Re: [PATCH 1/2] of: of_node_get()/of_node_put() nodes held in phandle cache

From: Frank Rowand
Date: Fri Dec 14 2018 - 18:05:06 EST

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@xxxxxxxxx> wrote:
>>> From: Frank Rowand <frank.rowand@xxxxxxxx>
>>> 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