inet: fully convert sk->sk_rx_dst to RCU rules

syzbot reported various issues around early demux,
one being included in this changelog [1]

sk->sk_rx_dst is using RCU protection without clearly
documenting it.

And following sequences in tcp_v4_do_rcv()/tcp_v6_do_rcv()
are not following standard RCU rules.

[a]    dst_release(dst);
[b]    sk->sk_rx_dst = NULL;

They look wrong because a delete operation of RCU protected
pointer is supposed to clear the pointer before
the call_rcu()/synchronize_rcu() guarding actual memory freeing.

In some cases indeed, dst could be freed before [b] is done.

We could cheat by clearing sk_rx_dst before calling
dst_release(), but this seems the right time to stick
to standard RCU annotations and debugging facilities.

[1]
BUG: KASAN: use-after-free in dst_check include/net/dst.h:470 [inline]
BUG: KASAN: use-after-free in tcp_v4_early_demux+0x95b/0x960 net/ipv4/tcp_ipv4.c:1792
Read of size 2 at addr ffff88807f1cb73a by task syz-executor.5/9204

CPU: 0 PID: 9204 Comm: syz-executor.5 Not tainted 5.16.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_address_description.constprop.0.cold+0x8d/0x320 mm/kasan/report.c:247
 __kasan_report mm/kasan/report.c:433 [inline]
 kasan_report.cold+0x83/0xdf mm/kasan/report.c:450
 dst_check include/net/dst.h:470 [inline]
 tcp_v4_early_demux+0x95b/0x960 net/ipv4/tcp_ipv4.c:1792
 ip_rcv_finish_core.constprop.0+0x15de/0x1e80 net/ipv4/ip_input.c:340
 ip_list_rcv_finish.constprop.0+0x1b2/0x6e0 net/ipv4/ip_input.c:583
 ip_sublist_rcv net/ipv4/ip_input.c:609 [inline]
 ip_list_rcv+0x34e/0x490 net/ipv4/ip_input.c:644
 __netif_receive_skb_list_ptype net/core/dev.c:5508 [inline]
 __netif_receive_skb_list_core+0x549/0x8e0 net/core/dev.c:5556
 __netif_receive_skb_list net/core/dev.c:5608 [inline]
 netif_receive_skb_list_internal+0x75e/0xd80 net/core/dev.c:5699
 gro_normal_list net/core/dev.c:5853 [inline]
 gro_normal_list net/core/dev.c:5849 [inline]
 napi_complete_done+0x1f1/0x880 net/core/dev.c:6590
 virtqueue_napi_complete drivers/net/virtio_net.c:339 [inline]
 virtnet_poll+0xca2/0x11b0 drivers/net/virtio_net.c:1557
 __napi_poll+0xaf/0x440 net/core/dev.c:7023
 napi_poll net/core/dev.c:7090 [inline]
 net_rx_action+0x801/0xb40 net/core/dev.c:7177
 __do_softirq+0x29b/0x9c2 kernel/softirq.c:558
 invoke_softirq kernel/softirq.c:432 [inline]
 __irq_exit_rcu+0x123/0x180 kernel/softirq.c:637
 irq_exit_rcu+0x5/0x20 kernel/softirq.c:649
 common_interrupt+0x52/0xc0 arch/x86/kernel/irq.c:240
 asm_common_interrupt+0x1e/0x40 arch/x86/include/asm/idtentry.h:629
RIP: 0033:0x7f5e972bfd57
Code: 39 d1 73 14 0f 1f 80 00 00 00 00 48 8b 50 f8 48 83 e8 08 48 39 ca 77 f3 48 39 c3 73 3e 48 89 13 48 8b 50 f8 48 89 38 49 8b 0e <48> 8b 3e 48 83 c3 08 48 83 c6 08 eb bc 48 39 d1 72 9e 48 39 d0 73
RSP: 002b:00007fff8a413210 EFLAGS: 00000283
RAX: 00007f5e97108990 RBX: 00007f5e97108338 RCX: ffffffff81d3aa45
RDX: ffffffff81d3aa45 RSI: 00007f5e97108340 RDI: ffffffff81d3aa45
RBP: 00007f5e97107eb8 R08: 00007f5e97108d88 R09: 0000000093c2e8d9
R10: 0000000000000000 R11: 0000000000000000 R12: 00007f5e97107eb0
R13: 00007f5e97108338 R14: 00007f5e97107ea8 R15: 0000000000000019
 </TASK>

