Re: [ 06/89] device_cgroup: fix RCU usage

[Ben Hutchings]

On Thu, 2012-12-06 at 17:36 -0200, Herton Ronaldo Krzesinski wrote:
> On Mon, Dec 03, 2012 at 02:31:52PM +0000, Ben Hutchings wrote:
> > 3.2-stable review patch.  If anyone has any objections, please let me know.
> > 
> > ------------------
> > 
> > From: Tejun Heo <tj@xxxxxxxxxx>
> > 
> > commit 201e72acb2d3821e2de9ce6091e98859c316b29a upstream.
> > 
> > dev_cgroup->exceptions is protected with devcgroup_mutex for writes
> > and RCU for reads; however, RCU usage isn't correct.
> > 
> > * dev_exception_clean() doesn't use RCU variant of list_del() and
> >   kfree().  The function can race with may_access() and may_access()
> >   may end up dereferencing already freed memory.  Use list_del_rcu()
> >   and kfree_rcu() instead.
> > 
> > * may_access() may be called only with RCU read locked but doesn't use
> >   RCU safe traversal over ->exceptions.  Use list_for_each_entry_rcu().
> > 
> > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> > Acked-by: Serge E. Hallyn <serge.hallyn@xxxxxxxxxx>
> > Cc: Aristeu Rozanski <aris@xxxxxxxxxx>
> > Cc: Li Zefan <lizefan@xxxxxxxxxx>
> > [bwh: Backported to 3.2:
> >  - Adjust context
> >  - Exception list is called whitelist]
> > Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> 
> I belive this backport is uneeded for 3.2 (or any stable <= 3.6), since
> may_access_whitelist (may_access now in mainline) isn't called under
> rcu_read_lock.

I think you're right.  There are other readers that do rely on
rcu_read_lock(), but I don't think any of them can race with
devcgroup_destroy() (and if they can, it would have to use
kfree_rcu(dev_cgroup) as well).

You were too late for 3.2.35 but I'll revert this in the next update.

Ben.

> > ---
> >  security/device_cgroup.c |    6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > --- a/security/device_cgroup.c
> > +++ b/security/device_cgroup.c
> > @@ -202,8 +202,8 @@ static void devcgroup_destroy(struct cgr
> >  
> >  	dev_cgroup = cgroup_to_devcgroup(cgroup);
> >  	list_for_each_entry_safe(wh, tmp, &dev_cgroup->whitelist, list) {
> > -		list_del(&wh->list);
> > -		kfree(wh);
> > +		list_del_rcu(&wh->list);
> > +		kfree_rcu(wh, rcu);
> >  	}
> >  	kfree(dev_cgroup);
> >  }
> > @@ -278,7 +278,7 @@ static int may_access_whitelist(struct d
> >  {
> >  	struct dev_whitelist_item *whitem;
> >  
> > -	list_for_each_entry(whitem, &c->whitelist, list) {
> > +	list_for_each_entry_rcu(whitem, &c->whitelist, list) {
> >  		if (whitem->type & DEV_ALL)
> >  			return 1;
> >  		if ((refwh->type & DEV_BLOCK) && !(whitem->type & DEV_BLOCK))
> > 
> > 
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> > 
> 

-- 
Ben Hutchings
Computers are not intelligent.	They only think they are.

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