Re: [syzbot] [bpf?] WARNING in __bpf_prog_ret0_warn

KaFai Wan
2025-05-23T14:48:02Z

On Sat, 2025-05-03 at 20:47 -0700, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    8bac8898fe39 Merge tag 'mmc-v6.15-rc1' of
> git://git.kernel..
> git tree:       upstream
> console output:
> https://syzkaller.appspot.com/x/log.txt?x=10f03774580000
> kernel config:
> https://syzkaller.appspot.com/x/.config?x=541aa584278da96c
> dashboard link:
> https://syzkaller.appspot.com/bug?extid=0903f6d7f285e41cdf10
> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils
> for Debian) 2.40
> syz repro:
> https://syzkaller.appspot.com/x/repro.syz?x=1550ca70580000
> C reproducer:
> https://syzkaller.appspot.com/x/repro.c?x=17d10f74580000
>
> Downloadable assets:
> disk image (non-bootable):
> https://storage.googleapis.com/syzbot-assets/7feb34a89c2a/non_bootable_disk-8bac8898.raw.xz
> vmlinux:
> https://storage.googleapis.com/syzbot-assets/5f7c2d7e1cd1/vmlinux-8bac8898.xz
> kernel image:
> https://storage.googleapis.com/syzbot-assets/77a157d2769a/bzImage-8bac8898.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the
> commit:
> Reported-by: syzbot+0903f6d7f285e41cdf10@syzkaller.appspotmail.com
>
> ------------[ cut here ]------------
> WARNING: CPU: 3 PID: 217 at kernel/bpf/core.c:2357
> __bpf_prog_ret0_warn+0xa/0x20 kernel/bpf/core.c:2357
> Modules linked in:
> CPU: 3 UID: 0 PID: 217 Comm: kworker/u32:6 Not tainted 6.15.0-rc4-
> syzkaller-00040-g8bac8898fe39 #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-
> debian-1.16.3-2~bpo12+1 04/01/2014
> Workqueue: ipv6_addrconf addrconf_dad_work
> RIP: 0010:__bpf_prog_ret0_warn+0xa/0x20 kernel/bpf/core.c:2357
> Code: f3 0f 1e fa e8 a7 c7 f0 ff 31 c0 c3 cc cc cc cc 90 90 90 90 90
> 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa e8 87 c7 f0 ff 90 <0f>
> 0b 90 31 c0 c3 cc cc cc cc 66 66 2e 0f 1f 84 00 00 00 00 00 90
> RSP: 0018:ffffc900031f6c18 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: ffffc9000006e000 RCX: 1ffff9200000dc06
> RDX: ffff8880234ba440 RSI: ffffffff81ca6979 RDI: ffff888031e93040
> RBP: ffffc900031f6cb8 R08: 0000000000000001 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: ffff88802b61e010
> R13: ffff888031e93040 R14: 00000000000000a0 R15: ffff88802c3d4800
> FS:  0000000000000000(0000) GS:ffff8880d6ce2000(0000)
> knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000055557b6d2ca8 CR3: 000000002473e000 CR4: 0000000000352ef0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  bpf_dispatcher_nop_func include/linux/bpf.h:1316 [inline]
>  __bpf_prog_run include/linux/filter.h:718 [inline]
>  bpf_prog_run include/linux/filter.h:725 [inline]
>  cls_bpf_classify+0x74a/0x1110 net/sched/cls_bpf.c:105
>  tc_classify include/net/tc_wrapper.h:197 [inline]
>  __tcf_classify net/sched/cls_api.c:1764 [inline]
>  tcf_classify+0x7ef/0x1380 net/sched/cls_api.c:1860
>  htb_classify net/sched/sch_htb.c:245 [inline]
>  htb_enqueue+0x2f6/0x12d0 net/sched/sch_htb.c:624
>  dev_qdisc_enqueue net/core/dev.c:3984 [inline]
>  __dev_xmit_skb net/core/dev.c:4080 [inline]
>  __dev_queue_xmit+0x2142/0x43e0 net/core/dev.c:4595
>  dev_queue_xmit include/linux/netdevice.h:3350 [inline]
>  neigh_hh_output include/net/neighbour.h:523 [inline]
>  neigh_output include/net/neighbour.h:537 [inline]
>  ip_finish_output2+0xc38/0x21a0 net/ipv4/ip_output.c:235
>  __ip_finish_output net/ipv4/ip_output.c:313 [inline]
>  __ip_finish_output+0x49e/0x950 net/ipv4/ip_output.c:295
>  ip_finish_output+0x35/0x380 net/ipv4/ip_output.c:323
>  NF_HOOK_COND include/linux/netfilter.h:303 [inline]
>  ip_output+0x13b/0x2a0 net/ipv4/ip_output.c:433
>  dst_output include/net/dst.h:459 [inline]
>  ip_local_out+0x33e/0x4a0 net/ipv4/ip_output.c:129
>  iptunnel_xmit+0x5d5/0xa00 net/ipv4/ip_tunnel_core.c:82
>  geneve_xmit_skb drivers/net/geneve.c:921 [inline]
>  geneve_xmit+0x2bc5/0x5610 drivers/net/geneve.c:1046
>  __netdev_start_xmit include/linux/netdevice.h:5203 [inline]
>  netdev_start_xmit include/linux/netdevice.h:5212 [inline]
>  xmit_one net/core/dev.c:3776 [inline]
>  dev_hard_start_xmit+0x93/0x740 net/core/dev.c:3792
>  __dev_queue_xmit+0x7eb/0x43e0 net/core/dev.c:4629
>  dev_queue_xmit include/linux/netdevice.h:3350 [inline]
>  neigh_hh_output include/net/neighbour.h:523 [inline]
>  neigh_output include/net/neighbour.h:537 [inline]
>  ip6_finish_output2+0xe98/0x2020 net/ipv6/ip6_output.c:141
>  __ip6_finish_output net/ipv6/ip6_output.c:215 [inline]
>  ip6_finish_output+0x3f9/0x1360 net/ipv6/ip6_output.c:226
>  NF_HOOK_COND include/linux/netfilter.h:303 [inline]
>  ip6_output+0x1f9/0x540 net/ipv6/ip6_output.c:247
>  dst_output include/net/dst.h:459 [inline]
>  NF_HOOK include/linux/netfilter.h:314 [inline]
>  NF_HOOK include/linux/netfilter.h:308 [inline]
>  mld_sendpack+0x9e9/0x1220 net/ipv6/mcast.c:1868
>  mld_send_initial_cr.part.0+0x1a1/0x260 net/ipv6/mcast.c:2285
>  mld_send_initial_cr include/linux/refcount.h:291 [inline]
>  ipv6_mc_dad_complete+0x22c/0x2b0 net/ipv6/mcast.c:2293
>  addrconf_dad_completed+0xd8a/0x10d0 net/ipv6/addrconf.c:4341
>  addrconf_dad_work+0x84d/0x14e0 net/ipv6/addrconf.c:4269
>  process_one_work+0x9cc/0x1b70 kernel/workqueue.c:3238
>  process_scheduled_works kernel/workqueue.c:3319 [inline]
>  worker_thread+0x6c8/0xf10 kernel/workqueue.c:3400
>  kthread+0x3c2/0x780 kernel/kthread.c:464
>  ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:153
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
>
>
> ---
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>

