Re: [syzbot ci] Re: io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER

[Caleb Sander Mateos <csander@purestorage.com>]

On Mon, Dec 15, 2025 at 9:21 PM syzbot ci
<syzbot+ci3ff889516a0b26a2@syzkaller.appspotmail.com> wrote:
>
> syzbot ci has tested the following series
>
> [v5] io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER
> https://lore.kernel.org/all/20251215200909.3505001-1-csander@purestorage.com
> * [PATCH v5 1/6] io_uring: use release-acquire ordering for IORING_SETUP_R_DISABLED
> * [PATCH v5 2/6] io_uring: clear IORING_SETUP_SINGLE_ISSUER for IORING_SETUP_SQPOLL
> * [PATCH v5 3/6] io_uring: ensure io_uring_create() initializes submitter_task
> * [PATCH v5 4/6] io_uring: use io_ring_submit_lock() in io_iopoll_req_issued()
> * [PATCH v5 5/6] io_uring: factor out uring_lock helpers
> * [PATCH v5 6/6] io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER
>
> and found the following issue:
> KASAN: slab-use-after-free Read in task_work_add
>
> Full report is available here:
> https://ci.syzbot.org/series/bce89909-ebf2-45f6-be49-bbd46e33e966
>
> ***
>
> KASAN: slab-use-after-free Read in task_work_add
>
> tree:      torvalds
> URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
> base:      d358e5254674b70f34c847715ca509e46eb81e6f
> arch:      amd64
> compiler:  Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> config:    https://ci.syzbot.org/builds/db5ac991-f49c-460f-80e4-2a33be76fe7c/config
> syz repro: https://ci.syzbot.org/findings/ddbf1feb-6618-4c0f-9a16-15b856f20d71/syz_repro
>
> ==================================================================
> BUG: KASAN: slab-use-after-free in task_work_add+0xd7/0x440 kernel/task_work.c:73
> Read of size 8 at addr ffff88816a8826f8 by task kworker/u9:2/54
>
> CPU: 0 UID: 0 PID: 54 Comm: kworker/u9:2 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> Workqueue: iou_exit io_ring_exit_work
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
>  print_address_description mm/kasan/report.c:378 [inline]
>  print_report+0xca/0x240 mm/kasan/report.c:482
>  kasan_report+0x118/0x150 mm/kasan/report.c:595
>  task_work_add+0xd7/0x440 kernel/task_work.c:73
>  io_ring_ctx_lock_nested io_uring/io_uring.h:271 [inline]
>  io_ring_ctx_lock io_uring/io_uring.h:282 [inline]
>  io_req_caches_free+0x342/0x3e0 io_uring/io_uring.c:2869
>  io_ring_ctx_free+0x56a/0x8e0 io_uring/io_uring.c:2908

The call to io_req_caches_free() comes after the
put_task_struct(ctx->submitter_task) call in io_ring_ctx_free(), so I
guess the task_struct may have already been freed when
io_ring_ctx_lock() is called. Should be simple enough to fix by just
moving the put_task_struct() call to the end of io_ring_ctx_free().

Looking at this made me realize one other small bug, it's incorrect to
assume that if task_work_add() fails because the submitter_task has
exited, the uring lock has been acquired successfully. Even though
submitter_task will no longer be using the uring lock, other tasks
could. So this path needs to acquire the uring_lock mutex, similar to
the IORING_SETUP_SINGLE_ISSUER && IORING_SETUP_R_DISABLED case.

