linux-kernel - Re: [2.6 patch] acpi/ec.c: fix use-after-free

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Date:	Wed, 24 Oct 2007 19:26:04 +0200
From:	Adrian Bunk <bunk@...nel.org>
To:	Alexey Starikovskiy <aystarik@...il.com>
Cc:	Alexey Starikovskiy <astarikovskiy@...e.de>,
	Len Brown <len.brown@...el.com>, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [2.6 patch] acpi/ec.c: fix use-after-free

On Wed, Oct 24, 2007 at 09:15:18PM +0400, Alexey Starikovskiy wrote:
> Adrian,
> 
> commit 30c08574da0ead1a47797ce028218ce5b2de61c7 can not introduce use-after-free.
> 
> Please check...


Commit 30c08574da0ead1a47797ce028218ce5b2de61c7 did:

<--  snip  -->

 	list_for_each_entry(handler, &ec->list, node) {
                if (query_bit == handler->query_bit) {
                        list_del(&handler->node);
                        kfree(handler);
-                       break;
                }
	}

<--  snip  -->


If you look at the definition of list_for_each_entry() in 
include/linux/list.h:

<--  snip  -->

#define list_for_each_entry(pos, head, member)                          \
        for (pos = list_entry((head)->next, typeof(*pos), member);      \
             prefetch(pos->member.next), &pos->member != (head);        \
             pos = list_entry(pos->member.next, typeof(*pos), member))
                              ^^^^^^^^^^^^^^^^

<--  snip  -->


Without the "break", "handler" is being dereferenced after it was freed.


> Regards,
> Alex.
> Adrian Bunk wrote:
> > This patch fixes a use-after-free introduced by
> > commit 30c08574da0ead1a47797ce028218ce5b2de61c7.
> > 
> > Spotted by the Coverity checker.
> > 
> > Signed-off-by: Adrian Bunk <bunk@...nel.org>
> > 
> > ---
> > --- linux-2.6/drivers/acpi/ec.c.old	2007-10-23 19:39:47.000000000 +0200
> > +++ linux-2.6/drivers/acpi/ec.c	2007-10-23 19:34:55.000000000 +0200
> > @@ -434,11 +442,11 @@
> >  EXPORT_SYMBOL_GPL(acpi_ec_add_query_handler);
> >  
> >  void acpi_ec_remove_query_handler(struct acpi_ec *ec, u8 query_bit)
> >  {
> > -	struct acpi_ec_query_handler *handler;
> > +	struct acpi_ec_query_handler *handler, *tmp;
> >  	mutex_lock(&ec->lock);
> > -	list_for_each_entry(handler, &ec->list, node) {
> > +	list_for_each_entry_safe(handler, tmp, &ec->list, node) {
> >  		if (query_bit == handler->query_bit) {
> >  			list_del(&handler->node);
> >  			kfree(handler);
> >  		}
> > 


cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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