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

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jan 11, 2024 at 9:00 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 11.01.24 21:20, Suren Baghdasaryan wrote:
> > On Thu, Jan 11, 2024 at 6:58 PM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 11.01.24 19:34, Suren Baghdasaryan wrote:
> >>> On Thu, Jan 11, 2024 at 8:44 AM Suren Baghdasaryan <surenb@google.com> wrote:
> >>>>
> >>>> On Thu, Jan 11, 2024 at 8:40 AM Suren Baghdasaryan <surenb@google.com> wrote:
> >>>>>
> >>>>> On Thu, Jan 11, 2024 at 8:25 AM syzbot
> >>>>> <syzbot+705209281e36404998f6@syzkaller.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@redhat.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@syzkaller.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().

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