netdev - Re: [PATCH net] netfilter: nf_reject: don't leak dst refcount for loopback packets

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Message-ID: <CANn89i+EQTt8eaBT0=1U=1JjOb5K5_hH=OhESo9_1hnU5XZU1g@mail.gmail.com>
Date: Wed, 20 Aug 2025 06:16:54 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Florian Westphal <fw@...len.de>
Cc: netdev@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, netfilter-devel@...r.kernel.org, 
	pablo@...filter.org
Subject: Re: [PATCH net] netfilter: nf_reject: don't leak dst refcount for
 loopback packets

On Wed, Aug 20, 2025 at 5:37 AM Florian Westphal <fw@...len.de> wrote:
>
> recent patches to add a WARN() when replacing skb dst entry found an
> old bug:
>
> WARNING: include/linux/skbuff.h:1165 skb_dst_check_unset include/linux/skbuff.h:1164 [inline]
> WARNING: include/linux/skbuff.h:1165 skb_dst_set include/linux/skbuff.h:1210 [inline]
> WARNING: include/linux/skbuff.h:1165 nf_reject_fill_skb_dst+0x2a4/0x330 net/ipv4/netfilter/nf_reject_ipv4.c:234
> [..]
> Call Trace:
>  nf_send_unreach+0x17b/0x6e0 net/ipv4/netfilter/nf_reject_ipv4.c:325
>  nft_reject_inet_eval+0x4bc/0x690 net/netfilter/nft_reject_inet.c:27
>  expr_call_ops_eval net/netfilter/nf_tables_core.c:237 [inline]
>  ..
>
> This is because blamed commit forgot about loopback packets.
> Such packets already have a dst_entry attached, even at PRE_ROUTING stage.
>
> Instead of checking hook just check if the skb already has a route
> attached to it.
>
> Fixes: f53b9b0bdc59 ("netfilter: introduce support for reject at prerouting stage")
> Signed-off-by: Florian Westphal <fw@...len.de>
> ---
>  Sending this instead of a pull request. the only other two
>  candidates for -net are still under review.
>
>  Let me know if you prefer a normal pull request even in this case.
>  Thanks!
>

Great, I was looking at an internal syzbot report with this exact issue.



WARNING: CPU: 1 PID: 5922 at ./include/linux/skbuff.h:1165
skb_dst_check_unset include/linux/skbuff.h:1164 [inline]
WARNING: CPU: 1 PID: 5922 at ./include/linux/skbuff.h:1165 skb_dst_set
include/linux/skbuff.h:1211 [inline]
WARNING: CPU: 1 PID: 5922 at ./include/linux/skbuff.h:1165
nf_reject6_fill_skb_dst net/ipv6/netfilter/nf_reject_ipv6.c:264
[inline]
WARNING: CPU: 1 PID: 5922 at ./include/linux/skbuff.h:1165
nf_send_unreach6+0x828/0xa20 net/ipv6/netfilter/nf_reject_ipv6.c:401
Modules linked in:
CPU: 1 UID: 0 PID: 5922 Comm: kworker/1:3 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine,
BIOS Google 07/12/2025
Workqueue: wg-crypt-wg0 wg_packet_tx_worker
RIP: 0010:skb_dst_check_unset include/linux/skbuff.h:1164 [inline]
RIP: 0010:skb_dst_set include/linux/skbuff.h:1211 [inline]
RIP: 0010:nf_reject6_fill_skb_dst
net/ipv6/netfilter/nf_reject_ipv6.c:264 [inline]
RIP: 0010:nf_send_unreach6+0x828/0xa20 net/ipv6/netfilter/nf_reject_ipv6.c:401
Code: 85 f6 74 0a e8 a9 6c 7a f7 e9 c8 fc ff ff e8 9f 6c 7a f7 4c 8b
7c 24 18 e9 34 fa ff ff e8 90 6c 7a f7 eb 9b e8 89 6c 7a f7 90 <0f> 0b
90 e9 c7 fb ff ff 48 85 db 0f 84 81 00 00 00 4c 8d a4 24 20
RSP: 0018:ffffc90000a083c0 EFLAGS: 00010246
RAX: ffffffff8a453fa7 RBX: ffff88802e6888c0 RCX: ffff88802fc3da00
RDX: 0000000000000100 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90000a08568 R08: ffff888078550b83 R09: 1ffff1100f0aa170
R10: dffffc0000000000 R11: ffffed100f0aa171 R12: ffff888079bb4101
R13: dffffc0000000001 R14: 1ffff11005cd1123 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff888125d1b000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fd4758e56c0 CR3: 00000000776ca000 CR4: 00000000003526f0
Call Trace:
<IRQ>
nft_reject_inet_eval+0x441/0x690 net/netfilter/nft_reject_inet.c:44
expr_call_ops_eval net/netfilter/nf_tables_core.c:237 [inline]
nft_do_chain+0x40c/0x1920 net/netfilter/nf_tables_core.c:285
nft_do_chain_inet+0x25d/0x340 net/netfilter/nft_chain_filter.c:161
nf_hook_entry_hookfn include/linux/netfilter.h:158 [inline]
nf_hook_slow+0xc5/0x220 net/netfilter/core.c:623
nf_hook include/linux/netfilter.h:273 [inline]
NF_HOOK+0x206/0x3a0 include/linux/netfilter.h:316
__netif_receive_skb_one_core net/core/dev.c:5979 [inline]
__netif_receive_skb+0xd3/0x380 net/core/dev.c:6092
process_backlog+0x60e/0x14f0 net/core/dev.c:6444
__napi_poll+0xc7/0x360 net/core/dev.c:7494
napi_poll net/core/dev.c:7557 [inline]
net_rx_action+0x707/0xe30 net/core/dev.c:7684
handle_softirqs+0x283/0x870 kernel/softirq.c:579
do_softirq+0xec/0x180 kernel/softirq.c:480
</IRQ>
<TASK>
__local_bh_enable_ip+0x17d/0x1c0 kernel/softirq.c:407
wg_socket_send_skb_to_peer+0x16b/0x1d0 drivers/net/wireguard/socket.c:184
wg_packet_create_data_done drivers/net/wireguard/send.c:251 [inline]
wg_packet_tx_worker+0x1c8/0x7c0 drivers/net/wireguard/send.c:276
process_one_work kernel/workqueue.c:3236 [inline]
process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3319
worker_thread+0x8a0/0xda0 kernel/workqueue.c:3400
kthread+0x711/0x8a0 kernel/kthread.c:463
ret_from_fork+0x3f9/0x770 arch/x86/kernel/process.c:148
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>