[PATCH net-next 4/4] net: Fix UDP length overflow with PMTU discover and big MTU

[Alice Mikityanska <alice.kernel@fastmail.im>]

From: Alice Mikityanska <alice@isovalent.com>

This commit bounds cork->base.fragsize to IP(6)_MAX_MTU to avoid a
possible overflow of UDP length that triggers a WARN in
udp_set_len_short when setsockopt IP(V6)_MTU_DISCOVER is set to
IPV6_PMTUDISC_DO or IP(V6)_PMTUDISC_PROBE, and a large packet is sent
over a netdev with an unusually large MTU.

Steps to reproduce (included in the new selftest):

1. Set device MTU bigger than IP6_MAX_MTU (or IP_MAX_MTU + 20).
   cork->base.fragsize will be set to that MTU in ip(6)_setup_cork.
2. Set IP(V6)_MTU_DISCOVER to IP(V6)_PMTUDISC_PROBE or IPV6_PMTUDISC_DO.
   It lets maxnonfragsize be set to device MTU (cork->fragsize) in
   __ip(6)_append_data, rather than to IP(6)_MAX_MTU.
3. Send 65528 bytes of payload (+8 bytes of UDP header, +20/40 bytes of
   IPv4/IPv6 header). Device MTU allows it (it's only one byte bigger
   than IP6_MAX_MTU or IP_MAX_MTU + IPv4 header, and the device MTU is
   bigger than that).
4. The UDP length in the built packet is 65536, which overflows the
   16-bit length field and triggers the WARN in udp_set_len_short.

Note: IP_PMTUDISC_DO with IPv4 is safe, because ip_dst_mtu_maybe_forward
always clamps at IP_MAX_MTU, unlike ip6_dst_mtu_maybe_forward.

Reported-by: syzbot+ce13c07d96d04716eaa2@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a6a966c.86abc875.e5c3d.0054.GAE@google.com/
Signed-off-by: Alice Mikityanska <alice@isovalent.com>
Assisted-by: Claude:claude-sonnet-4.6
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
---
 net/ipv4/ip_output.c                         |  2 +
 net/ipv6/ip6_output.c                        |  2 +-
 tools/testing/selftests/net/Makefile         |  1 +
 tools/testing/selftests/net/cork_fragsize.py | 77 ++++++++++++++++++++
 4 files changed, 81 insertions(+), 1 deletion(-)
 create mode 100755 tools/testing/selftests/net/cork_fragsize.py

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index e6dd1e5b8c32..2bc997c4fa7b 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1299,6 +1299,8 @@ static int ip_setup_cork(struct sock *sk, struct inet_cork *cork,
 
 	cork->fragsize = ip_sk_use_pmtu(sk) ?
 			 dst4_mtu(&rt->dst) : READ_ONCE(rt->dst.dev->mtu);
+	if (cork->fragsize > IP_MAX_MTU)
+		cork->fragsize = IP_MAX_MTU;
 
 	if (!inetdev_valid_mtu(cork->fragsize))
 		return -ENETUNREACH;
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 368e4fa3b43c..91c92cb8e529 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1430,7 +1430,7 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork,
 	if (frag_size && frag_size < mtu)
 		mtu = frag_size;
 
-	cork->base.fragsize = mtu;
+	cork->base.fragsize = min(mtu, IP6_MAX_MTU);
 	cork->base.gso_size = ipc6->gso_size;
 	cork->base.tx_flags = 0;
 	cork->base.mark = ipc6->sockc.mark;
diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index b43ddf192ecc..628203fda424 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/tools/testing/selftests/net/Makefile
@@ -25,6 +25,7 @@ TEST_PROGS := \
 	cmsg_so_mark.sh \
 	cmsg_so_priority.sh \
 	cmsg_time.sh \
+	cork_fragsize.py \
 	double_udp_encap.sh \
 	drop_monitor_tests.sh \
 	ecmp_rehash.sh \
diff --git a/tools/testing/selftests/net/cork_fragsize.py b/tools/testing/selftests/net/cork_fragsize.py
new file mode 100755
index 000000000000..7a1bb069227a
--- /dev/null
+++ b/tools/testing/selftests/net/cork_fragsize.py
@@ -0,0 +1,77 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+
+# Test possible UDP length overflow in udp_send_skb/udp_v6_send_skb.
+
+from lib.py import ksft_run, ksft_exit, ksft_true
+from lib.py import ip, NetNS, NetNSEnter
+import errno
+import socket
+import subprocess
+
+
+IP_MTU_DISCOVER = 10
+IP_PMTUDISC_PROBE = 3
+IPV6_MTU_DISCOVER = 23
+IPV6_PMTUDISC_DO = 2
+
+
+def check_dmesg_clean(func) -> bool:
+    dmesg = subprocess.Popen(['dmesg'], stdout=subprocess.PIPE)
+    result = subprocess.run(['grep', '-q', f'WARNING:.*{func}'], stdin=dmesg.stdout)
+    dmesg.wait()
+    return result.returncode != 0 and dmesg.returncode == 0
+
+
+def test_ipv6() -> None:
+    with NetNS() as ns:
+        ip('link add dummy type dummy', ns=ns)
+        ip('link set dummy mtu 65576', ns=ns)
+        ip('link set dummy up', ns=ns)
+        ip('-6 addr add fd00::1/64 dev dummy nodad', ns=ns)
+        ip('-6 neigh add fd00::2 lladdr 02:00:00:00:00:02 dev dummy nud permanent', ns=ns)
+
+        with NetNSEnter(ns):
+            with socket.socket(socket.AF_INET6, socket.SOCK_DGRAM) as fd:
+                fd.setsockopt(socket.IPPROTO_IPV6, IPV6_MTU_DISCOVER, IPV6_PMTUDISC_DO)
+                try:
+                    fd.sendto(b' ' * 65528, ('fd00::2', 1234))
+                except OSError as e:
+                    # Ignore EMSGSIZE: it happens on kernels with the fix.
+                    if e.errno != errno.EMSGSIZE:
+                        raise
+
+        ip('link del dummy', ns=ns)
+
+    ksft_true(check_dmesg_clean('udp_v6_send_skb'), 'WARNING detected in dmesg')
+
+
+def test_ipv4() -> None:
+    with NetNS() as ns:
+        ip('link add dummy type dummy', ns=ns)
+        ip('link set dummy mtu 65556', ns=ns)
+        ip('link set dummy up', ns=ns)
+        ip('addr add 10.0.0.1/24 dev dummy', ns=ns)
+        ip('neigh add 10.0.0.2 lladdr 02:00:00:00:00:02 dev dummy nud permanent', ns=ns)
+
+        with NetNSEnter(ns):
+            with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as fd:
+                fd.setsockopt(socket.IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_PROBE)
+                try:
+                    fd.sendto(b' ' * 65528, ('10.0.0.2', 1234))
+                except OSError as e:
+                    # Ignore EMSGSIZE: the check happens after the WARN is printed.
+                    if e.errno != errno.EMSGSIZE:
+                        raise
+
+        ip('link del dummy', ns=ns)
+
+    ksft_true(check_dmesg_clean('udp_send_skb'), 'WARNING detected in dmesg')
+
+
+if __name__ == "__main__":
+    ksft_run([
+        test_ipv6,
+        test_ipv4,
+    ])
+    ksft_exit()
-- 
2.55.0