From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: [PATCH] exec: do not leave bprm->interp on stack Date: Thu, 25 Oct 2012 05:16:21 +0100 Message-ID: <20121025041620.GH2616@ZenIV.linux.org.uk> References: <20121024232032.GA31129@www.outflux.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-kernel@vger.kernel.org, Andrew Morton , Josh Triplett , Serge Hallyn , linux-fsdevel@vger.kernel.org, halfdog To: Kees Cook Return-path: Content-Disposition: inline In-Reply-To: <20121024232032.GA31129@www.outflux.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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.