[syzbot] WARNING: suspicious RCU usage in mas_walk

[syzbot]

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+2ee188...@syzkaller.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!

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>


---
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[Christian Brauner]

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+2ee188...@syzkaller.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.h...@oracle.com>
gets Cced.

[Liam Howlett]

* Christian Brauner <bra...@kernel.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+2ee188...@syzkaller.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.h...@oracle.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.

[Dmitry Vyukov]

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.