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

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Message-ID: <CAHk-=wj=jthBt0XOjqstiYYROGhJAM0xrsWBVQgGMk79JBaKDg@mail.gmail.com>
Date:   Sun, 24 Oct 2021 09:54:22 -1000
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     syzbot <syzbot+75639e6a0331cd61d3e2@...kaller.appspotmail.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        jordy@...dyzomer.github.io, jordy@...ing.systems,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [syzbot] WARNING: refcount bug in memfd_secret

On Sat, Oct 23, 2021 at 9:35 AM syzbot
<syzbot+75639e6a0331cd61d3e2@...kaller.appspotmail.com> wrote:
>
> syzbot found the following issue on:
>
> HEAD commit:    9c0c4d24ac00 Merge tag 'block-5.15-2021-10-22' of git://gi..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=115a0328b00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=59f3ef2b4077575
> dashboard link: https://syzkaller.appspot.com/bug?extid=75639e6a0331cd61d3e2
> compiler:       Debian clang version 11.0.1-2, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13a035c2b00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14ae869f300000
>
> The issue was bisected to:
>
> commit 110860541f443f950c1274f217a1a3e298670a33

I think that commit is actually just buggy.

"secretmem_users" is not actually a reference count. There's no "magic
happens when it goes down to zero".

It's purely a count of the number of existing users, and incrementing
it from zero is not a probolem at all - it is in fact expected.

Sure, zero means "we can hibernate", so zero and overflow are somewhat
special, but not special enough to cause these kinds of issues.

I have reverted this commit in my tree, because honestly, the whole
"try to overflow exactly, and hibernate" threat model just isn't worth
this all.

If people really care, I can suggest

 - use "atomic_long_t" instead. Let's face it, 32-bit isn't
interesting any more, and 64-bit doesn't overflow.

 - make up some new "atomic_inc_nooverflow()" thing or whatever.

but for now this is just reverted.

             Linus