[PATCH net] icmp: fix NULL pointer dereference in icmp_tag_validation()

[bestswngs@gmail.com]

From: Weiming Shi <bestswngs@gmail.com>

icmp_tag_validation() unconditionally dereferences the result of
rcu_dereference(inet_protos[proto]) without checking for NULL.
The inet_protos[] array is sparse -- only about 15 of 256 protocol
numbers have registered handlers. When ip_no_pmtu_disc is set to 3
(hardened PMTU mode) and the kernel receives an ICMP Fragmentation
Needed error with a quoted inner IP header containing an unregistered
protocol number, the NULL dereference causes a kernel panic in
softirq context.

 Oops: general protection fault, probably for non-canonical address 0xdffffc0000000002: 0000 [#1] SMP KASAN NOPTI
 KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
 RIP: 0010:icmp_unreach (net/ipv4/icmp.c:1085 net/ipv4/icmp.c:1143)
 Call Trace:
  <IRQ>
  icmp_rcv (net/ipv4/icmp.c:1527)
  ip_protocol_deliver_rcu (net/ipv4/ip_input.c:207)
  ip_local_deliver_finish (net/ipv4/ip_input.c:242)
  ip_local_deliver (net/ipv4/ip_input.c:262)
  ip_rcv (net/ipv4/ip_input.c:573)
  __netif_receive_skb_one_core (net/core/dev.c:6164)
  process_backlog (net/core/dev.c:6628)
  handle_softirqs (kernel/softirq.c:561)
  </IRQ>

Add a NULL check before accessing icmp_strict_tag_validation. If the
protocol has no registered handler, return false since it cannot
perform strict tag validation.

Fixes: 8ed1dc44d3e9 ("ipv4: introduce hardened ip_no_pmtu_disc mode")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
---
 net/ipv4/icmp.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index a62b4c4033cc..568bd1e95d44 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -1079,10 +1079,12 @@ static void icmp_socket_deliver(struct sk_buff *skb, u32 info)
 
 static bool icmp_tag_validation(int proto)
 {
+	const struct net_protocol *ipprot;
 	bool ok;
 
 	rcu_read_lock();
-	ok = rcu_dereference(inet_protos[proto])->icmp_strict_tag_validation;
+	ipprot = rcu_dereference(inet_protos[proto]);
+	ok = ipprot ? ipprot->icmp_strict_tag_validation : false;
 	rcu_read_unlock();
 	return ok;
 }
-- 
2.43.0