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Subject: Re: [PATCH 1/2] of: of_node_get()/of_node_put() nodes held in phandle
 cache
To:     Rob Herring <robh+dt@kernel.org>
Cc:     mwb@linux.vnet.ibm.com,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Tyrel Datwyler <tyreld@linux.vnet.ibm.com>,
        tlfalcon@linux.vnet.ibm.com, minkim@us.ibm.com,
        devicetree@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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 <35cab334-0856-44e1-b18e-22668011b429@gmail.com>
From:   Frank Rowand <frowand.list@gmail.com>
Message-ID: <cb7caa9a-abcb-93c2-a3e9-185133f1241a@gmail.com>
Date:   Fri, 14 Dec 2018 15:04:58 -0800
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On 12/14/18 2:47 PM, Frank Rowand wrote:
> On 12/14/18 9:15 AM, Rob Herring wrote:
>> On Fri, Dec 14, 2018 at 12:43 AM <frowand.list@gmail.com> wrote:
>>>
>>> From: Frank Rowand <frank.rowand@sony.com>
>>>
>>> The phandle cache contains struct device_node pointers.  The refcount
>>> of the pointers was not incremented while in the cache, allowing use
>>> after free error after kfree() of the node.  Add the proper increment
>>> and decrement of the use count.
>>
>> Since we pre-populate the cache at boot, all the nodes will have a ref
>> count and will never be freed unless we happen to repopulate the whole

>> cache. That doesn't seem ideal. The node pointer is not "in use" just
>> because it is in the cache.

I forgot to reply to this sentence.

The node pointers are "in use" because of_find_node_by_phandle() will
use the pointers to access the phandle field.  This is a use after
free bug if the node has been kfree()'ed.


>>
>> Rob
>>
> 
> This patch also adds of_node_put() so that the refcount will go to zero
> when the node is removed as part of an overlay remove, if the node was
> added by an overlay.
> 
> Patch 2/2 adds the free cache entry call to __of_detach_node(), so the
> refcount will go to zero when the node is removed for dynamic use cases
> other than overlays.  (For overlays, all nodes are instead removed from
> the cache before __of_detach_node() is called.)
> 
> -Frank
>