Allocated by task 13:
 kasan_save_stack+0x1e/0x50 mm/kasan/common.c:38
 kasan_set_track mm/kasan/common.c:46 [inline]
 set_alloc_info mm/kasan/common.c:434 [inline]
 __kasan_slab_alloc+0x90/0xc0 mm/kasan/common.c:467
 kasan_slab_alloc include/linux/kasan.h:259 [inline]
 slab_post_alloc_hook mm/slab.h:519 [inline]
 slab_alloc_node mm/slub.c:3234 [inline]
 slab_alloc mm/slub.c:3242 [inline]
 kmem_cache_alloc+0x202/0x3a0 mm/slub.c:3247
 dst_alloc+0x146/0x1f0 net/core/dst.c:92
 rt_dst_alloc+0x73/0x430 net/ipv4/route.c:1613
 ip_route_input_slow+0x1817/0x3a20 net/ipv4/route.c:2340
 ip_route_input_rcu net/ipv4/route.c:2470 [inline]
 ip_route_input_noref+0x116/0x2a0 net/ipv4/route.c:2415
 ip_rcv_finish_core.constprop.0+0x288/0x1e80 net/ipv4/ip_input.c:354
 ip_list_rcv_finish.constprop.0+0x1b2/0x6e0 net/ipv4/ip_input.c:583
 ip_sublist_rcv net/ipv4/ip_input.c:609 [inline]
 ip_list_rcv+0x34e/0x490 net/ipv4/ip_input.c:644
 __netif_receive_skb_list_ptype net/core/dev.c:5508 [inline]
 __netif_receive_skb_list_core+0x549/0x8e0 net/core/dev.c:5556
 __netif_receive_skb_list net/core/dev.c:5608 [inline]
 netif_receive_skb_list_internal+0x75e/0xd80 net/core/dev.c:5699
 gro_normal_list net/core/dev.c:5853 [inline]
 gro_normal_list net/core/dev.c:5849 [inline]
 napi_complete_done+0x1f1/0x880 net/core/dev.c:6590
 virtqueue_napi_complete drivers/net/virtio_net.c:339 [inline]
 virtnet_poll+0xca2/0x11b0 drivers/net/virtio_net.c:1557
 __napi_poll+0xaf/0x440 net/core/dev.c:7023
 napi_poll net/core/dev.c:7090 [inline]
 net_rx_action+0x801/0xb40 net/core/dev.c:7177
 __do_softirq+0x29b/0x9c2 kernel/softirq.c:558

Freed by task 13:
 kasan_save_stack+0x1e/0x50 mm/kasan/common.c:38
 kasan_set_track+0x21/0x30 mm/kasan/common.c:46
 kasan_set_free_info+0x20/0x30 mm/kasan/generic.c:370
 ____kasan_slab_free mm/kasan/common.c:366 [inline]
 ____kasan_slab_free mm/kasan/common.c:328 [inline]
 __kasan_slab_free+0xff/0x130 mm/kasan/common.c:374
 kasan_slab_free include/linux/kasan.h:235 [inline]
 slab_free_hook mm/slub.c:1723 [inline]
 slab_free_freelist_hook+0x8b/0x1c0 mm/slub.c:1749
 slab_free mm/slub.c:3513 [inline]
 kmem_cache_free+0xbd/0x5d0 mm/slub.c:3530
 dst_destroy+0x2d6/0x3f0 net/core/dst.c:127
 rcu_do_batch kernel/rcu/tree.c:2506 [inline]
 rcu_core+0x7ab/0x1470 kernel/rcu/tree.c:2741
 __do_softirq+0x29b/0x9c2 kernel/softirq.c:558

Last potentially related work creation:
 kasan_save_stack+0x1e/0x50 mm/kasan/common.c:38
 __kasan_record_aux_stack+0xf5/0x120 mm/kasan/generic.c:348
 __call_rcu kernel/rcu/tree.c:2985 [inline]
 call_rcu+0xb1/0x740 kernel/rcu/tree.c:3065
 dst_release net/core/dst.c:177 [inline]
 dst_release+0x79/0xe0 net/core/dst.c:167
 tcp_v4_do_rcv+0x612/0x8d0 net/ipv4/tcp_ipv4.c:1712
 sk_backlog_rcv include/net/sock.h:1030 [inline]
 __release_sock+0x134/0x3b0 net/core/sock.c:2768
 release_sock+0x54/0x1b0 net/core/sock.c:3300
 tcp_sendmsg+0x36/0x40 net/ipv4/tcp.c:1441
 inet_sendmsg+0x99/0xe0 net/ipv4/af_inet.c:819
 sock_sendmsg_nosec net/socket.c:704 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:724
 sock_write_iter+0x289/0x3c0 net/socket.c:1057
 call_write_iter include/linux/fs.h:2162 [inline]
 new_sync_write+0x429/0x660 fs/read_write.c:503
 vfs_write+0x7cd/0xae0 fs/read_write.c:590
 ksys_write+0x1ee/0x250 fs/read_write.c:643
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

