linux-kernel - Re: [PATCH] ELF: warn if process starts with executable stack

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Message-ID: <20191118213603.GA24086@avx2>
Date:   Tue, 19 Nov 2019 00:36:03 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        security@...nel.org, ben.dooks@...ethink.co.uk
Subject: Re: [PATCH] ELF: warn if process starts with executable stack

On Mon, Nov 18, 2019 at 12:54:57PM -0800, Andrew Morton wrote:
> On Mon, 18 Nov 2019 17:51:15 +0300 Alexey Dobriyan <adobriyan@...il.com> wrote:
> 
> > PT_GNU_STACK is fail open design,
> 
> Not sure what this means.  Please expand on the motivation for this
> change.
> 
> > at least warn people that something
> > isn't right.
> 
> People who use an executable stack get a kernel splat.  How is that
> useful?

There were two stories about silent downgrade to an executable stack:

1)
compiling .S file and linking it to normal code:

	$ cat f.S
	.intel_syntax noprefix
	.text
	.globl f
	f:
	        ret

will silently add PT_GNU_STACK segment with RWE permissions

2)
closures with nested functions will require executable stack
https://nullprogram.com/blog/2019/11/15/

> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -762,6 +762,13 @@ int setup_arg_pages(struct linux_binprm *bprm,
> >  		goto out_unlock;
> >  	BUG_ON(prev != vma);
> >  
> > +#ifdef CONFIG_MMU

This code is already under CONFIG_MMU. I'll resend.