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