linux-kernel - Re: [syzbot] WARNING: refcount bug in sys_memfd

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Message-ID: <CACT4Y+bRDjZ=p_HCZK3odrjSfFa5LL8aeO8TpS_3DgY4w-7+=Q@mail.gmail.com>
Date:   Fri, 22 Oct 2021 18:29:05 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Jordy Zomer <jordy@...ing.systems>
Cc:     syzbot <syzbot+b904a1de3ec43711eba5@...kaller.appspotmail.com>,
        Mike Rapoport <rppt@...nel.org>, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] WARNING: refcount bug in sys_memfd_secret

On Fri, 22 Oct 2021 at 18:25, Jordy Zomer <jordy@...ing.systems> wrote:
>
> After a quick scan, it appears to be a false-positive.
>
> This because nothing appears to be being freed.
>
> In any case, you probably don't want warnings everywhere.
>
> I believe we should probably do something along the lines of:
>
> if (refcount_read(&secretmem_users) == 0) {
>     refcount_set(&secretmem_users, 1);
> } else {
>     refcount_inc(&secretmem_users);
> }
>
> Does this appear to be a feasible patch? :)

I don't think multithreading work this way :)
Imagine 2 threads reading refcount_read(&secretmem_users) == 0 and
then both doing refcount_set(&secretmem_users, 1).

> Best Regards,
>
> Jordy
>
> > On 10/22/2021 11:07 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
> >
> >
> > On Fri, 22 Oct 2021 at 17:02, syzbot
> > <syzbot+b904a1de3ec43711eba5@...kaller.appspotmail.com> wrote:
> > >
> > > Hello,
> > >
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    64222515138e Merge tag 'drm-fixes-2021-10-22' of git://ano..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=178e86c4b00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=be398dd7862f4b36
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=b904a1de3ec43711eba5
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > >
> > > 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+b904a1de3ec43711eba5@...kaller.appspotmail.com
> >
> > +Mike, Jordy for secretmem.c
> >
> > > ------------[ cut here ]------------
> > > refcount_t: addition on 0; use-after-free.
> > > WARNING: CPU: 2 PID: 32193 at lib/refcount.c:25 refcount_warn_saturate+0x169/0x1e0 lib/refcount.c:25
> > > Modules linked in:
> > > CPU: 2 PID: 32193 Comm: syz-executor.0 Not tainted 5.15.0-rc6-syzkaller #0
> > > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014
> > > RIP: 0010:refcount_warn_saturate+0x169/0x1e0 lib/refcount.c:25
> > > Code: 09 31 ff 89 de e8 f7 b9 9b fd 84 db 0f 85 36 ff ff ff e8 3a b2 9b fd 48 c7 c7 c0 65 e3 89 c6 05 6f 6c 7f 09 01 e8 7e 4a 19 05 <0f> 0b e9 17 ff ff ff e8 1b b2 9b fd 0f b6 1d 54 6c 7f 09 31 ff 89
> > > RSP: 0018:ffffc90009f9ff10 EFLAGS: 00010286
> > > RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> > > RDX: 0000000000040000 RSI: ffffffff815dbf58 RDI: fffff520013f3fd4
> > > RBP: 0000000000000002 R08: 0000000000000000 R09: 0000000000000001
> > > R10: ffffffff815d5cce R11: 0000000000000000 R12: 0000000000000000
> > > R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> > > FS:  00007f476a1e7700(0000) GS:ffff88802cd00000(0000) knlGS:0000000000000000
> > > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > CR2: 0000001b32c24000 CR3: 00000000494c4000 CR4: 0000000000150ee0
> > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > > Call Trace:
> > >  __refcount_add include/linux/refcount.h:199 [inline]
> > >  __refcount_inc include/linux/refcount.h:250 [inline]
> > >  refcount_inc include/linux/refcount.h:267 [inline]
> > >  __do_sys_memfd_secret mm/secretmem.c:221 [inline]
> > >  __se_sys_memfd_secret mm/secretmem.c:194 [inline]
> > >  __x64_sys_memfd_secret+0x182/0x1e0 mm/secretmem.c:194
> > >  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: 0033:0x7f476cc71a39
> > > Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 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 bc ff ff ff f7 d8 64 89 01 48
> > > RSP: 002b:00007f476a1e7188 EFLAGS: 00000246 ORIG_RAX: 00000000000001bf
> > > RAX: ffffffffffffffda RBX: 00007f476cd74f60 RCX: 00007f476cc71a39
> > > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> > > RBP: 00007f476cccbe8f R08: 0000000000000000 R09: 0000000000000000
> > > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> > > R13: 00007ffc5b5eb85f R14: 00007f476a1e7300 R15: 0000000000022000
> > >
> > >
> > > ---
> > > 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@...glegroups.com.
> > >
> > > syzbot will keep track of this issue. See:
> > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> > >
> > > --
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