Re: [syzbot] [mm?] kernel BUG in move_pages

[Suren Baghdasaryan]

On Thu, Jan 11, 2024 at 9:13 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Thu, Jan 11, 2024 at 1:06 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >
> > On 11.01.24 22:04, Suren Baghdasaryan wrote:
> > > On Thu, Jan 11, 2024 at 9:00 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >>
> > >> On 11.01.24 21:20, Suren Baghdasaryan wrote:
> > >>> On Thu, Jan 11, 2024 at 6:58 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >>>>
> > >>>> On 11.01.24 19:34, Suren Baghdasaryan wrote:
> > >>>>> On Thu, Jan 11, 2024 at 8:44 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > >>>>>>
> > >>>>>> On Thu, Jan 11, 2024 at 8:40 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > >>>>>>>
> > >>>>>>> On Thu, Jan 11, 2024 at 8:25 AM syzbot
> > >>>>>>> <syzbot+705209281e36404998f6@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > >>>>>>>> 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@syzkaller.appspotmailcom
> > >>>>>>>> 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.

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