Re: [syzbot] [mm?] KASAN: slab-use-after-free Read in mas_next_slot (2)

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

OK on second thoughts, there is one additional thing we need to do on each
loop to avoid observing the same VMA, either the prior logic of checking
directly or a vma_next().

So this may be a consequence of that.

I will respin the series to make life easier...

On Thu, Jul 17, 2025 at 05:18:17AM +0100, Lorenzo Stoakes wrote:
> This looks to be unrelated to my patch and some issue with syzbot (it's doing
> weird injection stuff).
>
> As I said, I have tested the change with reproducer locally and it fixes the
> issue, and I have been able to reliably observe that (note, without any of the
> below stuff happening).
>
> Thanks
>
> On Wed, Jul 16, 2025 at 08:55:03PM -0700, syzbot wrote:
> > Hello,
> >
> > syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> > INFO: task hung in exit_mm
> >
> > INFO: task syz.0.16:6665 blocked for more than 143 seconds.
> >       Not tainted 6.16.0-rc6-next-20250716-syzkaller-ge8352908bdcd-dirty #0
> >       Blocked by coredump.
> > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > task:syz.0.16        state:D stack:26920 pid:6665  tgid:6665  ppid:6577   task_flags:0x40044c flags:0x00004004
> > Call Trace:
> >  <TASK>
> >  context_switch kernel/sched/core.c:5314 [inline]
> >  __schedule+0x16fd/0x4cf0 kernel/sched/core.c:6697
> >  __schedule_loop kernel/sched/core.c:6775 [inline]
> >  schedule+0x165/0x360 kernel/sched/core.c:6790
> >  schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:6847
> >  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:423 [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
> >  get_signal+0x1286/0x1340 kernel/signal.c:3034
> >  arch_do_signal_or_restart+0x9a/0x750 arch/x86/kernel/signal.c:337
> >  exit_to_user_mode_loop+0x75/0x110 kernel/entry/common.c:40
> >  exit_to_user_mode_prepare include/linux/irq-entry-common.h:208 [inline]
> >  syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
> >  syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
> >  do_syscall_64+0x2bd/0x3b0 arch/x86/entry/syscall_64.c:100
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > RIP: 0033:0x7f524bb8e963
> > RSP: 002b:00007ffc99164708 EFLAGS: 00000246 ORIG_RAX: 0000000000000009
> > RAX: fffffffffffffffc RBX: 00007f524b5ff6c0 RCX: 00007f524bb8e963
> > RDX: 0000000000000000 RSI: 0000000000021000 RDI: 0000000000000000
> > RBP: 0000000000000000 R08: 00000000ffffffff R09: 0000000000000000
> > R10: 0000000000020022 R11: 0000000000000246 R12: 00007ffc99164860
> > R13: ffffffffffffffc0 R14: 0000000000001000 R15: 0000000000000000
> >  </TASK>
> > INFO: task syz.1.17:6807 blocked for more than 144 seconds.
> >       Not tainted 6.16.0-rc6-next-20250716-syzkaller-ge8352908bdcd-dirty #0
> >       Blocked by coredump.
> > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > task:syz.1.17        state:D stack:26920 pid:6807  tgid:6807  ppid:6787   task_flags:0x40044c flags:0x00004004
> > Call Trace:
> >  <TASK>
> >  context_switch kernel/sched/core.c:5314 [inline]
> >  __schedule+0x16fd/0x4cf0 kernel/sched/core.c:6697
> >  __schedule_loop kernel/sched/core.c:6775 [inline]
> >  schedule+0x165/0x360 kernel/sched/core.c:6790
> >  schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:6847
> >  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:423 [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
> >  get_signal+0x1286/0x1340 kernel/signal.c:3034
> >  arch_do_signal_or_restart+0x9a/0x750 arch/x86/kernel/signal.c:337
> >  exit_to_user_mode_loop+0x75/0x110 kernel/entry/common.c:40
> >  exit_to_user_mode_prepare include/linux/irq-entry-common.h:208 [inline]
> >  syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
> >  syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
> >  do_syscall_64+0x2bd/0x3b0 arch/x86/entry/syscall_64.c:100
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > RIP: 0033:0x7efc6b58e963
> > RSP: 002b:00007ffe5b639e88 EFLAGS: 00000246 ORIG_RAX: 0000000000000009
> > RAX: fffffffffffffffc RBX: 00007efc6afff6c0 RCX: 00007efc6b58e963
> > RDX: 0000000000000000 RSI: 0000000000021000 RDI: 0000000000000000
> > RBP: 0000000000000000 R08: 00000000ffffffff R09: 0000000000000000
> > R10: 0000000000020022 R11: 0000000000000246 R12: 00007ffe5b639fe0
