linux-kernel - Re: [PATCH 0/2] maple_tree: Fix mas

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Date:   Thu, 21 Sep 2023 20:23:11 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        maple-tree@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, pedro.falcato@...il.com,
        stable <stable@...nel.org>
Subject: Re: [PATCH 0/2] maple_tree: Fix mas_prev() state regression.

On Thu, Sep 21, 2023 at 02:53:30PM -0400, Liam R. Howlett wrote:
> * Andrew Morton <akpm@...ux-foundation.org> [230921 14:25]:
> > On Thu, 21 Sep 2023 14:12:34 -0400 "Liam R. Howlett" <Liam.Howlett@...cle.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.