The buggy address belongs to the object at ffff88807f1cb700
 which belongs to the cache ip_dst_cache of size 176
The buggy address is located 58 bytes inside of
 176-byte region [ffff88807f1cb700, ffff88807f1cb7b0)
The buggy address belongs to the page:
page:ffffea0001fc72c0 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x7f1cb
flags: 0xfff00000000200(slab|node=0|zone=1|lastcpupid=0x7ff)
raw: 00fff00000000200 dead000000000100 dead000000000122 ffff8881413bb780
raw: 0000000000000000 0000000000100010 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0x112a20(GFP_ATOMIC|__GFP_NOWARN|__GFP_NORETRY|__GFP_HARDWALL), pid 5, ts 108466983062, free_ts 108048976062
 prep_new_page mm/page_alloc.c:2418 [inline]
 get_page_from_freelist+0xa72/0x2f50 mm/page_alloc.c:4149
 __alloc_pages+0x1b2/0x500 mm/page_alloc.c:5369
 alloc_pages+0x1a7/0x300 mm/mempolicy.c:2191
 alloc_slab_page mm/slub.c:1793 [inline]
 allocate_slab mm/slub.c:1930 [inline]
 new_slab+0x32d/0x4a0 mm/slub.c:1993
 ___slab_alloc+0x918/0xfe0 mm/slub.c:3022
 __slab_alloc.constprop.0+0x4d/0xa0 mm/slub.c:3109
 slab_alloc_node mm/slub.c:3200 [inline]
 slab_alloc mm/slub.c:3242 [inline]
 kmem_cache_alloc+0x35c/0x3a0 mm/slub.c:3247
 dst_alloc+0x146/0x1f0 net/core/dst.c:92
 rt_dst_alloc+0x73/0x430 net/ipv4/route.c:1613
 __mkroute_output net/ipv4/route.c:2564 [inline]
 ip_route_output_key_hash_rcu+0x921/0x2d00 net/ipv4/route.c:2791
 ip_route_output_key_hash+0x18b/0x300 net/ipv4/route.c:2619
 __ip_route_output_key include/net/route.h:126 [inline]
 ip_route_output_flow+0x23/0x150 net/ipv4/route.c:2850
 ip_route_output_key include/net/route.h:142 [inline]
 geneve_get_v4_rt+0x3a6/0x830 drivers/net/geneve.c:809
 geneve_xmit_skb drivers/net/geneve.c:899 [inline]
 geneve_xmit+0xc4a/0x3540 drivers/net/geneve.c:1082
 __netdev_start_xmit include/linux/netdevice.h:4994 [inline]
 netdev_start_xmit include/linux/netdevice.h:5008 [inline]
 xmit_one net/core/dev.c:3590 [inline]
 dev_hard_start_xmit+0x1eb/0x920 net/core/dev.c:3606
 __dev_queue_xmit+0x299a/0x3650 net/core/dev.c:4229
page last free stack trace:
 reset_page_owner include/linux/page_owner.h:24 [inline]
 free_pages_prepare mm/page_alloc.c:1338 [inline]
 free_pcp_prepare+0x374/0x870 mm/page_alloc.c:1389
 free_unref_page_prepare mm/page_alloc.c:3309 [inline]
 free_unref_page+0x19/0x690 mm/page_alloc.c:3388
 qlink_free mm/kasan/quarantine.c:146 [inline]
 qlist_free_all+0x5a/0xc0 mm/kasan/quarantine.c:165
 kasan_quarantine_reduce+0x180/0x200 mm/kasan/quarantine.c:272
 __kasan_slab_alloc+0xa2/0xc0 mm/kasan/common.c:444
 kasan_slab_alloc include/linux/kasan.h:259 [inline]
 slab_post_alloc_hook mm/slab.h:519 [inline]
 slab_alloc_node mm/slub.c:3234 [inline]
 kmem_cache_alloc_node+0x255/0x3f0 mm/slub.c:3270
 __alloc_skb+0x215/0x340 net/core/skbuff.c:414
 alloc_skb include/linux/skbuff.h:1126 [inline]
 alloc_skb_with_frags+0x93/0x620 net/core/skbuff.c:6078
 sock_alloc_send_pskb+0x783/0x910 net/core/sock.c:2575
 mld_newpack+0x1df/0x770 net/ipv6/mcast.c:1754
 add_grhead+0x265/0x330 net/ipv6/mcast.c:1857
 add_grec+0x1053/0x14e0 net/ipv6/mcast.c:1995
 mld_send_initial_cr.part.0+0xf6/0x230 net/ipv6/mcast.c:2242
 mld_send_initial_cr net/ipv6/mcast.c:1232 [inline]
 mld_dad_work+0x1d3/0x690 net/ipv6/mcast.c:2268
 process_one_work+0x9b2/0x1690 kernel/workqueue.c:2298
 worker_thread+0x658/0x11f0 kernel/workqueue.c:2445

