linux-kernel - Re: [PATCH 1/3] mm: ensure safe rb_subtree

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Message-ID: <50A0FC2F.9030207@redhat.com>
Date:	Mon, 12 Nov 2012 08:39:59 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Michel Lespinasse <walken@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Sasha Levin <levinsasha928@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/3] mm: ensure safe rb_subtree_gap update when inserting
 new VMA

On 11/12/2012 06:51 AM, Michel Lespinasse wrote:
> Using the trinity fuzzer, Sasha Levin uncovered a case where
> rb_subtree_gap wasn't correctly updated.
>
> Digging into this, the root cause was that vma insertions and removals
> require both an rbtree insert or erase operation (which may trigger
> tree rotations), and an update of the next vma's gap (which does not
> change the tree topology, but may require iterating on the node's
> ancestors to propagate the update). The rbtree rotations caused the
> rb_subtree_gap values to be updated in some of the internal nodes, but
> without upstream propagation. Then the subsequent update on the next
> vma didn't iterate as high up the tree as it should have, as it
> stopped as soon as it hit one of the internal nodes that had been
> updated as part of a tree rotation.
>
> The fix is to impose that all rb_subtree_gap values must be up to date
> before any rbtree insertion or erase, with the possible exception that
> the node being erased doesn't need to have an up to date rb_subtree_gap.
>
> This change: during vma insertion, make sure to update the rb_subtree_gap
> values for both the current and next vmas prior to rebalancing the rbtree
> to account for the just-inserted vma.
>
> (Thanks to Sasha Levin for uncovering the problem and to Hugh Dickins
> for coming up with a simpler test case)
>
> Reported-by: Sasha Levin <sasha.levin@...cle.com>
> Signed-off-by: Michel Lespinasse <walken@...gle.com>

Reviewed-by: Rik van Riel <riel@...hat.com>


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