linux-kernel - Re: [PATCH v2] execve: warn if process starts with executable stack

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Date:   Tue, 10 Dec 2019 17:47:26 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     dan.carpenter@...cle.com, will@...nel.org, ebiederm@...ssion.com,
        linux-arch@...r.kernel.org, security@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] execve: warn if process starts with executable stack

On Sun, 8 Dec 2019 20:19:18 +0300 Alexey Dobriyan <adobriyan@...il.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?