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

On Tue, Dec 10, 2019 at 05:47:26PM -0800, Andrew Morton wrote:

On Sun, 8 Dec 2019 20:19:18 +0300 Alexey Dobriyan [off-list ref] wrote:

quoted

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.

quoted

--- 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