Re: [PATCH] exec: warn if process starts with executable stack

[Will Deacon <will@kernel.org>]

On Tue, Nov 19, 2019 at 12:52:27AM +0300, Alexey Dobriyan wrote:
> There were few episodes of silent downgrade to an executable stack:
> 
> 1) linking innocent looking assembly file
> 
> 	$ 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.
> 
> While without a double 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 much problems.
> 
> If the system is old or CPU is old, then there will be an early warning
> against init and/or support personnel will write that "uh-oh, our Enterprise
> Software absolutely requires executable stack" and close tickets and customers
> will nod heads and life moves on.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> ---
> 
>  fs/exec.c |    5 +++++
>  1 file changed, 5 insertions(+)
> 
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -762,6 +762,11 @@ int setup_arg_pages(struct linux_binprm *bprm,
>  		goto out_unlock;
>  	BUG_ON(prev != vma);
>  
> +	if (vm_flags & VM_EXEC) {
> +		pr_warn_once("process '%s'/%u started with executable stack\n",
> +			     current->comm, current->pid);
> +	}

Given that this is triggerable by userspace, is there a concern about PID
namespaces here?

Will