> > R13: ffffffffffffffc0 R14: 0000000000001000 R15: 0000000000000000
> >  </TASK>
> >
> > Showing all locks held in the system:
> > 1 lock held by khungtaskd/31:
> >  #0: ffffffff8e13e2e0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
> >  #0: ffffffff8e13e2e0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:841 [inline]
> >  #0: ffffffff8e13e2e0 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x2e/0x180 kernel/locking/lockdep.c:6770
> > 3 locks held by kworker/0:3/981:
> > 3 locks held by kworker/u8:9/3028:
> >  #0: ffff8880b8739f98 (&rq->__lock){-.-.}-{2:2}, at: raw_spin_rq_lock_nested+0x2a/0x140 kernel/sched/core.c:606
> >  #1: ffff8880b8724008 (per_cpu_ptr(&psi_seq, cpu)){-.-.}-{0:0}, at: psi_task_switch+0x53/0x880 kernel/sched/psi.c:937
> >  #2: ffff8880b8725918 (&base->lock){-.-.}-{2:2}, at: lock_timer_base kernel/time/timer.c:1004 [inline]
> >  #2: ffff8880b8725918 (&base->lock){-.-.}-{2:2}, at: __mod_timer+0x1ae/0xf30 kernel/time/timer.c:1085
> > 2 locks held by getty/5607:
> >  #0: ffff88814df960a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x25/0x70 drivers/tty/tty_ldisc.c:243
> >  #1: ffffc9000332e2f0 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x43e/0x1400 drivers/tty/n_tty.c:2222
> > 1 lock held by syz.0.16/6665:
> >  #0: ffff8880242d4260 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock include/linux/mmap_lock.h:423 [inline]
> >  #0: ffff8880242d4260 (&mm->mmap_lock){++++}-{4:4}, at: exit_mm+0xcc/0x2c0 kernel/exit.c:557
> > 1 lock held by syz.0.16/6666:
> > 1 lock held by syz.1.17/6807:
> >  #0: ffff88807b8c57e0 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock include/linux/mmap_lock.h:423 [inline]
> >  #0: ffff88807b8c57e0 (&mm->mmap_lock){++++}-{4:4}, at: exit_mm+0xcc/0x2c0 kernel/exit.c:557
> > 1 lock held by syz.1.17/6808:
> > 1 lock held by syz.2.18/6831:
> >  #0: ffff88807e36c260 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock include/linux/mmap_lock.h:423 [inline]
> >  #0: ffff88807e36c260 (&mm->mmap_lock){++++}-{4:4}, at: exit_mm+0xcc/0x2c0 kernel/exit.c:557
> > 1 lock held by syz.2.18/6832:
> > 1 lock held by syz.3.19/6858:
> >  #0: ffff88807b8c2ce0 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock include/linux/mmap_lock.h:423 [inline]
> >  #0: ffff88807b8c2ce0 (&mm->mmap_lock){++++}-{4:4}, at: exit_mm+0xcc/0x2c0 kernel/exit.c:557
> > 3 locks held by syz.3.19/6859:
> > 1 lock held by syz.4.20/6888:
> >  #0: ffff88801a476d60 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock include/linux/mmap_lock.h:423 [inline]
> >  #0: ffff88801a476d60 (&mm->mmap_lock){++++}-{4:4}, at: exit_mm+0xcc/0x2c0 kernel/exit.c:557
> > 1 lock held by syz.4.20/6889:
> > 1 lock held by syz.5.21/6925:
> >  #0: ffff88801a472220 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock include/linux/mmap_lock.h:423 [inline]
> >  #0: ffff88801a472220 (&mm->mmap_lock){++++}-{4:4}, at: exit_mm+0xcc/0x2c0 kernel/exit.c:557
> > 1 lock held by syz.5.21/6926:
> > 1 lock held by syz.6.22/6955:
> >  #0: ffff88807f93b7a0 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock include/linux/mmap_lock.h:423 [inline]
> >  #0: ffff88807f93b7a0 (&mm->mmap_lock){++++}-{4:4}, at: exit_mm+0xcc/0x2c0 kernel/exit.c:557
> > 1 lock held by syz.6.22/6956:
> > 1 lock held by syz.7.24/6990:
> >  #0: ffff88807c9ec260 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock include/linux/mmap_lock.h:423 [inline]
> >  #0: ffff88807c9ec260 (&mm->mmap_lock){++++}-{4:4}, at: exit_mm+0xcc/0x2c0 kernel/exit.c:557
> > 1 lock held by syz.7.24/6992:
> > 2 locks held by dhcpcd/6995:
> >  #0: ffff88805e42b808 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: inode_lock include/linux/fs.h:869 [inline]
> >  #0: ffff88805e42b808 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: __sock_release net/socket.c:648 [inline]
> >  #0: ffff88805e42b808 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: sock_close+0x9b/0x240 net/socket.c:1439
> >  #1: ffffffff8e143e38 (rcu_state.exp_mutex){+.+.}-{4:4}, at: exp_funnel_lock kernel/rcu/tree_exp.h:311 [inline]
> >  #1: ffffffff8e143e38 (rcu_state.exp_mutex){+.+.}-{4:4}, at: synchronize_rcu_expedited+0x2f6/0x730 kernel/rcu/tree_exp.h:967
> > 1 lock held by dhcpcd/6996:
