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

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Date:   Sun, 24 Oct 2021 21:09:58 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     syzbot <syzbot+75639e6a0331cd61d3e2@...kaller.appspotmail.com>,
        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 Sun, Oct 24, 2021 at 09:54:22AM -1000, Linus Torvalds wrote:
> 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.

There was a separate thread on an earlier version of this report.
https://lore.kernel.org/linux-mm/YXU7%2FiRjf9v77gon@casper.infradead.org/
I agree with you and suggested that if anybody really cares (I mean,
you need a multi-TB machine to produce this problem) that we simply do
what we did with the page refcount:

+++ b/mm/secretmem.c
@@ -203,6 +203,8 @@ SYSCALL_DEFINE1(memfd_secret, unsigned int, flags)

        if (flags & ~(SECRETMEM_FLAGS_MASK | O_CLOEXEC))
                return -EINVAL;
+       if (atomic_read(&secretmem_users) < 0)
+               return -ENFILE;

        fd = get_unused_fd_flags(flags & O_CLOEXEC);
        if (fd < 0)

Mike didn't particularly like that answer though.