linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Willy Tarreau <w@1wt.eu>
To: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	dan.carpenter@oracle.com, will@kernel.org, ebiederm@xmission.com,
	linux-arch@vger.kernel.org, security@kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] execve: warn if process starts with executable stack
Date: Wed, 11 Dec 2019 10:59:37 +0100	[thread overview]
Message-ID: <20191211095937.GB31670@1wt.eu> (raw)
In-Reply-To: <20191211072225.GB3700@avx2>

On Wed, Dec 11, 2019 at 10:22:25AM +0300, Alexey Dobriyan wrote:
> On Tue, Dec 10, 2019 at 05:47:26PM -0800, Andrew Morton wrote:
> > On Sun, 8 Dec 2019 20:19:18 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote:
> > 
> > > There were few episodes of silent downgrade to an executable stack over
> > > years:
> > > 
> > > 1) linking innocent looking assembly file will silently add executable
> > >    stack if proper linker options is not given as well:
> > > 
> > > 	$ cat f.S
> > > 	.intel_syntax noprefix
> > > 	.text
> > > 	.globl f
> > > 	f:
> > > 	        ret
> > > 
> > > 	$ cat main.c
> > > 	void f(void);
> > > 	int main(void)
> > > 	{
> > > 	        f();
> > > 	        return 0;
> > > 	}
> > > 
> > > 	$ gcc main.c f.S
> > > 	$ readelf -l ./a.out
> > > 	  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
> > >                          0x0000000000000000 0x0000000000000000  RWE    0x10
> > > 			 					 ^^^
> > > 
> > > 2) converting C99 nested function into a closure
> > > https://nullprogram.com/blog/2019/11/15/
> > > 
> > > 	void intsort2(int *base, size_t nmemb, _Bool invert)
> > > 	{
> > > 	    int cmp(const void *a, const void *b)
> > > 	    {
> > > 	        int r = *(int *)a - *(int *)b;
> > > 	        return invert ? -r : r;
> > > 	    }
> > > 	    qsort(base, nmemb, sizeof(*base), cmp);
> > > 	}
> > > 
> > > will silently require stack trampolines while non-closure version will not.
> > > 
> > > Without doubt this behaviour is documented somewhere, add a warning so that
> > > developers and users can at least notice. After so many years of x86_64 having
> > > proper executable stack support it should not cause too many problems.
> > 
> > hm, OK, let's give it a trial run.
> > 
> > > --- a/fs/exec.c
> > > +++ b/fs/exec.c
> > > @@ -761,6 +761,11 @@ int setup_arg_pages(struct linux_binprm *bprm,
> > >  		goto out_unlock;
> > >  	BUG_ON(prev != vma);
> > >  
> > > +	if (unlikely(vm_flags & VM_EXEC)) {
> > > +		pr_warn_once("process '%pD4' started with executable stack\n",
> > > +			     bprm->file);
> > > +	}
> > > +
> > >  	/* Move stack pages down in memory. */
> > >  	if (stack_shift) {
> > >  		ret = shift_arg_pages(vma, stack_shift);
> > 
> > What are poor users supposed to do if this message comes out? 
> > Hopefully google the message and end up at this thread.  What do you
> > want to tell them?
> 
> Me? Nothing :-) They hopefully should file tickets against distros and ISV,
> post egregious examples to oss-security.
> 
> Like they already do against this warning!
> > ACPI: [Firmware Bug]: BIOS _OSI(Linux) query ignored

Alexey, Andrew is right. Your message gives no instruction and users are
already flooded with messages they got used to ignore. A warning is made
to catch attention so it should give instructions. It can either say
"this application relies on insecure capabilities and might not work
anymore in the future, you should report this to its author", or "report
this to kernel developers if you think this warning is inappropriate".
Otherwise it will just go to /dev/null with all warning about bad blocks
on USB sticks and CPU core throttling under high temperature.

Willy

  reply	other threads:[~2019-12-11  9:59 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-12-08 17:19 Alexey Dobriyan
2019-12-11  1:47 ` Andrew Morton
2019-12-11  7:22   ` Alexey Dobriyan
2019-12-11  9:59     ` Willy Tarreau [this message]
2019-12-11 18:19       ` Alexey Dobriyan
2019-12-11 18:24         ` Willy Tarreau
2019-12-12 21:25           ` Alexey Dobriyan
2019-12-13  9:56             ` Dan Carpenter
2019-12-13 10:23               ` Willy Tarreau
2020-02-25 21:52   ` Kees Cook

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20191211095937.GB31670@1wt.eu \
    --to=w@1wt.eu \
    --cc=adobriyan@gmail.com \
    --cc=akpm@linux-foundation.org \
    --cc=dan.carpenter@oracle.com \
    --cc=ebiederm@xmission.com \
    --cc=linux-arch@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=security@kernel.org \
    --cc=will@kernel.org \
    --subject='Re: [PATCH v2] execve: warn if process starts with executable stack' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).