linux-kernel - Re: [PATCH] fs/userfaultfd: Fix maple tree iterator in userfaultfd

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Message-ID: <20221107220401.dkhto2mp66tnmzuc@revolver>
Date:   Mon, 7 Nov 2022 22:04:11 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Linus Torvalds <torvalds@...uxfoundation.org>
CC:     "maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "syzbot+0d2014e4da2ccced5b41@...kaller.appspotmail.com" 
        <syzbot+0d2014e4da2ccced5b41@...kaller.appspotmail.com>
Subject: Re: [PATCH] fs/userfaultfd: Fix maple tree iterator in
 userfaultfd_unregister()

* Linus Torvalds <torvalds@...uxfoundation.org> [221107 16:14]:
> On Mon, Nov 7, 2022 at 12:12 PM Liam Howlett <liam.howlett@...cle.com> wrote:
> >
> > When iterating the VMAs, the maple state needs to be invalidated if the
> > tree is modified by a split or merge to ensure the maple tree node
> > contained in the maple state is still valid.  These invalidations were
> > missed, so add them to the paths which alter the tree.
> 
> I have applied this as an obvious fix, but I would *really* want to
> also see longer-term
> 
>  - I'd really like the 'mas' operations to have 'vma' specializations
> that get the type safety right
> 
>  - that mas_pause() name is horrible, please let's just fix it to
> something sensible in this context
> 
>  - moving the iterator invalidation into split_vma() and vma_merge()
> or at least have some other means of not having these mistakes
> 

Yes, we intend to do all these things in the 6.2 merge window.

> From what I can tell, things like mprotect() and mlock() - end up not
> using the iterator at all because of this issue. Instead they seem to
> just do
> 
>                 vma = find_vma(current->mm, prev->vm_end);
> 
> despite having actually started out with the whole iterator state.
> Except for 'apply_mlockall_flags()' that randomly does end up usign
> the iterator (and has that mas_pause() as a result).
> 
> So it would make *sense* to have "mlock_fixup()" take a MA_STATE
> instead of "vma, &prev" as arguments, but it doesn't.
> 
> I dunno. Maybe there's some other reason for this very non-intuitive
> mix of "sometimes iterators, sometimes not, and always horrible
> naming".
> 
>                 Linus


I will have a deeper look at the mprotect(), mlock() areas as well.  My
first pass on this was to just replace the easier loops with iterators.

Thanks,
Liam