Re: [syzbot] [mm?] WARNING: suspicious RCU usage in mas_walk (2)

[Liam R. Howlett]

* syzbot <syzbot+8645fe63c4d22c8d27b8@xxxxxxxxxxxxxxxxxxxxxxxxx> [230726 02:57]:
> syzbot has bisected this issue to:
> 
> commit a52f58b34afe095ebc5823684eb264404dad6f7b
> Author: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Date:   Mon Jul 24 18:54:10 2023 +0000
> 
>     mm: handle faults that merely update the accessed bit under the VMA lock
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1783585ea80000
> start commit:   [unknown] 
> git tree:       linux-next
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=1443585ea80000
> console output: https://syzkaller.appspot.com/x/log.txt?x=1043585ea80000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f481ab36ce878b84
> dashboard link: https://syzkaller.appspot.com/bug?extid=8645fe63c4d22c8d27b8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1697cec9a80000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1566986ea80000
> 
> Reported-by: syzbot+8645fe63c4d22c8d27b8@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: a52f58b34afe ("mm: handle faults that merely update the accessed bit under the VMA lock")
> 
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection

This is caused by walking the maple tree without holding the mmap or rcu
read lock when per-vma locking is used for the page fault.

We could wrap the find_mergeable_anon_vma() walk with an rcu read lock,
but I am unsure if that's the correct way to handle this as the anon_vma
lock is taken later in __anon_vma_prepare().  Note that the anon_vma
lock is per-anon_vma, so we cannot just relocate that lock.

I'm wondering if we need find_mergeable_anon_vma() to take a read lock
on the VMA which contains the anon_vma to ensure it doesn't go away?
Maybe a find_and_lock_mergeable_anon_vma() and return a locked anon_vma?
Basically lock_vma_under_rcu(), anon_vma_lock_write(), vma_end_read().

Thoughts?

Thanks,
Liam