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

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Date: Thu, 11 Jan 2024 22:06:20 +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 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 :) ).

-- 
Cheers,

David / dhildenb