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Date:   Mon, 24 May 2021 15:46:02 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     syzbot <syzbot+7b2b13f4943374609532@syzkaller.appspotmail.com>,
        rcu@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Jens Axboe <axboe@kernel.dk>, bpf <bpf@vger.kernel.org>,
        Christian Brauner <christian@brauner.io>,
        Daniel Borkmann <daniel@iogearbox.net>,
        John Fastabend <john.fastabend@gmail.com>,
        Martin KaFai Lau <kafai@fb.com>,
        KP Singh <kpsingh@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Song Liu <songliubraving@fb.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Yonghong Song <yhs@fb.com>
Subject: Re: [syzbot] KASAN: use-after-free Read in
 check_all_holdout_tasks_trace
Message-ID: <20210524224602.GA1963972@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <000000000000f034fc05c2da6617@google.com>
 <CACT4Y+ZGkye_MnNr92qQameXVEHNc1QkpmNrG3W8Yd1Xg_hfhw@mail.gmail.com>
 <20210524041350.GJ4441@paulmck-ThinkPad-P17-Gen-1>
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On Sun, May 23, 2021 at 09:13:50PM -0700, Paul E. McKenney wrote:
> On Sun, May 23, 2021 at 08:51:56AM +0200, Dmitry Vyukov wrote:
> > On Fri, May 21, 2021 at 7:29 PM syzbot
> > <syzbot+7b2b13f4943374609532@syzkaller.appspotmail.com> wrote:
> > >
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    f18ba26d libbpf: Add selftests for TC-BPF management API
> > > git tree:       bpf-next
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=17f50d1ed00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=8ff54addde0afb5d
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=7b2b13f4943374609532
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+7b2b13f4943374609532@syzkaller.appspotmail.com
> > 
> > This looks rcu-related. +rcu mailing list
> 
> I think I see a possible cause for this, and will say more after some
> testing and after becoming more awake Monday morning, Pacific time.

No joy.  From what I can see, within RCU Tasks Trace, the calls to
get_task_struct() are properly protected (either by RCU or by an earlier
get_task_struct()), and the calls to put_task_struct() are balanced by
those to get_task_struct().

I could of course have missed something, but at this point I am suspecting
an unbalanced put_task_struct() has been added elsewhere.

As always, extra eyes on this code would be a good thing.

