From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Xiang Mei <xmei5@asu.edu>,
	Weiming Shi <bestswngs@gmail.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.1 439/481] icmp: fix NULL pointer dereference in icmp_tag_validation()
Date: Mon, 23 Mar 2026 14:47:01 +0100
Message-ID: <20260323134535.908037663@linuxfoundation.org>
In-Reply-To: <20260323134525.256603107@linuxfoundation.org>
References: <20260323134525.256603107@linuxfoundation.org>
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6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Weiming Shi <bestswngs@gmail.com>

[ Upstream commit 614aefe56af8e13331e50220c936fc0689cf5675 ]

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>
Link: https://patch.msgid.link/20260318130558.1050247-4-bestswngs@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 374ec3aba66e3..309d22f2858cc 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -864,10 +864,12 @@ 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;
 }
-- 
2.51.0