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Date:   Thu, 21 Sep 2023 20:23:11 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     "Liam R. Howlett" <Liam.Howlett@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        maple-tree@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, pedro.falcato@gmail.com,
        stable <stable@kernel.org>
Subject: Re: [PATCH 0/2] maple_tree: Fix mas_prev() state regression.
Message-ID: <ZQyYH+5pnDc1KYj0@casper.infradead.org>
References: <20230921181236.509072-1-Liam.Howlett@oracle.com>
 <20230921112521.fc07c658804f4e188179da77@linux-foundation.org>
 <20230921185330.j5jw3oms4tc6crkf@revolver>
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On Thu, Sep 21, 2023 at 02:53:30PM -0400, Liam R. Howlett wrote:
> * Andrew Morton <akpm@linux-foundation.org> [230921 14:25]:
> > On Thu, 21 Sep 2023 14:12:34 -0400 "Liam R. Howlett" <Liam.Howlett@oracle.com> wrote:
> > 
> > > Pedro Falcato contacted me on IRC with an mprotect regression which was
> > > bisected back to the iterator changes for maple tree.  Root cause
> > > analysis showed the mas_prev() running off the end of the VMA space
> > > (previous from 0) followed by mas_find(), would skip the first value.
> > > 
> > > This patch set introduces maple state underflow/overflow so the sequence
> > > of calls on the maple state will return what the user expects.
> > 
> > It isn't clear what are the user-visible effects of this flaw?  Please
> > send this along and I'll paste it in.
> 
> 
> User may notice that mas_prev() or mas_next() calls that result in going
> outside of the limit passed to the call will cause incorrect returns on
> subsequent calls using that maple state, such as mas_find() skipping an
> entry.

When Andrew says "User visible" he means "userspace visible".  Not
"in kernel user visible".  What are the _consequences_.

I'd say that if the user maps something at address 0, mprotect() can
then fail to ... or something.