If it were reproducible, I would of course suggest bisection.  :-/

							Thanx, Paul

> > > ==================================================================
> > > BUG: KASAN: use-after-free in check_all_holdout_tasks_trace+0x302/0x420 kernel/rcu/tasks.h:1084
> > > Read of size 1 at addr ffff88802767a05c by task rcu_tasks_trace/12
> > >
> > > CPU: 0 PID: 12 Comm: rcu_tasks_trace Not tainted 5.12.0-syzkaller #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > > Call Trace:
> > >  __dump_stack lib/dump_stack.c:79 [inline]
> > >  dump_stack+0x141/0x1d7 lib/dump_stack.c:120
> > >  print_address_description.constprop.0.cold+0x5b/0x2f8 mm/kasan/report.c:233
> > >  __kasan_report mm/kasan/report.c:419 [inline]
> > >  kasan_report.cold+0x7c/0xd8 mm/kasan/report.c:436
> > >  check_all_holdout_tasks_trace+0x302/0x420 kernel/rcu/tasks.h:1084
> > >  rcu_tasks_wait_gp+0x594/0xa60 kernel/rcu/tasks.h:358
> > >  rcu_tasks_kthread+0x31c/0x6a0 kernel/rcu/tasks.h:224
> > >  kthread+0x3b1/0x4a0 kernel/kthread.c:313
> > >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
> > >
> > > Allocated by task 8477:
> > >  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:38
> > >  kasan_set_track mm/kasan/common.c:46 [inline]
> > >  set_alloc_info mm/kasan/common.c:428 [inline]
> > >  __kasan_slab_alloc+0x84/0xa0 mm/kasan/common.c:461
> > >  kasan_slab_alloc include/linux/kasan.h:236 [inline]
> > >  slab_post_alloc_hook mm/slab.h:524 [inline]
> > >  slab_alloc_node mm/slub.c:2912 [inline]
> > >  kmem_cache_alloc_node+0x269/0x3e0 mm/slub.c:2948
> > >  alloc_task_struct_node kernel/fork.c:171 [inline]
> > >  dup_task_struct kernel/fork.c:865 [inline]
> > >  copy_process+0x5c8/0x7120 kernel/fork.c:1947
> > >  kernel_clone+0xe7/0xab0 kernel/fork.c:2503
> > >  __do_sys_clone+0xc8/0x110 kernel/fork.c:2620
> > >  do_syscall_64+0x3a/0xb0 arch/x86/entry/common.c:47
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > >
> > > Freed by task 12:
> > >  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:38
> > >  kasan_set_track+0x1c/0x30 mm/kasan/common.c:46
> > >  kasan_set_free_info+0x20/0x30 mm/kasan/generic.c:357
> > >  ____kasan_slab_free mm/kasan/common.c:360 [inline]
> > >  ____kasan_slab_free mm/kasan/common.c:325 [inline]
> > >  __kasan_slab_free+0xfb/0x130 mm/kasan/common.c:368
> > >  kasan_slab_free include/linux/kasan.h:212 [inline]
> > >  slab_free_hook mm/slub.c:1581 [inline]
> > >  slab_free_freelist_hook+0xdf/0x240 mm/slub.c:1606
> > >  slab_free mm/slub.c:3166 [inline]
> > >  kmem_cache_free+0x8a/0x740 mm/slub.c:3182
> > >  __put_task_struct+0x26f/0x400 kernel/fork.c:747
> > >  trc_wait_for_one_reader kernel/rcu/tasks.h:935 [inline]
> > >  check_all_holdout_tasks_trace+0x179/0x420 kernel/rcu/tasks.h:1081
> > >  rcu_tasks_wait_gp+0x594/0xa60 kernel/rcu/tasks.h:358
> > >  rcu_tasks_kthread+0x31c/0x6a0 kernel/rcu/tasks.h:224
> > >  kthread+0x3b1/0x4a0 kernel/kthread.c:313
> > >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
> > >
> > > Last potentially related work creation:
> > >  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:38
> > >  kasan_record_aux_stack+0xe5/0x110 mm/kasan/generic.c:345
> > >  __call_rcu kernel/rcu/tree.c:3038 [inline]
> > >  call_rcu+0xb1/0x750 kernel/rcu/tree.c:3113
> > >  put_task_struct_rcu_user+0x7f/0xb0 kernel/exit.c:180
> > >  release_task+0xca1/0x1690 kernel/exit.c:226
> > >  wait_task_zombie kernel/exit.c:1108 [inline]
> > >  wait_consider_task+0x2fb5/0x3b40 kernel/exit.c:1335
> > >  do_wait_thread kernel/exit.c:1398 [inline]
> > >  do_wait+0x724/0xd40 kernel/exit.c:1515
> > >  kernel_wait4+0x14c/0x260 kernel/exit.c:1678
> > >  __do_sys_wait4+0x13f/0x150 kernel/exit.c:1706
> > >  do_syscall_64+0x3a/0xb0 arch/x86/entry/common.c:47
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > >
