Re: possible deadlock in shmem_mfill_atomic_pte

[Hugh Dickins]

On Mon, 13 Apr 2020, Yang Shi wrote:
> On Tue, Mar 31, 2020 at 10:21 AM syzbot
> <syzbot+e27980339d305f2dbfd9@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > Hello,
> >
> > syzbot found the following crash on:
> >
> > HEAD commit:    527630fb Merge tag 'clk-fixes-for-linus' of git://git.kern..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1214875be00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=27392dd2975fd692
> > dashboard link: https://syzkaller.appspot.com/bug?extid=e27980339d305f2dbfd9
> > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> >
> > Unfortunately, I don't have any reproducer for this crash yet.
> >
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+e27980339d305f2dbfd9@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > WARNING: possible irq lock inversion dependency detected
> > 5.6.0-rc7-syzkaller #0 Not tainted
> > --------------------------------------------------------
> > syz-executor.0/10317 just changed the state of lock:
> > ffff888021d16568 (&(&info->lock)->rlock){+.+.}, at: spin_lock include/linux/spinlock.h:338 [inline]
> > ffff888021d16568 (&(&info->lock)->rlock){+.+.}, at: shmem_mfill_atomic_pte+0x1012/0x21c0 mm/shmem.c:2407
> > but this lock was taken by another, SOFTIRQ-safe lock in the past:
> >  (&(&xa->xa_lock)->rlock#5){..-.}
> >
> >
> > and interrupts could create inverse lock ordering between them.
> >
> >
> > other info that might help us debug this:
> >  Possible interrupt unsafe locking scenario:
> >
> >        CPU0                    CPU1
> >        ----                    ----
> >   lock(&(&info->lock)->rlock);
> >                                local_irq_disable();
> >                                lock(&(&xa->xa_lock)->rlock#5);
> >                                lock(&(&info->lock)->rlock);
> >   <Interrupt>
> >     lock(&(&xa->xa_lock)->rlock#5);
> >
> >  *** DEADLOCK ***
> 
> This looks possible. shmem_mfill_atomic_pte() acquires info->lock with
> irq enabled.
> 
> The below patch should be able to fix it:

I agree, thank you: please send to akpm with your signoff and

Reported-by: syzbot+e27980339d305f2dbfd9@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
Acked-by: Hugh Dickins <hughd@xxxxxxxxxx>

I bet that 4.11 commit was being worked on before 4.8 reversed the
ordering of info->lock and tree_lock, changing spin_lock(&info->lock)s
to spin_lock_irq*(&info->lock)s - this one is the only hold-out; and
not using userfaultfd, I wouldn't have seen the lockdep report.

> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index d722eb8..762da6a 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2399,11 +2399,11 @@ static int shmem_mfill_atomic_pte(struct
> mm_struct *dst_mm,
> 
>         lru_cache_add_anon(page);
> 
> -       spin_lock(&info->lock);
> +       spin_lock_irq(&info->lock);
>         info->alloced++;
>         inode->i_blocks += BLOCKS_PER_PAGE;
>         shmem_recalc_inode(inode);
> -       spin_unlock(&info->lock);
> +       spin_unlock_irq(&info->lock);
> 
>         inc_mm_counter(dst_mm, mm_counter_file(page));
>         page_add_file_rmap(page, false);