Subject: Re: [PATCH net] inet: fully convert sk->sk_rx_dst to RCU rules

From: Carlos Llamas <[email-masked]>
Date: Thu, 20 Oct 2022 21:05:34 +0000

On Mon, Dec 20, 2021 at 06:33:30AM -0800, Eric Dumazet wrote:
> From: Eric Dumazet <[email-masked]>
> 
> 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: 41063e9dd119 ("ipv4: Early TCP socket demux.")
> Signed-off-by: Eric Dumazet <[email-masked]>
> ---
>  include/net/sock.h   |  2 +-
>  net/ipv4/af_inet.c   |  2 +-
>  net/ipv4/tcp.c       |  3 +--
>  net/ipv4/tcp_input.c |  2 +-
>  net/ipv4/tcp_ipv4.c  | 11 +++++++----
>  net/ipv4/udp.c       |  6 +++---
>  net/ipv6/tcp_ipv6.c  | 11 +++++++----
>  net/ipv6/udp.c       |  4 ++--
>  8 files changed, 23 insertions(+), 18 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index bea21ff70e74d906216f4eaa2d5a712d12551216..d47e9658da28545c1f6afd9db0cf136b3e13d7b6 100644
> --- a/include/net/sock.h
> +++ b/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;
>  
> diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> index 0189e3cd4a7df2dc2ea7121182ee290e0164df90..6b5956500436187d5e9b801ebb6f8f52ba0db090 100644
> --- a/net/ipv4/af_inet.c
> +++ b/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);
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index bbb3d39c69afc2d5a42c6ace8d473657861da61f..2bb28bfd83bf621b5d9f3fb7ce5695195697b43a 100644
> --- a/net/ipv4/tcp.c
> +++ b/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;
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 246ab7b5e857eb9e802c4805075e89c98cf00636..0ce46849ec3d4595699dd54229919b2d66b70257 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/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.
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index 13d868c43284584ee0c58ddfd411bb52c8b0c830..084df223b5dff8089a615a4a8d392b620fc0a28a 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/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;
>  	}
>  }
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 23b05e28490b0a0a690a837027f26167e353f8ce..15c6b450b8dba44d5f344a554eea6b991c1ea5f1 100644
> --- a/net/ipv4/udp.c
> +++ b/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);
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 551fce49841d7f53a111b0435855634cece2b40a..680e6481b9672040ccb41fd08ab80166575bef50 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/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);
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index e43b31d25fb61c7875f3bb8a93eb74da244d912a..a2caca6ccf114546f9e4ea854ad67208b2f3873e 100644
> --- a/net/ipv6/udp.c
> +++ b/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);
> -- 
> 2.34.1.173.g76aa8bc2d0-goog
> 
> 

Eric, this patch was picked for v5.15 stable and I wonder whether this
needs to be backported to older branches too. The Fixes commit quoted
here seems to go back all the way to v3.5. Would you know?

--
Carlos Llamas