Thanks,
Caleb

>  io_ring_exit_work+0xff9/0x1220 io_uring/io_uring.c:3113
>  process_one_work kernel/workqueue.c:3257 [inline]
>  process_scheduled_works+0xad1/0x1770 kernel/workqueue.c:3340
>  worker_thread+0x8a0/0xda0 kernel/workqueue.c:3421
>  kthread+0x711/0x8a0 kernel/kthread.c:463
>  ret_from_fork+0x599/0xb30 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
>  </TASK>
>
> Allocated by task 7671:
>  kasan_save_stack mm/kasan/common.c:56 [inline]
>  kasan_save_track+0x3e/0x80 mm/kasan/common.c:77
>  unpoison_slab_object mm/kasan/common.c:339 [inline]
>  __kasan_slab_alloc+0x6c/0x80 mm/kasan/common.c:365
>  kasan_slab_alloc include/linux/kasan.h:252 [inline]
>  slab_post_alloc_hook mm/slub.c:4953 [inline]
>  slab_alloc_node mm/slub.c:5263 [inline]
>  kmem_cache_alloc_node_noprof+0x43c/0x720 mm/slub.c:5315
>  alloc_task_struct_node kernel/fork.c:184 [inline]
>  dup_task_struct+0x57/0x9a0 kernel/fork.c:915
>  copy_process+0x4ea/0x3950 kernel/fork.c:2052
>  kernel_clone+0x21e/0x820 kernel/fork.c:2651
>  __do_sys_clone3 kernel/fork.c:2953 [inline]
>  __se_sys_clone3+0x256/0x2d0 kernel/fork.c:2932
>  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
>
> Freed by task 6024:
>  kasan_save_stack mm/kasan/common.c:56 [inline]
>  kasan_save_track+0x3e/0x80 mm/kasan/common.c:77
>  kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:584
>  poison_slab_object mm/kasan/common.c:252 [inline]
>  __kasan_slab_free+0x5c/0x80 mm/kasan/common.c:284
>  kasan_slab_free include/linux/kasan.h:234 [inline]
>  slab_free_hook mm/slub.c:2540 [inline]
>  slab_free mm/slub.c:6668 [inline]
>  kmem_cache_free+0x197/0x620 mm/slub.c:6779
>  rcu_do_batch kernel/rcu/tree.c:2605 [inline]
>  rcu_core+0xd70/0x1870 kernel/rcu/tree.c:2857
>  handle_softirqs+0x27d/0x850 kernel/softirq.c:622
>  __do_softirq kernel/softirq.c:656 [inline]
>  invoke_softirq kernel/softirq.c:496 [inline]
>  __irq_exit_rcu+0xca/0x1f0 kernel/softirq.c:723
>  irq_exit_rcu+0x9/0x30 kernel/softirq.c:739
>  instr_sysvec_call_function_single arch/x86/kernel/smp.c:266 [inline]
>  sysvec_call_function_single+0xa3/0xc0 arch/x86/kernel/smp.c:266
>  asm_sysvec_call_function_single+0x1a/0x20 arch/x86/include/asm/idtentry.h:704
>
> Last potentially related work creation:
>  kasan_save_stack+0x3e/0x60 mm/kasan/common.c:56
>  kasan_record_aux_stack+0xbd/0xd0 mm/kasan/generic.c:556
>  __call_rcu_common kernel/rcu/tree.c:3119 [inline]
>  call_rcu+0x157/0x9c0 kernel/rcu/tree.c:3239
>  rcu_do_batch kernel/rcu/tree.c:2605 [inline]
>  rcu_core+0xd70/0x1870 kernel/rcu/tree.c:2857
>  handle_softirqs+0x27d/0x850 kernel/softirq.c:622
>  run_ksoftirqd+0x9b/0x100 kernel/softirq.c:1063
>  smpboot_thread_fn+0x542/0xa60 kernel/smpboot.c:160
>  kthread+0x711/0x8a0 kernel/kthread.c:463
>  ret_from_fork+0x599/0xb30 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
>
> Second to last potentially related work creation:
>  kasan_save_stack+0x3e/0x60 mm/kasan/common.c:56
>  kasan_record_aux_stack+0xbd/0xd0 mm/kasan/generic.c:556
>  __call_rcu_common kernel/rcu/tree.c:3119 [inline]
>  call_rcu+0x157/0x9c0 kernel/rcu/tree.c:3239
>  context_switch kernel/sched/core.c:5259 [inline]
>  __schedule+0x14c4/0x5000 kernel/sched/core.c:6863
>  preempt_schedule_irq+0xb5/0x150 kernel/sched/core.c:7190
>  irqentry_exit+0x5d8/0x660 kernel/entry/common.c:216
>  asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
>
> The buggy address belongs to the object at ffff88816a881d40
>  which belongs to the cache task_struct of size 7232
> The buggy address is located 2488 bytes inside of
>  freed 7232-byte region [ffff88816a881d40, ffff88816a883980)
>
> The buggy address belongs to the physical page:
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x16a880
