On Wed, Nov 20, 2019 at 02:28:37PM -0600, Eric W. Biederman wrote:
> Will Deacon <will@xxxxxxxxxx> writes:
>
> > 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@xxxxxxxxx>
> >> ---
> >>
> >> 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?
>
> In what sense? Are you thinking about the printing of the pid?
>
> Pretty much by fiat and by definition the kernel log always print things
> in the initial pid namespace. Which this printk does.
Ok, fair enough. Just wanted to make sure it was ok, since we're not using
a task_pid_nr*() accessor and it might have been overlooked.
Will
Will
