linux-kernel - Re: [PATCH] exec: fix linux_binprm::exec in transfer_args_to

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Date: Fri, 22 Mar 2024 09:39:43 -0700
From: Max Filippov <jcmvbkbc@...il.com>
To: Kees Cook <keescook@...omium.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	linux-fsdevel@...r.kernel.org, Eric Biederman <ebiederm@...ssion.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Rich Felker <dalias@...c.org>, stable@...r.kernel.org
Subject: Re: [PATCH] exec: fix linux_binprm::exec in transfer_args_to_stack()

On Thu, Mar 21, 2024 at 8:48 PM Kees Cook <keescook@...omium.org> wrote:
> On Thu, Mar 21, 2024 at 12:52:16PM -0700, Max Filippov wrote:
> > On Thu, Mar 21, 2024 at 10:05 AM Kees Cook <keescook@...omium.org> wrote:
> > > What's the best way to test this? (Is there a qemu setup I can use to
> > > see the before/after of AT_EXECFN?)
> >
> > I put a readme with the steps to build such system here:
> >   http://jcmvbkbc.org/~dumb/tmp/202403211236/README
> > it uses a prebuilt rootfs image and a 6.8 kernel branch with two
> > patches on top of it: one adds a dts and a defconfig and the other
> > is this fix. The rootfs boots successfully with this fix, but panics
> > if this fix is removed.
>
> Does musl have something like the LD_SHOW_AUXV env variable. With glibc,
> I usually explore auxv like so:
>
> $ LD_SHOW_AUXV=1 uname -a | grep EXECFN
> AT_EXECFN:            /usr/bin/uname

I couldn't find anything like that in either musl or uClibc-ng.
So I updated the above rootfs and put the following program
into it as /bin/test-auxv:

#include <stdio.h>
#include <sys/auxv.h>

int main()
{
       unsigned long p = getauxval(AT_EXECFN);
       fprintf(stderr, "AT_EXECFN: 0x%lx\n", p);
       if (p)
               fprintf(stderr, "%s\n", (const char *)p);
       return 0;
}

While looking at it I also noticed that /proc/<pid>/auxv is empty
on NOMMU, looks like there will be yet another fix for that.

-- 
Thanks.
-- Max