linux-kernel - Re: [syzbot] WARNING: suspicious RCU usage in mas

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Date:   Mon, 2 May 2022 14:57:36 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        syzbot <syzbot+2ee18845e89ae76342c5@...kaller.appspotmail.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "syzkaller-bugs@...glegroups.com" <syzkaller-bugs@...glegroups.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [syzbot] WARNING: suspicious RCU usage in mas_walk

On Fri, 29 Apr 2022 at 20:01, Liam Howlett <liam.howlett@...cle.com> wrote:
>
> * Christian Brauner <brauner@...nel.org> [220429 08:05]:
> > On Thu, Apr 28, 2022 at 10:41:27PM -0700, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    bdc61aad77fa Add linux-next specific files for 20220428
> > > git tree:       linux-next
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=15bb3dc2f00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=87767e89da13a759
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=2ee18845e89ae76342c5
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1118a5f6f00000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=125bd212f00000
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+2ee18845e89ae76342c5@...kaller.appspotmail.com
> > >
> > > =============================
> > > WARNING: suspicious RCU usage
> > > 5.18.0-rc4-next-20220428-syzkaller #0 Not tainted
> > > -----------------------------
> > > lib/maple_tree.c:844 suspicious rcu_dereference_check() usage!
> >
> > I _think_ for maple tree stuff you want to somehow ensure that
> > Liam Howlett <liam.howlett@...cle.com>
> > gets Cced.
>
> Thanks for sending this along.  Yes, it would be best if I was somehow
> emailed for maple tree things.  I did add myself to maintainers for this
> file.

Hi,

Are you sure you want this persistently? And not biased by the fact
that this code is just added?
syzkaller does not blame anything in lib/*.c b/c the assumption is
that for things like memcpy, atomic ops, vsprintf, xarray most bugs
are really in the callers.
If we start blaming lib/maple_tree.c then later ii will be your
responsibility to reroute bugs when callers use if after freeing, etc
to these callers.



> > > other info that might help us debug this:
> > >
> > >
> > > rcu_scheduler_active = 2, debug_locks = 1
> > > 5 locks held by syz-executor842/4211:
> > >  #0: ffff88807f0ae460 (sb_writers#8){.+.+}-{0:0}, at: ksys_write+0x127/0x250 fs/read_write.c:644
> > >  #1: ffff88801df04488 (&of->mutex){+.+.}-{3:3}, at: kernfs_fop_write_iter+0x28c/0x610 fs/kernfs/file.c:282
> > >  #2: ffff8881453b9a00 (kn->active#106){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x2b0/0x610 fs/kernfs/file.c:283
> > >  #3: ffffffff8bedc528 (ksm_thread_mutex){+.+.}-{3:3}, at: run_store+0xd1/0xa60 mm/ksm.c:2917
> > >  #4: ffff88801e5e8fd8 (&mm->mmap_lock#2){++++}-{3:3}, at: mmap_read_lock include/linux/mmap_lock.h:117 [inline]
> > >  #4: ffff88801e5e8fd8 (&mm->mmap_lock#2){++++}-{3:3}, at: unmerge_and_remove_all_rmap_items mm/ksm.c:989 [inline]
> > >  #4: ffff88801e5e8fd8 (&mm->mmap_lock#2){++++}-{3:3}, at: run_store+0x2a5/0xa60 mm/ksm.c:2923
> > >
> > > stack backtrace:
> > > CPU: 0 PID: 4211 Comm: syz-executor842 Not tainted 5.18.0-rc4-next-20220428-syzkaller #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > > Call Trace:
> > >  <TASK>
> > >  __dump_stack lib/dump_stack.c:88 [inline]
> > >  dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
> > >  mas_root lib/maple_tree.c:844 [inline]
> > >  mas_start lib/maple_tree.c:1331 [inline]
> > >  mas_state_walk lib/maple_tree.c:3745 [inline]
> > >  mas_walk+0x45e/0x670 lib/maple_tree.c:4923
> > >  mas_find+0x442/0xc90 lib/maple_tree.c:5861
> > >  vma_find include/linux/mm.h:664 [inline]
> > >  vma_next include/linux/mm.h:673 [inline]
> > >  unmerge_and_remove_all_rmap_items mm/ksm.c:990 [inline]
> > >  run_store+0x2ed/0xa60 mm/ksm.c:2923
> > >  kobj_attr_store+0x50/0x80 lib/kobject.c:824
> > >  sysfs_kf_write+0x110/0x160 fs/sysfs/file.c:136
> > >  kernfs_fop_write_iter+0x3f8/0x610 fs/kernfs/file.c:291
> > >  call_write_iter include/linux/fs.h:2059 [inline]
> > >  new_sync_write+0x38a/0x560 fs/read_write.c:504
> > >  vfs_write+0x7c0/0xac0 fs/read_write.c:591
> > >  ksys_write+0x127/0x250 fs/read_write.c:644
> > >  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > >  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xae
> > > RIP: 0033:0x7f6a91306e79
> > > Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 b1 14 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 c0 ff ff ff f7 d8 64 89 01 48
> > > RSP: 002b:00007ffddeb8cde8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> > > RAX: ffffffffffffffda RBX: 00000000000f4240 RCX: 00007f6a91306e79
> > > RDX: 0000000000000002 RSI: 0000000020000000 RDI: 0000000000000003
> > > RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000001
> > > R10: 0000000020117000 R11: 0000000000000246 R12: 000000000000cf84
> > > R13: 00007ffddeb8cdfc R14: 00007ffddeb8ce10 R15: 00007ffddeb8ce00
> > >  </TASK>