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

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Date: Thu, 11 Jan 2024 22:00:43 +0100
From: David Hildenbrand <david@...hat.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: syzbot <syzbot+705209281e36404998f6@...kaller.appspotmail.com>,
 Peter Xu <peterx@...hat.com>, aarcange@...hat.com,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] kernel BUG in move_pages

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)

-- 
Cheers,

David / dhildenb