Message-ID: <fb41c7e1-c8de-40fc-9470-2e742e358ba9@suse.cz>
Date: Fri, 19 Dec 2025 17:11:00 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: syzbot <syzbot+b1546ad4a95331b2101e@...kaller.appspotmail.com>,
 ast@...nel.org, bpf@...r.kernel.org, davem@...emloft.net,
 edumazet@...gle.com, horms@...nel.org, houtao1@...wei.com,
 jkangas@...hat.com, kuba@...nel.org, linux-kernel@...r.kernel.org,
 martin.lau@...nel.org, netdev@...r.kernel.org, pabeni@...hat.com,
 sgarzare@...hat.com, syzkaller-bugs@...glegroups.com,
 virtualization@...ts.linux.dev, wangfushuai@...du.com,
 Linux-RT-Users <linux-rt-users@...r.kernel.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Peter Zijlstra <peterz@...radead.org>, Alexei Starovoitov <ast@...nel.org>,
 RCU <rcu@...r.kernel.org>, "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [syzbot] [net?] [virt?] BUG: sleeping function called from
 invalid context in __bpf_stream_push_str

On 12/8/25 12:21, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    559e608c4655 Merge tag 'ntfs3_for_6.19' of https://github...
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=164fdcc2580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=74c2ec4187efdce
> dashboard link: https://syzkaller.appspot.com/bug?extid=b1546ad4a95331b2101e
> compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1446301a580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=112c3f42580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/7d28798cb263/disk-559e608c.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/239e800627b8/vmlinux-559e608c.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/e89da2cc9887/bzImage-559e608c.xz

> The issue was bisected to:
> 
> commit 0db4941d9dae159d887e7e2eac7e54e60c3aac87
> Author: Fushuai Wang <wangfushuai@...du.com>
> Date:   Tue Oct 7 07:40:11 2025 +0000
> 
>     bpf: Use rcu_read_lock_dont_migrate in bpf_sk_storage.c

(came here because reviewing a proposed fix:
https://lore.kernel.org/all/20251219085755.139846-1-swarajgaikwad1925@gmail.com/
)

The bisection result seem weird to me, it changes some usages of
migrate_disable(); + rcu_read_lock(); to  rcu_read_lock_dont_migrate();
However with CONFIG_PREEMPT_RCU=y which the syzbot .config has, this should
be equivalent? (Also I'm not sure the affected paths are even in the backtrace?)

In either case with CONFIG_PREEMPT_RCU=y, rcu_read_lock() should not be
disabling preemption?

> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=10cd3c1a580000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=12cd3c1a580000
> console output: https://syzkaller.appspot.com/x/log.txt?x=14cd3c1a580000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+b1546ad4a95331b2101e@...kaller.appspotmail.com
> Fixes: 0db4941d9dae ("bpf: Use rcu_read_lock_dont_migrate in bpf_sk_storage.c")
> 
> BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:48
> in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 6128, name: syz.3.73
> preempt_count: 2, expected: 0
> RCU nest depth: 1, expected: 1
> 3 locks held by syz.3.73/6128:
>  #0: ffff8880493da398 (sk_lock-AF_VSOCK){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1700 [inline]
>  #0: ffff8880493da398 (sk_lock-AF_VSOCK){+.+.}-{0:0}, at: vsock_connect+0x152/0xd40 net/vmw_vsock/af_vsock.c:1546
>  #1: ffffffff8d5aeba0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
>  #1: ffffffff8d5aeba0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:867 [inline]
>  #1: ffffffff8d5aeba0 (rcu_read_lock){....}-{1:3}, at: __bpf_trace_run kernel/trace/bpf_trace.c:2074 [inline]
>  #1: ffffffff8d5aeba0 (rcu_read_lock){....}-{1:3}, at: bpf_trace_run9+0x1ec/0x510 kernel/trace/bpf_trace.c:2123
>  #2: ffff8880b893fd48 (&s->lock_key#14){+.+.}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:44 [inline]
>  #2: ffff8880b893fd48 (&s->lock_key#14){+.+.}-{3:3}, at: ___slab_alloc+0x12f/0x1400 mm/slub.c:4516
> Preemption disabled at:
> [<ffffffff82179f5a>] class_preempt_constructor include/linux/preempt.h:468 [inline]
> [<ffffffff82179f5a>] __migrate_enable include/linux/sched.h:2378 [inline]
> [<ffffffff82179f5a>] migrate_enable include/linux/sched.h:2429 [inline]
> [<ffffffff82179f5a>] __slab_alloc+0xea/0x1f0 mm/slub.c:4777

Wait, so it's slab code itself disabling preemption, in migrate_enable()?
But there's guard(preempt)(); so it should be enabled again.

