4/8] netfilter: ip6t_rt: fix zero-address non-strict match out-of-bounds read

[Pablo Neira Ayuso <pablo@netfilter.org>]

From: Luxiao Xu <rakukuip@gmail.com>

rt_mt6_check() permits rules to be configured with rtinfo->addrnr == 0
even when address matching (IP6T_RT_FST_MASK) is requested.

In the IP6T_RT_FST_NSTRICT path, rt_mt6() evaluates packet routing
addresses against rtinfo->addrs[i] and terminates backwards at the bottom
of the loop:

    if (ipv6_addr_equal(ap, &rtinfo->addrs[i])) {
        i++;
    }
    if (i == rtinfo->addrnr)
        break;

When addrnr is 0, if the first packet address matches rtinfo->addrs[0],
i is incremented to 1. Because i is now strictly greater than addrnr (0),
the loop termination condition (i == rtinfo->addrnr) is bypassed and will
never be satisfied.

If a crafted IPv6 packet contains matching routing addresses, i will
advance past IP6T_RT_HOPS (16). The subsequent call to ipv6_addr_equal()
reads beyond struct ip6t_rt, triggering UBSAN/KASAN out-of-bounds warnings
or kernel panics.

Fix this by:
1. Rejecting rules in rt_mt6_check() where IP6T_RT_FST_MASK is set but
   rtinfo->addrnr is zero.
2. In rt_mt6(), moving the termination condition (i < rtinfo->addrnr)
   into the for-loop header condition and removing the backwards break
   at the end of the loop body.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Suggested-by: Florian Westphal <fw@strlen.de>
Assisted-by: LLM
Signed-off-by: Luxiao Xu <rakukuip@gmail.com>
Signed-off-by: Ren Wei <weir@nebusec.ai>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 net/ipv6/netfilter/ip6t_rt.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/netfilter/ip6t_rt.c b/net/ipv6/netfilter/ip6t_rt.c
index 8051425213dd..9880faf3cc7d 100644
--- a/net/ipv6/netfilter/ip6t_rt.c
+++ b/net/ipv6/netfilter/ip6t_rt.c
@@ -96,7 +96,8 @@ static bool rt_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 			unsigned int i = 0;
 
 			for (temp = 0;
-			     temp < (unsigned int)((hdrlen - 8) / 16);
+			     temp < (unsigned int)((hdrlen - 8) / 16) &&
+			     i < rtinfo->addrnr;
 			     temp++) {
 				ap = skb_header_pointer(skb,
 							ptr
@@ -112,8 +113,6 @@ static bool rt_mt6(const struct sk_buff *skb, struct xt_action_param *par)
 
 				if (ipv6_addr_equal(ap, &rtinfo->addrs[i]))
 					i++;
-				if (i == rtinfo->addrnr)
-					break;
 			}
 			if (i == rtinfo->addrnr)
 				return ret;
@@ -162,6 +161,12 @@ static int rt_mt6_check(const struct xt_mtchk_param *par)
 		pr_info_ratelimited("too many addresses specified\n");
 		return -EINVAL;
 	}
+
+	if ((rtinfo->flags & IP6T_RT_FST_MASK) && !rtinfo->addrnr) {
+		pr_info_ratelimited("address list match requested but addrnr is 0\n");
+		return -EINVAL;
+	}
+
 	if ((rtinfo->flags & (IP6T_RT_RES | IP6T_RT_FST_MASK)) &&
 	    (!(rtinfo->flags & IP6T_RT_TYP) ||
 	     (rtinfo->rt_type != 0) ||
-- 
2.47.3