[syzbot] [kernfs?] WARNING: suspicious RCU usage in kernfs_node_dentry

syzbot
2025-02-18T09:27:19Z

Hello,

syzbot found the following issue on:

HEAD commit: 253c82b3a2ce Add linux-next specific files for 20250217
git tree: linux-next
console+strace: https://syzkaller.appspot.com/x/log.txt?x=15141898580000
kernel config: https://syzkaller.appspot.com/x/.config?x=f984ff1d92cb4017
dashboard link: https://syzkaller.appspot.com/bug?extid=ecccecbc636b455f9084
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=175b7da4580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=17141898580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/af8b0f79abb4/disk-253c82b3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/463f7d416e09/vmlinux-253c82b3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/07101e97dd04/bzImage-253c82b3.xz

The issue was bisected to:

commit 5b2fabf7fe8f745ff214ff003e6067b64f172271
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Thu Feb 13 14:50:20 2025 +0000

kernfs: Acquire kernfs_rwsem in kernfs_node_dentry().

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=15a9fda4580000
final oops: https://syzkaller.appspot.com/x/report.txt?x=17a9fda4580000
console output: https://syzkaller.appspot.com/x/log.txt?x=13a9fda4580000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ecccecbc636b455f9084@syzkaller.appspotmail.com
Fixes: 5b2fabf7fe8f ("kernfs: Acquire kernfs_rwsem in kernfs_node_dentry().")

=============================
WARNING: suspicious RCU usage
6.14.0-rc2-next-20250217-syzkaller #0 Not tainted
-----------------------------
fs/kernfs/mount.c:243 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
3 locks held by syz-executor215/5837:
#0: ffff88802ab0b870 (&fc->uapi_mutex){+.+.}-{4:4}, at: __do_sys_fsconfig fs/fsopen.c:465 [inline]
#0: ffff88802ab0b870 (&fc->uapi_mutex){+.+.}-{4:4}, at: __se_sys_fsconfig+0x9b2/0xf60 fs/fsopen.c:344
#1: ffff8880355560e0 (&type->s_umount_key#46){+.+.}-{4:4}, at: __super_lock fs/super.c:56 [inline]
#1: ffff8880355560e0 (&type->s_umount_key#46){+.+.}-{4:4}, at: super_lock+0x196/0x400 fs/super.c:120
#2: ffff88801bef7148 (&root->kernfs_rwsem){++++}-{4:4}, at: class_rwsem_read_constructor include/linux/rwsem.h:241 [inline]
#2: ffff88801bef7148 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_node_dentry+0xc3/0x2d0 fs/kernfs/mount.c:223

stack backtrace:
CPU: 0 UID: 0 PID: 5837 Comm: syz-executor215 Not tainted 6.14.0-rc2-next-20250217-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 12/27/2024
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
lockdep_rcu_suspicious+0x226/0x340 kernel/locking/lockdep.c:6847
kernfs_node_dentry+0x24b/0x2d0 fs/kernfs/mount.c:243
cgroup_do_get_tree+0x248/0x390 kernel/cgroup/cgroup.c:2233
cgroup_get_tree+0xbb/0x230 kernel/cgroup/cgroup.c:2272
vfs_get_tree+0x90/0x2b0 fs/super.c:1759
vfs_cmd_create+0xa0/0x1f0 fs/fsopen.c:225
__do_sys_fsconfig fs/fsopen.c:467 [inline]
__se_sys_fsconfig+0xa33/0xf60 fs/fsopen.c:344
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f09e41a2de9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 61 18 00 00 90 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 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe4f61eba8 EFLAGS: 00000246 ORIG_RAX: 00000000000001af
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f09e41a2de9
RDX: 0000000000000000 RSI: 0000000000000006 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f09e41ec036
R13: 00007ffe4f61ebe0 R14: 00007ffe4f61ec20 R15: 0000000000000000
</TASK>

