Message-ID: <4ca54a2c-46c8-435b-af72-7f871236eeb2@lucifer.local>
Date: Tue, 22 Jul 2025 16:41:08 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: syzbot <syzbot+1c6607097b258f18065c@...kaller.appspotmail.com>
Cc: Liam.Howlett@...cle.com, akpm@...ux-foundation.org, brauner@...nel.org,
        jannh@...gle.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mjguzik@...il.com, oleg@...hat.com, pasha.tatashin@...een.com,
        pfalcato@...e.de, rostedt@...dmis.org, syzkaller-bugs@...glegroups.com,
        vbabka@...e.cz
Subject: Re: [syzbot] [kernel?] INFO: task hung in exit_mm (5)

This is testing against an old version of the code.

This version had a major issue with incorrect usage of the VMA iterator which is
now resolved, so it's highly likely this is resolved.

I can't reproduce this locally even on the specific linux-next version, or with
a RCU barrier inserted to make it more likely to hit... So let's see if syzbot
can manage it:

#syz test:  git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-unstable

Cheers, Lorenzo

On Tue, Jul 22, 2025 at 08:17:30AM -0700, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    d086c886ceb9 Add linux-next specific files for 20250718
> git tree:       linux-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=15c4c4f0580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=cc0cd9fdf69889c3
> dashboard link: https://syzkaller.appspot.com/bug?extid=1c6607097b258f18065c
> compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14f1e38c580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13c4c4f0580000
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/4435f80a19c4/disk-d086c886.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/9a7dc57a5ea3/vmlinux-d086c886.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/da9f2dc22ae1/bzImage-d086c886.xz
>
> The issue was bisected to:
>
> commit d22f1d61e7d950dccee81e3ed75ff2908aecd1a9
> Author: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Date:   Wed Jul 16 19:29:54 2025 +0000
>
>     mm/mremap: reset VMI on unmap
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=10e42f22580000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=12e42f22580000
> console output: https://syzkaller.appspot.com/x/log.txt?x=14e42f22580000
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+1c6607097b258f18065c@...kaller.appspotmail.com
> Fixes: d22f1d61e7d9 ("mm/mremap: reset VMI on unmap")
>
> INFO: task syz-executor942:5848 blocked for more than 143 seconds.
>       Not tainted 6.16.0-rc6-next-20250718-syzkaller #0
>       Blocked by coredump.
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> task:syz-executor942 state:D stack:25848 pid:5848  tgid:5848  ppid:5846   task_flags:0x40004c flags:0x00004002
> Call Trace:
>  <TASK>
>  context_switch kernel/sched/core.c:5351 [inline]
>  __schedule+0x1737/0x4d30 kernel/sched/core.c:6954
>  __schedule_loop kernel/sched/core.c:7036 [inline]
>  schedule+0x165/0x360 kernel/sched/core.c:7051
>  schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7108
>  rwsem_down_read_slowpath+0x5fd/0x8f0 kernel/locking/rwsem.c:1088
>  __down_read_common kernel/locking/rwsem.c:1263 [inline]
>  __down_read kernel/locking/rwsem.c:1276 [inline]
>  down_read+0x98/0x2e0 kernel/locking/rwsem.c:1541
>  mmap_read_lock include/linux/mmap_lock.h:412 [inline]
>  exit_mm+0xcc/0x2c0 kernel/exit.c:557
>  do_exit+0x648/0x2300 kernel/exit.c:947
>  do_group_exit+0x21c/0x2d0 kernel/exit.c:1100
>  __do_sys_exit_group kernel/exit.c:1111 [inline]
>  __se_sys_exit_group kernel/exit.c:1109 [inline]
>  __x64_sys_exit_group+0x3f/0x40 kernel/exit.c:1109
>  x64_sys_call+0x21f7/0x2200 arch/x86/include/generated/asm/syscalls_64.h:232
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f70d2bb1349
> RSP: 002b:00007ffd9c81a718 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f70d2bb1349
> RDX: 000000000000003c RSI: 00000000000000e7 RDI: 0000000000000000
> RBP: 00007f70d2c3d2b0 R08: ffffffffffffffb0 R09: 7fffffffffffffff