> > > Second to last potentially related work creation:
> > >  kasan_save_stack+0x1b/0x40 mm/kasan/common.c:38
> > >  kasan_record_aux_stack+0xe5/0x110 mm/kasan/generic.c:345
> > >  __call_rcu kernel/rcu/tree.c:3038 [inline]
> > >  call_rcu+0xb1/0x750 kernel/rcu/tree.c:3113
> > >  put_task_struct_rcu_user+0x7f/0xb0 kernel/exit.c:180
> > >  context_switch kernel/sched/core.c:4342 [inline]
> > >  __schedule+0x91e/0x23e0 kernel/sched/core.c:5147
> > >  preempt_schedule_common+0x45/0xc0 kernel/sched/core.c:5307
> > >  preempt_schedule_thunk+0x16/0x18 arch/x86/entry/thunk_64.S:35
> > >  try_to_wake_up+0xa12/0x14b0 kernel/sched/core.c:3489
> > >  wake_up_process kernel/sched/core.c:3552 [inline]
> > >  wake_up_q+0x96/0x100 kernel/sched/core.c:597
> > >  futex_wake+0x3e9/0x490 kernel/futex.c:1634
> > >  do_futex+0x326/0x1780 kernel/futex.c:3738
> > >  __do_sys_futex+0x2a2/0x470 kernel/futex.c:3796
> > >  do_syscall_64+0x3a/0xb0 arch/x86/entry/common.c:47
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > >
> > > The buggy address belongs to the object at ffff888027679c40
> > >  which belongs to the cache task_struct of size 6976
> > > The buggy address is located 1052 bytes inside of
> > >  6976-byte region [ffff888027679c40, ffff88802767b780)
> > > The buggy address belongs to the page:
> > > page:ffffea00009d9e00 refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff88802767b880 pfn:0x27678
> > > head:ffffea00009d9e00 order:3 compound_mapcount:0 compound_pincount:0
> > > flags: 0xfff00000010200(slab|head|node=0|zone=1|lastcpupid=0x7ff)
> > > raw: 00fff00000010200 ffffea000071e208 ffffea0000950808 ffff888140005140
> > > raw: ffff88802767b880 0000000000040003 00000001ffffffff 0000000000000000
> > > 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 243, ts 14372676818, free_ts 0
> > >  prep_new_page mm/page_alloc.c:2358 [inline]
> > >  get_page_from_freelist+0x1033/0x2b60 mm/page_alloc.c:3994
> > >  __alloc_pages+0x1b2/0x500 mm/page_alloc.c:5200
> > >  alloc_pages+0x18c/0x2a0 mm/mempolicy.c:2272
> > >  alloc_slab_page mm/slub.c:1644 [inline]
> > >  allocate_slab+0x2c5/0x4c0 mm/slub.c:1784
> > >  new_slab mm/slub.c:1847 [inline]
> > >  new_slab_objects mm/slub.c:2593 [inline]
> > >  ___slab_alloc+0x44c/0x7a0 mm/slub.c:2756
> > >  __slab_alloc.constprop.0+0xa7/0xf0 mm/slub.c:2796
> > >  slab_alloc_node mm/slub.c:2878 [inline]
> > >  kmem_cache_alloc_node+0x12f/0x3e0 mm/slub.c:2948
> > >  alloc_task_struct_node kernel/fork.c:171 [inline]
> > >  dup_task_struct kernel/fork.c:865 [inline]
> > >  copy_process+0x5c8/0x7120 kernel/fork.c:1947
> > >  kernel_clone+0xe7/0xab0 kernel/fork.c:2503
> > >  kernel_thread+0xb5/0xf0 kernel/fork.c:2555
> > >  call_usermodehelper_exec_work kernel/umh.c:174 [inline]
> > >  call_usermodehelper_exec_work+0xcc/0x180 kernel/umh.c:160
> > >  process_one_work+0x98d/0x1600 kernel/workqueue.c:2275
> > >  worker_thread+0x64c/0x1120 kernel/workqueue.c:2421
> > >  kthread+0x3b1/0x4a0 kernel/kthread.c:313
> > >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
> > > page_owner free stack trace missing
> > >
> > > Memory state around the buggy address:
> > >  ffff888027679f00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > >  ffff888027679f80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > > >ffff88802767a000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > >                                                     ^
> > >  ffff88802767a080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > >  ffff88802767a100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> > > ==================================================================
> > >
> > >
> > > ---
> > > 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.
> > >
> > > --
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