============================================
WARNING: possible recursive locking detected
6.14.0-rc2-next-20250217-syzkaller #0 Not tainted
--------------------------------------------
syz-executor215/5837 is trying to acquire lock:
ffff88801bef7148 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_dop_revalidate+0xa2/0x5d0 fs/kernfs/dir.c:1178

but task is already holding lock:
ffff88801bef7148 (&root->kernfs_rwsem){++++}-{4:4}, at: class_rwsem_read_constructor include/linux/rwsem.h:241 [inline]
ffff88801bef7148 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_node_dentry+0xc3/0x2d0 fs/kernfs/mount.c:223

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(&root->kernfs_rwsem);
lock(&root->kernfs_rwsem);

*** DEADLOCK ***

May be due to missing lock nesting notation

3 locks held by syz-executor215/5837:
#0: ffff88802ab0b870 (&fc->uapi_mutex){+.+.}-{4:4}, at: __do_sys_fsconfig fs/fsopen.c:465 [inline]
#0: ffff88802ab0b870 (&fc->uapi_mutex){+.+.}-{4:4}, at: __se_sys_fsconfig+0x9b2/0xf60 fs/fsopen.c:344
#1: ffff8880355560e0 (&type->s_umount_key#46){+.+.}-{4:4}, at: __super_lock fs/super.c:56 [inline]
#1: ffff8880355560e0 (&type->s_umount_key#46){+.+.}-{4:4}, at: super_lock+0x196/0x400 fs/super.c:120
#2: ffff88801bef7148 (&root->kernfs_rwsem){++++}-{4:4}, at: class_rwsem_read_constructor include/linux/rwsem.h:241 [inline]
#2: ffff88801bef7148 (&root->kernfs_rwsem){++++}-{4:4}, at: kernfs_node_dentry+0xc3/0x2d0 fs/kernfs/mount.c:223

stack backtrace:
CPU: 1 UID: 0 PID: 5837 Comm: syz-executor215 Not tainted 6.14.0-rc2-next-20250217-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 12/27/2024
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:94 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120
print_deadlock_bug+0x483/0x620 kernel/locking/lockdep.c:3039
check_deadlock kernel/locking/lockdep.c:3091 [inline]
validate_chain+0x15e2/0x5920 kernel/locking/lockdep.c:3893
__lock_acquire+0x1397/0x2100 kernel/locking/lockdep.c:5228
lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5851
down_read+0xb1/0xa40 kernel/locking/rwsem.c:1524
kernfs_dop_revalidate+0xa2/0x5d0 fs/kernfs/dir.c:1178
d_revalidate fs/namei.c:923 [inline]
lookup_dcache fs/namei.c:1651 [inline]
lookup_one_unlocked+0x23b/0x2d0 fs/namei.c:2972
lookup_one_positive_unlocked fs/namei.c:3003 [inline]
lookup_positive_unlocked+0x2b/0xb0 fs/namei.c:3043
kernfs_node_dentry+0x139/0x2d0 fs/kernfs/mount.c:244
cgroup_do_get_tree+0x248/0x390 kernel/cgroup/cgroup.c:2233
cgroup_get_tree+0xbb/0x230 kernel/cgroup/cgroup.c:2272
vfs_get_tree+0x90/0x2b0 fs/super.c:1759
vfs_cmd_create+0xa0/0x1f0 fs/fsopen.c:225
__do_sys_fsconfig fs/fsopen.c:467 [inline]
__se_sys_fsconfig+0xa33/0xf60 fs/fsopen.c:344
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f09e41a2de9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 61 18 00 00 90 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 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe4f61eba8 EFLAGS: 00000246 ORIG_RAX: 00000000000001af
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f09e41a2de9
RDX: 0000000000000000 RSI: 0000000000000006 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f09e41ec036
R13: 00007ffe4f61ebe0 R14: 00007ffe4f61ec20 R15: 0000000000000000
</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@googlegroups.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