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007f70d2c3d2b0
> R13: 0000000000000000 R14: 00007f70d2c3de40 R15: 00007f70d2b7f160
>  </TASK>
>
> Showing all locks held in the system:
> 1 lock held by khungtaskd/31:
>  #0: ffffffff8e13d8a0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
>  #0: ffffffff8e13d8a0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:841 [inline]
>  #0: ffffffff8e13d8a0 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x2e/0x180 kernel/locking/lockdep.c:6770
> 2 locks held by getty/5605:
>  #0: ffff888033a1b0a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x25/0x70 drivers/tty/tty_ldisc.c:243
>  #1: ffffc9000332b2f0 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x43e/0x1400 drivers/tty/n_tty.c:2222
> 1 lock held by syz-executor942/5848:
>  #0: ffff88807fbd57e0 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock include/linux/mmap_lock.h:412 [inline]
>  #0: ffff88807fbd57e0 (&mm->mmap_lock){++++}-{4:4}, at: exit_mm+0xcc/0x2c0 kernel/exit.c:557
> 1 lock held by syz-executor942/5849:
>
> =============================================
>
> NMI backtrace for cpu 0
> CPU: 0 UID: 0 PID: 31 Comm: khungtaskd Not tainted 6.16.0-rc6-next-20250718-syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
>  nmi_cpu_backtrace+0x39e/0x3d0 lib/nmi_backtrace.c:113
>  nmi_trigger_cpumask_backtrace+0x17a/0x300 lib/nmi_backtrace.c:62
>  trigger_all_cpu_backtrace include/linux/nmi.h:160 [inline]
>  check_hung_uninterruptible_tasks kernel/hung_task.c:328 [inline]
>  watchdog+0xf93/0xfe0 kernel/hung_task.c:491
>  kthread+0x70e/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>
> Sending NMI from CPU 0 to CPUs 1:
> NMI backtrace for cpu 1
> CPU: 1 UID: 0 PID: 5849 Comm: syz-executor942 Not tainted 6.16.0-rc6-next-20250718-syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
> RIP: 0010:mas_next_slot+0x7b4/0xcf0 lib/maple_tree.c:4718
> Code: 08 4c 89 f7 e8 7d 0c b3 f6 49 89 1e 48 8b 44 24 48 42 0f b6 04 20 84 c0 0f 85 f8 03 00 00 48 8b 44 24 10 44 0f b6 38 41 fe c7 <44> 88 38 48 8b 44 24 68 42 0f b6 04 20 84 c0 0f 85 f6 03 00 00 48
> RSP: 0018:ffffc9000413f320 EFLAGS: 00000206
> RAX: ffffc9000413f4fd RBX: 00001fff829eb000 RCX: ffff88807d0fbc00
> RDX: 0000000000000000 RSI: ffffffffffffffff RDI: 00001fff829eafff
> RBP: 0000000000000001 R08: ffff88807d0fbc00 R09: 0000000000000004
> R10: 0000000000000003 R11: 0000000000000000 R12: dffffc0000000000
> R13: ffff888076c6d200 R14: ffffc9000413f4c8 R15: 0000000000000005
> FS:  00007f70d2b6c6c0(0000) GS:ffff888125ce3000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00005634ac61d168 CR3: 0000000076c8c000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  mas_find+0xb0e/0xd30 lib/maple_tree.c:6062
>  vma_next include/linux/mm.h:864 [inline]
>  validate_mm+0xe1/0x4b0 mm/vma.c:652
>  vma_link+0x366/0x450 mm/vma.c:1799
>  copy_vma+0x70c/0x940 mm/vma.c:1880
>  copy_vma_and_data mm/mremap.c:1184 [inline]
>  move_vma+0x81f/0x1840 mm/mremap.c:1282
>  mremap_to+0x6d6/0x7a0 mm/mremap.c:1406
>  remap_move mm/mremap.c:1863 [inline]
>  do_mremap mm/mremap.c:1906 [inline]
>  __do_sys_mremap mm/mremap.c:1970 [inline]
>  __se_sys_mremap+0xa0b/0xef0 mm/mremap.c:1938
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f70d2bb32e9
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 51 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 b0 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f70d2b6c228 EFLAGS: 00000246 ORIG_RAX: 0000000000000019
> RAX: ffffffffffffffda RBX: 00007f70d2c3d308 RCX: 00007f70d2bb32e9
> RDX: 0000000000001000 RSI: 0000000000001000 RDI: 0000200000000000
> RBP: 00007f70d2c3d300 R08: 0000200000481000 R09: 00007f70d2b6c6c0
> R10: 0000000000000007 R11: 0000000000000246 R12: 00007f70d2c3d30c
> R13: 0000200000000000 R14: 0000200000a96000 R15: 00007ffd9c81a698
>  </TASK>
> INFO: NMI handler (nmi_cpu_backtrace_handler) took too long to run: 1.092 msecs
>
>
> ---
> 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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