> >  #0: ffff88805e42ca08 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: inode_lock include/linux/fs.h:869 [inline]
> >  #0: ffff88805e42ca08 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: __sock_release net/socket.c:648 [inline]
> >  #0: ffff88805e42ca08 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: sock_close+0x9b/0x240 net/socket.c:1439
> > 1 lock held by dhcpcd/6997:
> >  #0: ffff888078933208 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: inode_lock include/linux/fs.h:869 [inline]
> >  #0: ffff888078933208 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: __sock_release net/socket.c:648 [inline]
> >  #0: ffff888078933208 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: sock_close+0x9b/0x240 net/socket.c:1439
> > 2 locks held by dhcpcd/6998:
> >  #0: ffff888078930208 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: inode_lock include/linux/fs.h:869 [inline]
> >  #0: ffff888078930208 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: __sock_release net/socket.c:648 [inline]
> >  #0: ffff888078930208 (&sb->s_type->i_mutex_key#11){+.+.}-{4:4}, at: sock_close+0x9b/0x240 net/socket.c:1439
> >  #1: ffffffff8e143e38 (rcu_state.exp_mutex){+.+.}-{4:4}, at: exp_funnel_lock kernel/rcu/tree_exp.h:343 [inline]
> >  #1: ffffffff8e143e38 (rcu_state.exp_mutex){+.+.}-{4:4}, at: synchronize_rcu_expedited+0x3b9/0x730 kernel/rcu/tree_exp.h:967
> >
> > =============================================
> >
> > NMI backtrace for cpu 0
> > CPU: 0 UID: 0 PID: 31 Comm: khungtaskd Not tainted 6.16.0-rc6-next-20250716-syzkaller-ge8352908bdcd-dirty #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+0xfee/0x1030 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: 12 Comm: kworker/u8:0 Not tainted 6.16.0-rc6-next-20250716-syzkaller-ge8352908bdcd-dirty #0 PREEMPT(full)
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
> > Workqueue: events_unbound nsim_dev_trap_report_work
> > RIP: 0010:__this_cpu_preempt_check+0xe/0x20 lib/smp_processor_id.c:64
> > Code: 66 66 66 2e 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 48 89 fe 48 c7 c7 00 65 e3 8b <e9> bd fe ff ff cc cc cc cc cc cc cc cc cc cc cc cc cc 90 90 90 90
> > RSP: 0018:ffffc90000a08bc8 EFLAGS: 00000002
> > RAX: 0000000000000001 RBX: ffffffff822479bd RCX: da4b2af8b834fd00
> > RDX: ffff888029254d90 RSI: ffffffff8d994444 RDI: ffffffff8be36500
> > RBP: ffffc90000a08ed0 R08: 00000000c506ef33 R09: 00000000624b5ae2
> > R10: 000000000000000e R11: ffffffff81ac3010 R12: 0000000000000000
> > R13: ffffffff81a7e844 R14: ffff88801cecda00 R15: 0000000000000286
> > FS:  0000000000000000(0000) GS:ffff888125ce2000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 000055a7ec2b0660 CR3: 000000005fa88000 CR4: 00000000003526f0
> > Call Trace:
> >  <IRQ>
> >  lockdep_hardirqs_off+0x74/0x110 kernel/locking/lockdep.c:4514
> >  trace_hardirqs_off+0x12/0x40 kernel/trace/trace_preemptirq.c:104
> >  kasan_quarantine_put+0x3d/0x220 mm/kasan/quarantine.c:207
> >  kasan_slab_free include/linux/kasan.h:233 [inline]
> >  slab_free_hook mm/slub.c:2417 [inline]
> >  slab_free_after_rcu_debug+0x129/0x2a0 mm/slub.c:4730
> >  rcu_do_batch kernel/rcu/tree.c:2584 [inline]
> >  rcu_core+0xca5/0x1710 kernel/rcu/tree.c:2840
> >  handle_softirqs+0x286/0x870 kernel/softirq.c:579
> >  do_softirq+0xec/0x180 kernel/softirq.c:480
> >  </IRQ>
> >  <TASK>
> >  __local_bh_enable_ip+0x17d/0x1c0 kernel/softirq.c:407
> >  spin_unlock_bh include/linux/spinlock.h:396 [inline]
> >  nsim_dev_trap_report drivers/net/netdevsim/dev.c:833 [inline]
> >  nsim_dev_trap_report_work+0x7c7/0xb80 drivers/net/netdevsim/dev.c:864
> >  process_one_work kernel/workqueue.c:3239 [inline]
> >  process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3322
> >  worker_thread+0x8a0/0xda0 kernel/workqueue.c:3403
> >  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>
> >
> >
> > Tested on:
> >
> > commit:         e8352908 Add linux-next specific files for 20250716
> > git tree:       linux-next
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1523c58c580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=2594af20939db736
> > dashboard link: https://syzkaller.appspot.com/bug?extid=ebfd0e44b5c11034e1eb
> > compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> > patch:          https://syzkaller.appspot.com/x/patch.diff?x=10776382580000
> >