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Subject: Re: [syzbot] possible deadlock in hugetlb_fault
To:     Dmitry Vyukov <dvyukov@google.com>
CC:     Mike Kravetz <mike.kravetz@oracle.com>,
        syzbot <syzbot+ca56f14c500045350f93@syzkaller.appspotmail.com>,
        <akpm@linux-foundation.org>, <linux-kernel@vger.kernel.org>,
        <linux-mm@kvack.org>, <llvm@lists.linux.dev>, <nathan@kernel.org>,
        <ndesaulniers@google.com>, <songmuchun@bytedance.com>,
        <syzkaller-bugs@googlegroups.com>, <trix@redhat.com>
References: <0000000000008c742d05eca72d4d@google.com>
 <Y27jxKoo1BmgbDbl@monkey> <c7646d97-c04b-7795-a6f8-a6523945f89c@huawei.com>
 <CACT4Y+ZWNV6ApzEv0UrsF2T8JWmXez_-H-EGMii-S_2JbXv07Q@mail.gmail.com>
 <CACT4Y+YAQVAMoU_vS_b+x-Ew3d-ecj+FTHGW9-MPE04dCzZ37w@mail.gmail.com>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <4c5fa6b4-6278-ce2b-73c2-883eb734bdc7@huawei.com>
Date:   Mon, 14 Nov 2022 10:35:01 +0800
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On 2022/11/12 22:14, Dmitry Vyukov wrote:
> On Sat, 12 Nov 2022 at 12:33, Dmitry Vyukov <dvyukov@google.com> wrote:
>>
>> On Sat, 12 Nov 2022 at 05:02, 'Miaohe Lin' via syzkaller-bugs
>> <syzkaller-bugs@googlegroups.com> wrote:
>>>
>>> On 2022/11/12 8:07, Mike Kravetz wrote:
>>>> On 11/04/22 09:00, syzbot wrote:
>>>>> Hello,
>>>>>
>>>>> syzbot found the following issue on:
>>>>>
>>>>> HEAD commit:    f2f32f8af2b0 Merge tag 'for-6.1-rc3-tag' of git://git.kern..
>>>>> git tree:       upstream
>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=137d52ca880000
>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=d080a4bd239918dd
>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=ca56f14c500045350f93
>>>>> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
>>>>> userspace arch: i386
>>>>>
>>>>> Unfortunately, I don't have any reproducer for this issue yet.
>>>>>
>>>>> Downloadable assets:
>>>>> disk image: https://storage.googleapis.com/syzbot-assets/b4f72e7a4c11/disk-f2f32f8a.raw.xz
>>>>> vmlinux: https://storage.googleapis.com/syzbot-assets/3f88997ad7c9/vmlinux-f2f32f8a.xz
>>>>> kernel image: https://storage.googleapis.com/syzbot-assets/b4b5b3963e2d/bzImage-f2f32f8a.xz
>>>>>
>>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>>>> Reported-by: syzbot+ca56f14c500045350f93@syzkaller.appspotmail.com
>>>>>
>>>>> ======================================================
>>>>> WARNING: possible circular locking dependency detected
>>>>> 6.1.0-rc3-syzkaller-00152-gf2f32f8af2b0 #0 Not tainted
>>>>> ------------------------------------------------------
>>>>> syz-executor.2/5665 is trying to acquire lock:
>>>>> ffff88801c74c298 (&mm->mmap_lock#2){++++}-{3:3}, at: __might_fault+0xa1/0x170 mm/memory.c:5645
>>>>>
>>>>> but task is already holding lock:
>>>>> ffff88801c4f3078 (&vma_lock->rw_sema){++++}-{3:3}, at: hugetlb_vma_lock_read mm/hugetlb.c:6816 [inline]
>>>>> ffff88801c4f3078 (&vma_lock->rw_sema){++++}-{3:3}, at: hugetlb_fault+0x40a/0x2060 mm/hugetlb.c:5859
>>>>>
>>>>> which lock already depends on the new lock.
>>>>>
>>>>>
>>>>> the existing dependency chain (in reverse order) is:
>>>>>
>>>>> -> #1 (&vma_lock->rw_sema){++++}-{3:3}:
>>>>>        down_write+0x90/0x220 kernel/locking/rwsem.c:1562
>>>>>        hugetlb_vma_lock_write mm/hugetlb.c:6834 [inline]
>>>>>        __unmap_hugepage_range_final+0x97/0x340 mm/hugetlb.c:5202
>>>>>        unmap_single_vma+0x23d/0x2a0 mm/memory.c:1690
>>>>>        unmap_vmas+0x21e/0x370 mm/memory.c:1733
>>>>>        exit_mmap+0x189/0x7a0 mm/mmap.c:3090
>>>>>        __mmput+0x128/0x4c0 kernel/fork.c:1185
>>>>>        mmput+0x5c/0x70 kernel/fork.c:1207
>>>>>        exit_mm kernel/exit.c:516 [inline]
>>>>>        do_exit+0xa39/0x2a20 kernel/exit.c:807
>>>>>        do_group_exit+0xd0/0x2a0 kernel/exit.c:950
>>>>>        get_signal+0x21a1/0x2430 kernel/signal.c:2858
>>>>>        arch_do_signal_or_restart+0x82/0x2300 arch/x86/kernel/signal.c:869
>>>>>        exit_to_user_mode_loop kernel/entry/common.c:168 [inline]
>>>>>        exit_to_user_mode_prepare+0x15f/0x250 kernel/entry/common.c:203
>>>>>        __syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
>>>>>        syscall_exit_to_user_mode+0x19/0x50 kernel/entry/common.c:296
>>>>>        __do_fast_syscall_32+0x72/0xf0 arch/x86/entry/common.c:181
