Re: [syzbot] possible deadlock in hugetlb_fault

Dmitry Vyukov
2022-11-12T14:14:33Z

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

> > > 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?