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

[Alexey Dobriyan <adobriyan@gmail.com>]

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