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

[Suren Baghdasaryan]

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@redhatcom> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> >>>>>>>> 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.

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