Memory state around the buggy address:
 ffff88807f1cb600: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88807f1cb680: fb fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc
>ffff88807f1cb700: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                        ^
 ffff88807f1cb780: fb fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc
 ffff88807f1cb800: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

Fixes: 41063e9d ("ipv4: Early TCP socket demux.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20211220143330.680945-1-eric.dumazet@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/sock.h

@@ -431,7 +431,7 @@ struct sock {
 #ifdef CONFIG_XFRM
 	struct xfrm_policy __rcu *sk_policy[2];
 #endif
-	struct dst_entry	*sk_rx_dst;
+	struct dst_entry __rcu	*sk_rx_dst;
 	int			sk_rx_dst_ifindex;
 	u32			sk_rx_dst_cookie;
 

net/ipv4/af_inet.c

@@ -154,7 +154,7 @@ void inet_sock_destruct(struct sock *sk)
 
 	kfree(rcu_dereference_protected(inet->inet_opt, 1));
 	dst_release(rcu_dereference_protected(sk->sk_dst_cache, 1));
-	dst_release(sk->sk_rx_dst);
+	dst_release(rcu_dereference_protected(sk->sk_rx_dst, 1));
 	sk_refcnt_debug_dec(sk);
 }
 EXPORT_SYMBOL(inet_sock_destruct);

net/ipv4/tcp.c

@@ -3012,8 +3012,7 @@ int tcp_disconnect(struct sock *sk, int flags)
 	icsk->icsk_ack.rcv_mss = TCP_MIN_MSS;
 	memset(&tp->rx_opt, 0, sizeof(tp->rx_opt));
 	__sk_dst_reset(sk);
-	dst_release(sk->sk_rx_dst);
-	sk->sk_rx_dst = NULL;
+	dst_release(xchg((__force struct dst_entry **)&sk->sk_rx_dst, NULL));
 	tcp_saved_syn_free(tp);
 	tp->compressed_ack = 0;
 	tp->segs_in = 0;

net/ipv4/tcp_input.c

@@ -5787,7 +5787,7 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
 	trace_tcp_probe(sk, skb);
 
 	tcp_mstamp_refresh(tp);
-	if (unlikely(!sk->sk_rx_dst))
+	if (unlikely(!rcu_access_pointer(sk->sk_rx_dst)))
 		inet_csk(sk)->icsk_af_ops->sk_rx_dst_set(sk, skb);
 	/*
 	 *	Header prediction.

net/ipv4/tcp_ipv4.c

@@ -1701,7 +1701,10 @@ int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
 	struct sock *rsk;
 
 	if (sk->sk_state == TCP_ESTABLISHED) { /* Fast path */
-		struct dst_entry *dst = sk->sk_rx_dst;
+		struct dst_entry *dst;
+
+		dst = rcu_dereference_protected(sk->sk_rx_dst,
+						lockdep_sock_is_held(sk));
 
 		sock_rps_save_rxhash(sk, skb);
 		sk_mark_napi_id(sk, skb);
@@ -1709,8 +1712,8 @@ int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
 			if (sk->sk_rx_dst_ifindex != skb->skb_iif ||
 			    !INDIRECT_CALL_1(dst->ops->check, ipv4_dst_check,
 					     dst, 0)) {
+				RCU_INIT_POINTER(sk->sk_rx_dst, NULL);
 				dst_release(dst);
-				sk->sk_rx_dst = NULL;
 			}
 		}
 		tcp_rcv_established(sk, skb);
@@ -1786,7 +1789,7 @@ int tcp_v4_early_demux(struct sk_buff *skb)
 		skb->sk = sk;
 		skb->destructor = sock_edemux;
 		if (sk_fullsock(sk)) {
-			struct dst_entry *dst = READ_ONCE(sk->sk_rx_dst);
+			struct dst_entry *dst = rcu_dereference(sk->sk_rx_dst);
 
 			if (dst)
 				dst = dst_check(dst, 0);
@@ -2201,7 +2204,7 @@ void inet_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb)
 	struct dst_entry *dst = skb_dst(skb);
 
