Re: [syzbot] possible deadlock in hugetlb_fault

[Miaohe Lin]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
>>
>> ======================================================
>> 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).
> 
> 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?

Thanks,
Miaohe Lin