linux-kernel - Re: [syzbot] [mm?] KCSAN: data-race in mprotect_fixup / try_to_migrate

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Message-ID: <CANpmjNNeXjtatBD6Lrz2-1jiqLkpcuk0Ra-6uJKzz3=y86qVuA@mail.gmail.com>
Date: Wed, 5 Feb 2025 16:46:40 +0100
From: Marco Elver <elver@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Jann Horn <jannh@...gle.com>, 
	syzbot <syzbot+c2e5712cbb14c95d4847@...kaller.appspotmail.com>, 
	Liam.Howlett@...cle.com, akpm@...ux-foundation.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	syzkaller-bugs@...glegroups.com, vbabka@...e.cz, 
	"Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [syzbot] [mm?] KCSAN: data-race in mprotect_fixup / try_to_migrate_one

On Wed, 5 Feb 2025 at 16:11, 'Lorenzo Stoakes' via syzkaller-bugs
<syzkaller-bugs@...glegroups.com> wrote:
>
> On Wed, Feb 05, 2025 at 04:00:06PM +0100, Jann Horn wrote:
> > On Wed, Feb 5, 2025 at 12:41 PM syzbot
> > <syzbot+c2e5712cbb14c95d4847@...kaller.appspotmail.com> wrote:
> > > syzbot found the following issue on:
> > >
> > > HEAD commit:    d009de7d5428 Merge tag 'livepatching-for-6.14-rc2' of git:..
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=12b678a4580000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=9e757e3762bd630b
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=c2e5712cbb14c95d4847
> > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > >
> > > Unfortunately, I don't have any reproducer for this issue yet.
> > >
> > > Downloadable assets:
> > > disk image: https://storage.googleapis.com/syzbot-assets/9235000a1b88/disk-d009de7d.raw.xz
> > > vmlinux: https://storage.googleapis.com/syzbot-assets/098ef82f8ab3/vmlinux-d009de7d.xz
> > > kernel image: https://storage.googleapis.com/syzbot-assets/4f51f5eb5782/bzImage-d009de7d.xz
> > >
> > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > Reported-by: syzbot+c2e5712cbb14c95d4847@...kaller.appspotmail.com
> > >
> > > ==================================================================
> > > BUG: KCSAN: data-race in mprotect_fixup / try_to_migrate_one
[...]
> I hate that we have these landmines waiting for us. Be good to find a way
> to explicitly annotate this, or at least comment somehow.
>
> But agreed, probably adding a READ_ONCE()/WRITE_ONCE() is appropriate at
> least for the proximate thing.
>
> It's a wonder these things don't trigger more, except you need probably
> very precise timing to do it...

They do trigger, but we don't send all of them to LKML.
When we first introduced KCSAN, the notion of "data race" was still
poorly understood. At the time we decided to pre-review a number of
them (but our time to do so has been going down :-/), or let willing
maintainers deal with them directly. A number of articles followed,
such as:
 - https://lwn.net/Articles/816850/
 - https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt

And I think much of the community has indeed been "Calibrating your
fear of big bad optimizing compilers"
[https://lwn.net/Articles/799218/]. :-)

If you want to see more reports (you can try to search for ones
relevant to you):
https://syzkaller.appspot.com/upstream?manager=ci2-upstream-kcsan-gce
(see "moderation")