linux-kernel - Re: [PATCH RFC] rcu: assoc_array: Add critical section to avoid a bug

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Date:	Thu, 28 Aug 2014 12:32:31 +0100
From:	David Howells <dhowells@...hat.com>
To:	Andreea-Cristina Bernat <bernat.ada@...il.com>
Cc:	dhowells@...hat.com, shemming@...cade.com,
	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH RFC] rcu: assoc_array: Add critical section to avoid a bug

Andreea-Cristina Bernat <bernat.ada@...il.com> wrote:

> * The function "assoc_array_gc()" could be preempted between the call to
> "assoc_array_apply_edit()" call and the assignment
> "edit->array->nr_leaves_on_tree = nr_leaves_on_tree;", so the grace
> period could complete.

The bug is real, but this patch isn't the right solution.

The edit script should be considered inaccessible to a function the moment it
calls assoc_array_apply_edit() or assoc_array_cancel_edit().  I think this is
a better way.

David
---
 assoc_array.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

commit 10c26fcc224c0515e15272515e7b9006cb08adc8
Author: David Howells <dhowells@...hat.com>
Date:   Wed Aug 27 18:39:44 2014 +0100

    KEYS: Fix use-after-free in assoc_array_gc()
    
    An edit script should be considered inaccessible by a function once it has
    called assoc_array_apply_edit() or assoc_array_cancel_edit().
    
    However, assoc_array_gc() is accessing the edit script just after the
    gc_complete: label.
    
    Reported-by: Andreea-Cristina Bernat <bernat.ada@...il.com>
    Signed-off-by: David Howells <dhowells@...hat.com>

diff --git a/lib/assoc_array.c b/lib/assoc_array.c
index c0b1007011e1..ae146f0734eb 100644
--- a/lib/assoc_array.c
+++ b/lib/assoc_array.c
@@ -1735,7 +1735,7 @@ ascend_old_tree:
 gc_complete:
 	edit->set[0].to = new_root;
 	assoc_array_apply_edit(edit);
-	edit->array->nr_leaves_on_tree = nr_leaves_on_tree;
+	array->nr_leaves_on_tree = nr_leaves_on_tree;
 	return 0;
 
 enomem:
--
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