linux-kernel - Re: [PATCH 2/4] test: firmware_class: add asynchronous request trigger

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Date:	Wed, 9 Dec 2015 14:12:20 -0800
From:	Brian Norris <computersforpeace@...il.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	Shuah Khan <shuahkh@....samsung.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ming Lei <ming.lei@...onical.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"Luis R. Rodriguez" <mcgrof@...e.com>
Subject: Re: [PATCH 2/4] test: firmware_class: add asynchronous request
 trigger

On Wed, Dec 09, 2015 at 02:05:17PM -0800, Kees Cook wrote:
> On Wed, Dec 9, 2015 at 1:48 PM, Brian Norris
> <computersforpeace@...il.com> wrote:
> > I was also thinking, since use-after-free isn't necessarily immediately
> > obvious (this worked fine in my testing), that maybe we could poison the
> > buffer before kfree()'ing? Like:
> >
> >         name = ...;
> >         len = strlen(name);
> >
> >         ...
> >
> >         rc = request_firmware_nowait(...);
> >         if (rc) {
> >                 pr_info("...");
> >                 kfree(name);
> >                 goto out;
> >         }
> >         /*
> >          * Clear out the name, to test for race conditions with the
> >          * async request
> >          */
> >         memset(name, 0, len);
> >         kfree(name);
> 
> Hrm, well, I'm not against it, but I think running under KASan is
> probably the right way to find these things. But, might as well, just
> to notice any regressions.

Fair enough. The memset probably isn't that useful.

BTW, one reason I didn't notice my use-after-free is that the "use" was
under an error path that I didn't exercise. I need to remember to turn
my brain back on.

Brian
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