I think this issue is triggered because of CONFIG_BPF_JIT_ALWAYS_ON is
not set and /proc/sys/net/core/bpf_jit_enable is set to 1, causing the
arch to attempt JIT the prog, but jit failed due to FAULT_INJECTION.

When `bpf_jit_enable` set to 1, sets `fp->jit_requested = 1` to
indicate need to jit when create BPF program. During runtime selection,
set 'fp->bpf_func' to `__bpf_prog_ret0_warn` by default, and should
return an error when prog need to jit(`jit_needed`) but not jited.

Since CONFIG_BPF_JIT_ALWAYS_ON is not set and the BPF program contains
no kfuncs, the `jit_needed` set to false. As a result, incorrectly
treats the program as valid, when the program runs it calls
`__bpf_prog_ret0_warn` and triggers the WARN_ON_ONCE(1).

--
Thanks,
kafai

#syz test

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index ba6b6118cf50..4c951d60bef5 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2474,8 +2474,7 @@ struct bpf_prog *bpf_prog_select_runtime(struct
bpf_prog *fp, int *err)
if (fp->bpf_func)
goto finalize;

- if (IS_ENABLED(CONFIG_BPF_JIT_ALWAYS_ON) ||
- bpf_prog_has_kfunc_call(fp))
+ if (fp->jit_requested || bpf_prog_has_kfunc_call(fp))
jit_needed = true;

bpf_prog_select_func(fp);