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Date: Thu, 25 Oct 2012 05:16:21 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Serge Hallyn <serge.hallyn@canonical.com>,
        linux-fsdevel@vger.kernel.org, halfdog <me@halfdog.net>
Subject: Re: [PATCH] exec: do not leave bprm->interp on stack
Message-ID: <20121025041620.GH2616@ZenIV.linux.org.uk>
References: <20121024232032.GA31129@www.outflux.net>
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On Wed, Oct 24, 2012 at 04:20:32PM -0700, Kees Cook wrote:
> If a series of scripts are executed, each triggering module loading via
> unprintable bytes in the script header, kernel stack contents can leak
> into the command line.
> 
> Normally execution of binfmt_script and binfmt_misc happens
> recursively. However, when modules are enabled, and unprintable bytes
> exist in the bprm->buf, execution will restart after attempting to load
> matching binfmt modules. Unfortunately, the logic in binfmt_script and
> binfmt_misc does not expect to get restarted. They leave bprm->interp
> pointing to their local stack. This means on restart bprm->interp is
> left pointing into unused stack memory which can then be copied into
> the userspace argv areas.
> 
> This changes the logic to require allocation for any changes to the
> bprm->interp. To avoid adding a new kmalloc to every exec, the default
> value is left as-is. Only when passing through binfmt_script or
> binfmt_misc does an allocation take place.

I really don't like that.  It papers over the problem, but doesn't really
solve the underlying stupidity.  We have no good reason to retry a binfmt
we'd already attempted on this level of recursion.  And your patch doesn't
deal with that at all.