>>>>>        do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:203
>>>>>        entry_SYSENTER_compat_after_hwframe+0x70/0x82
>>>>>
>>>>> -> #0 (&mm->mmap_lock#2){++++}-{3:3}:
>>>>>        check_prev_add kernel/locking/lockdep.c:3097 [inline]
>>>>>        check_prevs_add kernel/locking/lockdep.c:3216 [inline]
>>>>>        validate_chain kernel/locking/lockdep.c:3831 [inline]
>>>>>        __lock_acquire+0x2a43/0x56d0 kernel/locking/lockdep.c:5055
>>>>>        lock_acquire kernel/locking/lockdep.c:5668 [inline]
>>>>>        lock_acquire+0x1df/0x630 kernel/locking/lockdep.c:5633
>>>>>        __might_fault mm/memory.c:5646 [inline]
>>>>>        __might_fault+0x104/0x170 mm/memory.c:5639
>>>>>        _copy_from_user+0x25/0x170 lib/usercopy.c:13
>>>>>        copy_from_user include/linux/uaccess.h:161 [inline]
>>>>>        snd_rawmidi_kernel_write1+0x366/0x880 sound/core/rawmidi.c:1549
>>>>>        snd_rawmidi_write+0x273/0xbb0 sound/core/rawmidi.c:1618
>>>>>        vfs_write+0x2d7/0xdd0 fs/read_write.c:582
>>>>>        ksys_write+0x1e8/0x250 fs/read_write.c:637
>>>>>        do_syscall_32_irqs_on arch/x86/entry/common.c:112 [inline]
>>>>>        __do_fast_syscall_32+0x65/0xf0 arch/x86/entry/common.c:178
>>>>>        do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:203
>>>>>        entry_SYSENTER_compat_after_hwframe+0x70/0x82
>>>>>
>>>>> other info that might help us debug this:
>>>>>
>>>>>  Possible unsafe locking scenario:
>>>>>
>>>>>        CPU0                    CPU1
>>>>>        ----                    ----
>>>>>   lock(&vma_lock->rw_sema);
>>>>>                                lock(&mm->mmap_lock#2);
>>>>>                                lock(&vma_lock->rw_sema);
>>>>>   lock(&mm->mmap_lock#2);
>>>>
>>>> I may not be reading the report correctly, but I can not see how we acquire the
>>>> hugetlb vma_lock before trying to acquire mmap_lock in stack 0.  We would not
>>>> acquire the vma_lock until we enter hugetlb fault processing (not in the stack).
>>
>> The unlock of vma_lock is conditional:
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/hugetlb.c?id=f2f32f8af2b0ca9d619e5183eae3eed431793baf#n6840
>>
>> and the condition is:
>>
>> static bool __vma_shareable_flags_pmd(struct vm_area_struct *vma)
>> {
>>     return vma->vm_flags & (VM_MAYSHARE | VM_SHARED) &&
>>          vma->vm_private_data;
>> }
>>
>> Is it possible that the condition has changed between vma lock and unlock?
>> What mutexes protect vma->vm_flags/vm_private_data?
>>
>> That would make the report perfectly sensible.
>>
>> FWIW the test case that was running is this, that's the syscalls that
>> were running concurrently:
>>
>> 07:56:56 executing program 2:
>> r0 = syz_open_dev$sndmidi(&(0x7f0000000040), 0x2, 0x141101)
>> r1 = dup(r0)
>> setsockopt$inet_sctp_SCTP_I_WANT_MAPPED_V4_ADDR(r1, 0x84, 0xc,
>> &(0x7f0000000080), 0x4) (async)
>> write$6lowpan_enable(r1, &(0x7f0000000000)='0', 0xc86ade39) (async)
>> mmap(&(0x7f0000000000/0xb36000)=nil, 0xb36000, 0x3, 0x68831,
>> 0xffffffffffffffff, 0x0) (async)
>> madvise(&(0x7f0000000000/0x600000)=nil, 0x600003, 0x4) (async, rerun: 32)
>> mremap(&(0x7f00007a0000/0x3000)=nil, 0x3000, 0x2000, 0x7,
>> &(0x7f0000835000/0x2000)=nil) (rerun: 32)
> 
> 
> This new bug report seems to confirm the hypothesis:
> 
> WARNING: locking bug in hugetlb_no_page
> https://syzkaller.appspot.com/bug?extid=d07c65298d2c15eafcb0

Thanks Dmitry!

I believe Mike found the root cause and his proposed patches in next-20221111 will fix the problem:
  https://lore.kernel.org/linux-mm/20221111232628.290160-1-mike.kravetz@oracle.com/

Let's keep looking. :) Thanks for your work!

Thanks,
Miaohe Lin


> 
> 
>>>> Adding Miaohe Lin on Cc due to previous help with vma_lock potential deadlock
>>>> situations.  Miaohe, does this make sense to you?
>>>>
>>>
>>> Hi Mike,
>>>   This doesn't make sense for me too. Stack #1 shows that syz-executor is releasing
>>> its address space while stack #0 shows another thread is serving the write syscall.
>>> In this case, mm->mm_users is 0 and all threads in this process should be serving
>>> do_exit()? But I could be easily wrong. Also I can't see how vma_lock is locked before
>>> trying to acquire mmap_lock in above stacks. Might this be a false positive?
> .
>