> head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> memcg:ffff8881726b0441
> anon flags: 0x57ff00000000040(head|node=1|zone=2|lastcpupid=0x7ff)
> page_type: f5(slab)
> raw: 057ff00000000040 ffff88816040a500 0000000000000000 0000000000000001
> raw: 0000000000000000 0000000080040004 00000000f5000000 ffff8881726b0441
> head: 057ff00000000040 ffff88816040a500 0000000000000000 0000000000000001
> head: 0000000000000000 0000000080040004 00000000f5000000 ffff8881726b0441
> head: 057ff00000000003 ffffea0005aa2001 00000000ffffffff 00000000ffffffff
> head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008
> page dumped because: kasan: bad access detected
> page_owner tracks the page as allocated
> page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 7291, tgid 7291 (syz.2.649), ts 88142964676, free_ts 88127352940
>  set_page_owner include/linux/page_owner.h:32 [inline]
>  post_alloc_hook+0x234/0x290 mm/page_alloc.c:1846
>  prep_new_page mm/page_alloc.c:1854 [inline]
>  get_page_from_freelist+0x2365/0x2440 mm/page_alloc.c:3915
>  __alloc_frozen_pages_noprof+0x181/0x370 mm/page_alloc.c:5210
>  alloc_pages_mpol+0x232/0x4a0 mm/mempolicy.c:2486
>  alloc_slab_page mm/slub.c:3075 [inline]
>  allocate_slab+0x86/0x3b0 mm/slub.c:3248
>  new_slab mm/slub.c:3302 [inline]
>  ___slab_alloc+0xf2b/0x1960 mm/slub.c:4656
>  __slab_alloc+0x65/0x100 mm/slub.c:4779
>  __slab_alloc_node mm/slub.c:4855 [inline]
>  slab_alloc_node mm/slub.c:5251 [inline]
>  kmem_cache_alloc_node_noprof+0x4ce/0x720 mm/slub.c:5315
>  alloc_task_struct_node kernel/fork.c:184 [inline]
>  dup_task_struct+0x57/0x9a0 kernel/fork.c:915
>  copy_process+0x4ea/0x3950 kernel/fork.c:2052
>  kernel_clone+0x21e/0x820 kernel/fork.c:2651
>  __do_sys_clone3 kernel/fork.c:2953 [inline]
>  __se_sys_clone3+0x256/0x2d0 kernel/fork.c:2932
>  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
> page last free pid 5275 tgid 5275 stack trace:
>  reset_page_owner include/linux/page_owner.h:25 [inline]
>  free_pages_prepare mm/page_alloc.c:1395 [inline]
>  __free_frozen_pages+0xbc8/0xd30 mm/page_alloc.c:2943
>  __slab_free+0x21b/0x2a0 mm/slub.c:6004
>  qlink_free mm/kasan/quarantine.c:163 [inline]
>  qlist_free_all+0x97/0x100 mm/kasan/quarantine.c:179
>  kasan_quarantine_reduce+0x148/0x160 mm/kasan/quarantine.c:286
>  __kasan_slab_alloc+0x22/0x80 mm/kasan/common.c:349
>  kasan_slab_alloc include/linux/kasan.h:252 [inline]
>  slab_post_alloc_hook mm/slub.c:4953 [inline]
>  slab_alloc_node mm/slub.c:5263 [inline]
>  kmem_cache_alloc_noprof+0x37d/0x710 mm/slub.c:5270
>  getname_flags+0xb8/0x540 fs/namei.c:146
>  getname include/linux/fs.h:2498 [inline]
>  do_sys_openat2+0xbc/0x200 fs/open.c:1426
>  do_sys_open fs/open.c:1436 [inline]
>  __do_sys_openat fs/open.c:1452 [inline]
>  __se_sys_openat fs/open.c:1447 [inline]
>  __x64_sys_openat+0x138/0x170 fs/open.c:1447
>  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
>
> Memory state around the buggy address:
>  ffff88816a882580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff88816a882600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >ffff88816a882680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>                                                                 ^
>  ffff88816a882700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff88816a882780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
>
>
> ***
>
> If these findings have caused you to resend the series or submit a
> separate fix, please add the following tag to your commit message:
>   Tested-by: syzbot@syzkaller.appspotmail.com
>
> ---
> This report is generated by a bot. It may contain errors.
> syzbot ci engineers can be reached at syzkaller@googlegroups.com.