ipv6: mcast: use rcu-safe version of ipv6_get_lladdr()

Some time ago 8965779d ("ipv6,mcast: always hold idev->lock before mca_lock")
switched ipv6_get_lladdr() to __ipv6_get_lladdr(), which is rcu-unsafe
version. That was OK, because idev->lock was held for these codepaths.

In 88e2ca30 ("mld: convert ifmcaddr6 to RCU") these external locks were
removed, so we probably need to restore the original rcu-safe call.

Otherwise, we occasionally get a machine crashed/stalled with the following
in dmesg:

[ 3405.966610][T230589] general protection fault, probably for non-canonical address 0xdead00000000008c: 0000 [#1] SMP NOPTI
[ 3405.982083][T230589] CPU: 44 PID: 230589 Comm: kworker/44:3 Tainted: G           O      5.15.19-cloudflare-2022.2.1 #1
[ 3405.998061][T230589] Hardware name: SUPA-COOL-SERV
[ 3406.009552][T230589] Workqueue: mld mld_ifc_work
[ 3406.017224][T230589] RIP: 0010:__ipv6_get_lladdr+0x34/0x60
[ 3406.025780][T230589] Code: 57 10 48 83 c7 08 48 89 e5 48 39 d7 74 3e 48 8d 82 38 ff ff ff eb 13 48 8b 90 d0 00 00 00 48 8d 82 38 ff ff ff 48 39 d7 74 22 <66> 83 78 32 20 77 1b 75 e4 89 ca 23 50 2c 75 dd 48 8b 50 08 48 8b
[ 3406.055748][T230589] RSP: 0018:ffff94e4b3fc3d10 EFLAGS: 00010202
[ 3406.065617][T230589] RAX: dead00000000005a RBX: ffff94e4b3fc3d30 RCX: 0000000000000040
[ 3406.077477][T230589] RDX: dead000000000122 RSI: ffff94e4b3fc3d30 RDI: ffff8c3a31431008
[ 3406.089389][T230589] RBP: ffff94e4b3fc3d10 R08: 0000000000000000 R09: 0000000000000000
[ 3406.101445][T230589] R10: ffff8c3a31430000 R11: 000000000000000b R12: ffff8c2c37887100
[ 3406.113553][T230589] R13: ffff8c3a39537000 R14: 00000000000005dc R15: ffff8c3a31431000
[ 3406.125730][T230589] FS:  0000000000000000(0000) GS:ffff8c3b9fc80000(0000) knlGS:0000000000000000
[ 3406.138992][T230589] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3406.149895][T230589] CR2: 00007f0dfea1db60 CR3: 000000387b5f2000 CR4: 0000000000350ee0
[ 3406.162421][T230589] Call Trace:
[ 3406.170235][T230589]  <TASK>
[ 3406.177736][T230589]  mld_newpack+0xfe/0x1a0
[ 3406.186686][T230589]  add_grhead+0x87/0xa0
[ 3406.195498][T230589]  add_grec+0x485/0x4e0
[ 3406.204310][T230589]  ? newidle_balance+0x126/0x3f0
[ 3406.214024][T230589]  mld_ifc_work+0x15d/0x450
[ 3406.223279][T230589]  process_one_work+0x1e6/0x380
[ 3406.232982][T230589]  worker_thread+0x50/0x3a0
[ 3406.242371][T230589]  ? rescuer_thread+0x360/0x360
[ 3406.252175][T230589]  kthread+0x127/0x150
[ 3406.261197][T230589]  ? set_kthread_struct+0x40/0x40
[ 3406.271287][T230589]  ret_from_fork+0x22/0x30
[ 3406.280812][T230589]  </TASK>
[ 3406.288937][T230589] Modules linked in: ... [last unloaded: kheaders]
[ 3406.476714][T230589] ---[ end trace 3525a7655f2f3b9e ]---

Fixes: 88e2ca30 ("mld: convert ifmcaddr6 to RCU")
Reported-by: David Pinilla Caparros <dpini@cloudflare.com>
Signed-off-by: Ignat Korchagin <ignat@cloudflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/addrconf.h

@@ -109,8 +109,6 @@ struct inet6_ifaddr *ipv6_get_ifaddr(struct net *net,
 int ipv6_dev_get_saddr(struct net *net, const struct net_device *dev,
 		       const struct in6_addr *daddr, unsigned int srcprefs,
 		       struct in6_addr *saddr);
-int __ipv6_get_lladdr(struct inet6_dev *idev, struct in6_addr *addr,
-		      u32 banned_flags);
 int ipv6_get_lladdr(struct net_device *dev, struct in6_addr *addr,
 		    u32 banned_flags);
 bool inet_rcv_saddr_equal(const struct sock *sk, const struct sock *sk2,

net/ipv6/addrconf.c

@@ -1839,7 +1839,7 @@ int ipv6_dev_get_saddr(struct net *net, const struct net_device *dst_dev,
 }
 EXPORT_SYMBOL(ipv6_dev_get_saddr);
 
-int __ipv6_get_lladdr(struct inet6_dev *idev, struct in6_addr *addr,
+static int __ipv6_get_lladdr(struct inet6_dev *idev, struct in6_addr *addr,
 			      u32 banned_flags)
 {
 	struct inet6_ifaddr *ifp;

net/ipv6/mcast.c

@@ -1759,7 +1759,7 @@ static struct sk_buff *mld_newpack(struct inet6_dev *idev, unsigned int mtu)
 	skb_reserve(skb, hlen);
 	skb_tailroom_reserve(skb, mtu, tlen);
 
-	if (__ipv6_get_lladdr(idev, &addr_buf, IFA_F_TENTATIVE)) {
+	if (ipv6_get_lladdr(dev, &addr_buf, IFA_F_TENTATIVE)) {
 		/* <draft-ietf-magma-mld-source-05.txt>:
 		 * use unspecified address as the source address
 		 * when a valid link-local address is not available.