Or is it the limitation of the reporting, that it doesn't know which
preemptions were re-enabled and which not?


> CPU: 1 UID: 0 PID: 6128 Comm: syz.3.73 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
>  __might_resched+0x44b/0x5d0 kernel/sched/core.c:8830
>  __rt_spin_lock kernel/locking/spinlock_rt.c:48 [inline]
>  rt_spin_lock+0xc7/0x3e0 kernel/locking/spinlock_rt.c:57
>  spin_lock include/linux/spinlock_rt.h:44 [inline]
>  ___slab_alloc+0x12f/0x1400 mm/slub.c:4516
>  __slab_alloc+0xc6/0x1f0 mm/slub.c:4774
>  __slab_alloc_node mm/slub.c:4850 [inline]
>  kmalloc_nolock_noprof+0x1be/0x440 mm/slub.c:5729
>  bpf_stream_elem_alloc kernel/bpf/stream.c:33 [inline]
>  __bpf_stream_push_str+0xa8/0x2b0 kernel/bpf/stream.c:50
>  bpf_stream_stage_printk+0x14e/0x1c0 kernel/bpf/stream.c:306
>  bpf_prog_report_may_goto_violation+0xc4/0x190 kernel/bpf/core.c:3203
>  bpf_check_timed_may_goto+0xaa/0xb0 kernel/bpf/core.c:3221
>  arch_bpf_timed_may_goto+0x21/0x40 arch/x86/net/bpf_timed_may_goto.S:40
>  bpf_prog_262a74d054ad2993+0x53/0x5f
>  bpf_dispatcher_nop_func include/linux/bpf.h:1376 [inline]
>  __bpf_prog_run include/linux/filter.h:723 [inline]
>  bpf_prog_run include/linux/filter.h:730 [inline]
>  __bpf_trace_run kernel/trace/bpf_trace.c:2075 [inline]
>  bpf_trace_run9+0x2de/0x510 kernel/trace/bpf_trace.c:2123
>  __bpf_trace_virtio_transport_alloc_pkt+0x2d7/0x340 include/trace/events/vsock_virtio_transport_common.h:39
>  __do_trace_virtio_transport_alloc_pkt include/trace/events/vsock_virtio_transport_common.h:39 [inline]
>  trace_virtio_transport_alloc_pkt include/trace/events/vsock_virtio_transport_common.h:39 [inline]
>  virtio_transport_alloc_skb+0x10af/0x1110 net/vmw_vsock/virtio_transport_common.c:311
>  virtio_transport_send_pkt_info+0x694/0x10b0 net/vmw_vsock/virtio_transport_common.c:390
>  virtio_transport_connect+0xa7/0x100 net/vmw_vsock/virtio_transport_common.c:1072
>  vsock_connect+0xaca/0xd40 net/vmw_vsock/af_vsock.c:1611
>  __sys_connect_file net/socket.c:2080 [inline]
>  __sys_connect+0x323/0x450 net/socket.c:2099
>  __do_sys_connect net/socket.c:2105 [inline]
>  __se_sys_connect net/socket.c:2102 [inline]
>  __x64_sys_connect+0x7a/0x90 net/socket.c:2102
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f0c4d91f749
> Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffd8ed26ac8 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
> RAX: ffffffffffffffda RBX: 00007f0c4db75fa0 RCX: 00007f0c4d91f749
> RDX: 0000000000000010 RSI: 0000200000000080 RDI: 0000000000000003
> RBP: 00007f0c4d9a3f91 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f0c4db75fa0 R14: 00007f0c4db75fa0 R15: 0000000000000003
>  </TASK>
> 
> 
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
> syzbot engineers can be reached at syzkaller@...glegroups.com.
> 
> syzbot will keep track of this issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> 
> If the report is already addressed, let syzbot know by replying with:
> #syz fix: exact-commit-title
> 
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.
> 
> If you want to overwrite report's subsystems, reply with:
> #syz set subsystems: new-subsystem
> (See the list of subsystem names on the web dashboard)
> 
> If the report is a duplicate of another one, reply with:
> #syz dup: exact-subject-of-another-report
> 
> If you want to undo deduplication, reply with:
> #syz undup
> 

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