ipmr: Always call ip{,6}_mr_forward() from RCU read-side critical section

These functions expect to be called from RCU read-side critical section,
but this only happens when invoked from the data path via
ip{,6}_mr_input(). They can also be invoked from process context in
response to user space adding a multicast route which resolves a cache
entry with queued packets [1][2].

Fix by adding missing rcu_read_lock() / rcu_read_unlock() in these call
paths.

[1]
WARNING: suspicious RCU usage
6.0.0-rc3-custom-15969-g049d233c8bcc-dirty #1387 Not tainted
-----------------------------
net/ipv4/ipmr.c:84 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by smcrouted/246:
 #0: ffffffff862389b0 (rtnl_mutex){+.+.}-{3:3}, at: ip_mroute_setsockopt+0x11c/0x1420

stack backtrace:
CPU: 0 PID: 246 Comm: smcrouted Not tainted 6.0.0-rc3-custom-15969-g049d233c8bcc-dirty #1387
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-1.fc36 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0x91/0xb9
 vif_dev_read+0xbf/0xd0
 ipmr_queue_xmit+0x135/0x1ab0
 ip_mr_forward+0xe7b/0x13d0
 ipmr_mfc_add+0x1a06/0x2ad0
 ip_mroute_setsockopt+0x5c1/0x1420
 do_ip_setsockopt+0x23d/0x37f0
 ip_setsockopt+0x56/0x80
 raw_setsockopt+0x219/0x290
 __sys_setsockopt+0x236/0x4d0
 __x64_sys_setsockopt+0xbe/0x160
 do_syscall_64+0x34/0x80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

[2]
WARNING: suspicious RCU usage
6.0.0-rc3-custom-15969-g049d233c8bcc-dirty #1387 Not tainted
-----------------------------
net/ipv6/ip6mr.c:69 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
1 lock held by smcrouted/246:
 #0: ffffffff862389b0 (rtnl_mutex){+.+.}-{3:3}, at: ip6_mroute_setsockopt+0x6b9/0x2630

stack backtrace:
CPU: 1 PID: 246 Comm: smcrouted Not tainted 6.0.0-rc3-custom-15969-g049d233c8bcc-dirty #1387
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.0-1.fc36 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0x91/0xb9
 vif_dev_read+0xbf/0xd0
 ip6mr_forward2.isra.0+0xc9/0x1160
 ip6_mr_forward+0xef0/0x13f0
 ip6mr_mfc_add+0x1ff2/0x31f0
 ip6_mroute_setsockopt+0x1825/0x2630
 do_ipv6_setsockopt+0x462/0x4440
 ipv6_setsockopt+0x105/0x140
 rawv6_setsockopt+0xd8/0x690
 __sys_setsockopt+0x236/0x4d0
 __x64_sys_setsockopt+0xbe/0x160
 do_syscall_64+0x34/0x80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

Fixes: ebc31979 ("ipmr: add rcu protection over (struct vif_device)->dev")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/ipmr.c

@@ -1004,7 +1004,9 @@ static void ipmr_cache_resolve(struct net *net, struct mr_table *mrt,
 
 			rtnl_unicast(skb, net, NETLINK_CB(skb).portid);
 		} else {
+			rcu_read_lock();
 			ip_mr_forward(net, mrt, skb->dev, skb, c, 0);
+			rcu_read_unlock();
 		}
 	}
 }

net/ipv6/ip6mr.c

@@ -1028,8 +1028,11 @@ static void ip6mr_cache_resolve(struct net *net, struct mr_table *mrt,
 				((struct nlmsgerr *)nlmsg_data(nlh))->error = -EMSGSIZE;
 			}
 			rtnl_unicast(skb, net, NETLINK_CB(skb).portid);
-		} else
+		} else {
+			rcu_read_lock();
 			ip6_mr_forward(net, mrt, skb->dev, skb, c);
+			rcu_read_unlock();
+		}
 	}
 }