[PATCH v1 net] ipmr: Call ipmr_fib_lookup() under RCU.

[Kuniyuki Iwashima <kuniyu@google.com>]

Yi Lai reported RCU splat in reg_vif_xmit() below. [0]

When CONFIG_IP_MROUTE_MULTIPLE_TABLES=n, ipmr_fib_lookup()
uses rcu_dereference() without explicit rcu_read_lock().

Although rcu_read_lock_bh() is already held by the caller
__dev_queue_xmit(), lockdep requires explicit rcu_read_lock()
for rcu_dereference().

Let's move up rcu_read_lock() in reg_vif_xmit() to
cover ipmr_fib_lookup().

[0]:
WARNING: suspicious RCU usage
7.1.0-rc2-next-20260504-9d0d467c3572 #1 Not tainted
 -----------------------------
net/ipv4/ipmr.c:329 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
2 locks held by syz.2.17/1779:
 #0: ffffffff87896440 (rcu_read_lock_bh){....}-{1:3}, at: local_bh_disable include/linux/bottom_half.h:20 [inline]
 #0: ffffffff87896440 (rcu_read_lock_bh){....}-{1:3}, at: rcu_read_lock_bh include/linux/rcupdate.h:891 [inline]
 #0: ffffffff87896440 (rcu_read_lock_bh){....}-{1:3}, at: __dev_queue_xmit+0x239/0x4140 net/core/dev.c:4792
 #1: ffff88801a199d18 (_xmit_PIMREG#2){+...}-{3:3}, at: spin_lock include/linux/spinlock.h:342 [inline]
 #1: ffff88801a199d18 (_xmit_PIMREG#2){+...}-{3:3}, at: __netif_tx_lock include/linux/netdevice.h:4795 [inline]
 #1: ffff88801a199d18 (_xmit_PIMREG#2){+...}-{3:3}, at: __dev_queue_xmit+0x1d5d/0x4140 net/core/dev.c:4865

stack backtrace:
CPU: 1 UID: 0 PID: 1779 Comm: syz.2.17 Not tainted 7.1.0-rc2-next-20260504-9d0d467c3572 #1 PREEMPT(lazy)
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x121/0x150 lib/dump_stack.c:120
 dump_stack+0x19/0x20 lib/dump_stack.c:129
 lockdep_rcu_suspicious+0x15b/0x1f0 kernel/locking/lockdep.c:6878
 ipmr_fib_lookup net/ipv4/ipmr.c:329 [inline]
 reg_vif_xmit+0x2ee/0x3c0 net/ipv4/ipmr.c:540
 __netdev_start_xmit include/linux/netdevice.h:5382 [inline]
 netdev_start_xmit include/linux/netdevice.h:5391 [inline]
 xmit_one net/core/dev.c:3889 [inline]
 dev_hard_start_xmit+0x170/0x700 net/core/dev.c:3905
 __dev_queue_xmit+0x1df1/0x4140 net/core/dev.c:4871
 dev_queue_xmit include/linux/netdevice.h:3423 [inline]
 packet_xmit+0x252/0x370 net/packet/af_packet.c:276
 packet_snd net/packet/af_packet.c:3082 [inline]
 packet_sendmsg+0x39ad/0x5650 net/packet/af_packet.c:3114
 sock_sendmsg_nosec net/socket.c:797 [inline]
 __sock_sendmsg net/socket.c:812 [inline]
 ____sys_sendmsg+0xa21/0xba0 net/socket.c:2716
 ___sys_sendmsg+0x121/0x1c0 net/socket.c:2770
 __sys_sendmsg+0x177/0x220 net/socket.c:2802
 __do_sys_sendmsg net/socket.c:2807 [inline]
 __se_sys_sendmsg net/socket.c:2805 [inline]
 __x64_sys_sendmsg+0x80/0xc0 net/socket.c:2805
 x64_sys_call+0x1d9c/0x21c0 arch/x86/include/generated/asm/syscalls_64.h:47
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc1/0x1020 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7f37e563ee5d
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 93 af 1b 00 f7 d8 64 89 01 48
RSP: 002b:00007ffe5caa7fa8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00000000005c5fa0 RCX: 00007f37e563ee5d
RDX: 0000000000000000 RSI: 00002000000012c0 RDI: 0000000000000004
RBP: 00000000005c5fa0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00000000005c5fac R15: 00000000005c5fa0
 </TASK>

Fixes: b3b6babf4751 ("ipmr: Free mr_table after RCU grace period.")
Reported-by: syzkaller <syzkaller@googlegroups.com>
Reported-by: Yi Lai <yi1.lai@intel.com>
Closes: https://lore.kernel.org/netdev/afrY34dLXNUboevf@ly-workstation/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/ipv4/ipmr.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index 05fb6eefe0be..2628cd3a93a6 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -537,15 +537,16 @@ static netdev_tx_t reg_vif_xmit(struct sk_buff *skb, struct net_device *dev)
 	};
 	int err;
 
+	rcu_read_lock();
 	err = ipmr_fib_lookup(net, &fl4, &mrt);
 	if (err < 0) {
+		rcu_read_unlock();
 		kfree_skb(skb);
 		return err;
 	}
 
 	DEV_STATS_ADD(dev, tx_bytes, skb->len);
 	DEV_STATS_INC(dev, tx_packets);
-	rcu_read_lock();
 
 	/* Pairs with WRITE_ONCE() in vif_add() and vif_delete() */
 	ipmr_cache_report(mrt, skb, READ_ONCE(mrt->mroute_reg_vif_num),
-- 
2.54.0.545.g6539524ca2-goog