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Message-ID: <20230727164757.e2di75xjybxncohn@revolver>
Date: Thu, 27 Jul 2023 12:47:57 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, syzkaller-bugs@...glegroups.com,
willy@...radead.org,
syzbot <syzbot+8645fe63c4d22c8d27b8@...kaller.appspotmail.com>
Subject: Re: [syzbot] [mm?] WARNING: suspicious RCU usage in mas_walk (2)
* syzbot <syzbot+8645fe63c4d22c8d27b8@...kaller.appspotmail.com> [230726 02:57]:
> syzbot has bisected this issue to:
>
> commit a52f58b34afe095ebc5823684eb264404dad6f7b
> Author: Matthew Wilcox (Oracle) <willy@...radead.org>
> 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@...kaller.appspotmail.com
> 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
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