 	if (dst && dst_hold_safe(dst)) {
-		sk->sk_rx_dst = dst;
+		rcu_assign_pointer(sk->sk_rx_dst, dst);
 		sk->sk_rx_dst_ifindex = skb->skb_iif;
 	}
 }

net/ipv4/udp.c

@@ -2250,7 +2250,7 @@ bool udp_sk_rx_dst_set(struct sock *sk, struct dst_entry *dst)
 	struct dst_entry *old;
 
 	if (dst_hold_safe(dst)) {
-		old = xchg(&sk->sk_rx_dst, dst);
+		old = xchg((__force struct dst_entry **)&sk->sk_rx_dst, dst);
 		dst_release(old);
 		return old != dst;
 	}
@@ -2440,7 +2440,7 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 		struct dst_entry *dst = skb_dst(skb);
 		int ret;
 
-		if (unlikely(sk->sk_rx_dst != dst))
+		if (unlikely(rcu_dereference(sk->sk_rx_dst) != dst))
 			udp_sk_rx_dst_set(sk, dst);
 
 		ret = udp_unicast_rcv_skb(sk, skb, uh);
@@ -2599,7 +2599,7 @@ int udp_v4_early_demux(struct sk_buff *skb)
 
 	skb->sk = sk;
 	skb->destructor = sock_efree;
-	dst = READ_ONCE(sk->sk_rx_dst);
+	dst = rcu_dereference(sk->sk_rx_dst);
 
 	if (dst)
 		dst = dst_check(dst, 0);

net/ipv6/tcp_ipv6.c

@@ -107,7 +107,7 @@ static void inet6_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb)
 	if (dst && dst_hold_safe(dst)) {
 		const struct rt6_info *rt = (const struct rt6_info *)dst;
 
-		sk->sk_rx_dst = dst;
+		rcu_assign_pointer(sk->sk_rx_dst, dst);
 		sk->sk_rx_dst_ifindex = skb->skb_iif;
 		sk->sk_rx_dst_cookie = rt6_get_cookie(rt);
 	}
@@ -1505,7 +1505,10 @@ static int tcp_v6_do_rcv(struct sock *sk, struct sk_buff *skb)
 		opt_skb = skb_clone(skb, sk_gfp_mask(sk, GFP_ATOMIC));
 
 	if (sk->sk_state == TCP_ESTABLISHED) { /* Fast path */
-		struct dst_entry *dst = sk->sk_rx_dst;
+		struct dst_entry *dst;
+
+		dst = rcu_dereference_protected(sk->sk_rx_dst,
+						lockdep_sock_is_held(sk));
 
 		sock_rps_save_rxhash(sk, skb);
 		sk_mark_napi_id(sk, skb);
@@ -1513,8 +1516,8 @@ static int tcp_v6_do_rcv(struct sock *sk, struct sk_buff *skb)
 			if (sk->sk_rx_dst_ifindex != skb->skb_iif ||
 			    INDIRECT_CALL_1(dst->ops->check, ip6_dst_check,
 					    dst, sk->sk_rx_dst_cookie) == NULL) {
+				RCU_INIT_POINTER(sk->sk_rx_dst, NULL);
 				dst_release(dst);
-				sk->sk_rx_dst = NULL;
 			}
 		}
 
@@ -1874,7 +1877,7 @@ INDIRECT_CALLABLE_SCOPE void tcp_v6_early_demux(struct sk_buff *skb)
 		skb->sk = sk;
 		skb->destructor = sock_edemux;
 		if (sk_fullsock(sk)) {
-			struct dst_entry *dst = READ_ONCE(sk->sk_rx_dst);
+			struct dst_entry *dst = rcu_dereference(sk->sk_rx_dst);
 
 			if (dst)
 				dst = dst_check(dst, sk->sk_rx_dst_cookie);

net/ipv6/udp.c

@@ -956,7 +956,7 @@ int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 		struct dst_entry *dst = skb_dst(skb);
 		int ret;
 
-		if (unlikely(sk->sk_rx_dst != dst))
+		if (unlikely(rcu_dereference(sk->sk_rx_dst) != dst))
 			udp6_sk_rx_dst_set(sk, dst);
 
 		if (!uh->check && !udp_sk(sk)->no_check6_rx) {
@@ -1070,7 +1070,7 @@ INDIRECT_CALLABLE_SCOPE void udp_v6_early_demux(struct sk_buff *skb)
 
 	skb->sk = sk;
 	skb->destructor = sock_efree;
-	dst = READ_ONCE(sk->sk_rx_dst);
+	dst = rcu_dereference(sk->sk_rx_dst);
 
 	if (dst)
 		dst = dst_check(dst, sk->sk_rx_dst_cookie);