linux-kernel - Re: [syzbot] [mm?] kernel BUG in move

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Message-ID: <CAJuCfpFxoDzQ_MBQe127pdWBRxugN2c7QsoemvHp+qCnfir+JQ@mail.gmail.com>
Date: Thu, 11 Jan 2024 17:44:57 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Stephen Rothwell <sfr@...b.auug.org.au>, akpm@...ux-foundation.org
Cc: syzbot <syzbot+705209281e36404998f6@...kaller.appspotmail.com>, 
	Peter Xu <peterx@...hat.com>, David Hildenbrand <david@...hat.com>, aarcange@...hat.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] kernel BUG in move_pages

On Thu, Jan 11, 2024 at 3:23 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Thu, Jan 11, 2024 at 9:13 PM Suren Baghdasaryan <surenb@...glecom> wrote:
> >
> > On Thu, Jan 11, 2024 at 1:06 PM David Hildenbrand <david@...hatcom> wrote:
> > >
> > > On 11.01.24 22:04, Suren Baghdasaryan wrote:
> > > > On Thu, Jan 11, 2024 at 9:00 PM David Hildenbrand <david@...hat.com> wrote:
> > > >>
> > > >> On 11.01.24 21:20, Suren Baghdasaryan wrote:
> > > >>> On Thu, Jan 11, 2024 at 6:58 PM David Hildenbrand <david@...hat.com> wrote:
> > > >>>>
> > > >>>> On 11.01.24 19:34, Suren Baghdasaryan wrote:
> > > >>>>> On Thu, Jan 11, 2024 at 8:44 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
> > > >>>>>>
> > > >>>>>> On Thu, Jan 11, 2024 at 8:40 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
> > > >>>>>>>
> > > >>>>>>> On Thu, Jan 11, 2024 at 8:25 AM syzbot
> > > >>>>>>> <syzbot+705209281e36404998f6@...kaller.appspotmail.com> wrote:
> > > >>>>>>>>
> > > >>>>>>>> Hello,
> > > >>>>>>>>
> > > >>>>>>>> syzbot found the following issue on:
> > > >>>>>>>>
> > > >>>>>>>> HEAD commit:    e2425464bc87 Add linux-next specific files for 20240105
> > > >>>>>>>> git tree:       linux-next
> > > >>>>>>>> console+strace: https://syzkaller.appspot.com/x/log.txt?x=14941cdee80000
> > > >>>>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=4056b9349f3da8c9
> > > >>>>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=705209281e36404998f6
> > > >>>>>>>> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> > > >>>>>>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=125d0a09e80000
> > > >>>>>>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15bc7331e80000
> > > >>>>>>>>
> > > >>>>>>>> Downloadable assets:
> > > >>>>>>>> disk image: https://storage.googleapis.com/syzbot-assets/2f738185e2cf/disk-e2425464.raw.xz
> > > >>>>>>>> vmlinux: https://storage.googleapis.com/syzbot-assets/b248fcf4ea46/vmlinux-e2425464.xz
> > > >>>>>>>> kernel image: https://storage.googleapis.com/syzbot-assets/a9945c8223f4/bzImage-e2425464.xz
> > > >>>>>>>>
> > > >>>>>>>> The issue was bisected to:
> > > >>>>>>>>
> > > >>>>>>>> commit adef440691bab824e39c1b17382322d195e1fab0
> > > >>>>>>>> Author: Andrea Arcangeli <aarcange@...hat.com>
> > > >>>>>>>> Date:   Wed Dec 6 10:36:56 2023 +0000
> > > >>>>>>>>
> > > >>>>>>>>        userfaultfd: UFFDIO_MOVE uABI
> > > >>>>>>>>
> > > >>>>>>>> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=11cb6ea9e80000
> > > >>>>>>>> final oops:     https://syzkaller.appspot.com/x/report.txt?x=13cb6ea9e80000
> > > >>>>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=15cb6ea9e80000
> > > >>>>>>>>
> > > >>>>>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > >>>>>>>> Reported-by: syzbot+705209281e36404998f6@...kaller.appspotmail.com
> > > >>>>>>>> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> > > >>>>>>>>
> > > >>>>>>>>     do_one_initcall+0x128/0x680 init/main.c:1237
> > > >>>>>>>>     do_initcall_level init/main.c:1299 [inline]
> > > >>>>>>>>     do_initcalls init/main.c:1315 [inline]
> > > >>>>>>>>     do_basic_setup init/main.c:1334 [inline]
> > > >>>>>>>>     kernel_init_freeable+0x692/0xc30 init/main.c:1552
> > > >>>>>>>>     kernel_init+0x1c/0x2a0 init/main.c:1442
> > > >>>>>>>>     ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
> > > >>>>>>>>     ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:242
> > > >>>>>>>> ------------[ cut here ]------------
> > > >>>>>>>> kernel BUG at include/linux/page-flags.h:1035!
> > > >>>>>>>> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> > > >>>>>>>> CPU: 0 PID: 5068 Comm: syz-executor191 Not tainted 6.7.0-rc8-next-20240105-syzkaller #0
> > > >>>>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/17/2023
> > > >>>>>>>> RIP: 0010:PageAnonExclusive include/linux/page-flags.h:1035 [inline]
> > > >>>>>>>
> > > >>>>>>>    From a quick look, I think the new ioctl is being used against a
> > > >>>>>>> file-backed page and that's why PageAnonExclusive() throws this error.
> > > >>>>>>> I'll confirm if this is indeed the case and will add checks for that
> > > >>>>>>> case. Thanks!
> > > >>>>>>
> > > >>>>>> Hmm. Looking at the reproducer it does not look like a file-backed
> > > >>>>>> memory... Anyways, I'm on it.
> > > >>>>>
> > > >>>>> Looks like the test is trying to move the huge_zero_page. Wonder how
> > > >>>>> we should handle this. Just fail or do something else? Adding David
> > > >>>>> and Peter for feedback.
> > > >>>>
> > > >>>> You'll need some special-casing to handle that. But it should be fairly
> > > >>>> easy.
> > > >>>
> > > >>> Ok, so should we treat zeropage the same as PAE and map destination
> > > >>> PTE/PMD to zeropage while clearing source PTE/PMD?
> > > >>
> > > >> Likely yes. So it's transparent for user space what we are moving. (this
> > > >> sounds like an easy case to not require a prior write access just to
> > > >> move it)
> > > >
> > > > Ok, working on it. split_huge_pmd() already knows how to split
> > > > huge_zero_page but I think I'll need special handling in both
> > > > move_pages_pte() and move_pages_huge_pmd().
> > >
> > > A PTE-mapped huge zeropage is just a page table populated with the
> > > ordinary shared zeropage. Are you moving the ordinary shared zeropage as
> > > well? If not, you should do so for consistency (or not do either :) ).
> >
> > Yes, I think I should move ordinary zeropages as well.
>
> I have a version that seems to work but I want to test it more and
> it's too heavy to be considered a quick fix for linux-next. I'll post
> a simple one-line fix which takes care of this crash and keeps the
> behavior for zeropages the same (ioctl returns -EBUSY). Later will
> post a separate patch to move huge and ordinary zeropages.

I posted a quick fix for this issue here:
https://lore.kernel.org/all/20240112013935.1474648-1-surenb@google.com/
It cleanly applies over linux-next, mm-stable and mm-unstable. Andrew,
Stephen, could you please pull the fix into your branches?
Thanks,
Suren.


>
> >
> > >
> > > --
> > > Cheers,
> > >
> > > David / dhildenb
> > >