On Thu, Nov 21, 2019 at 09:38:06AM +0000, Will Deacon wrote:
> 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.
PID is printed both as ->pid and a task_pid_vnr().
I'll just print filename, so that executable can be easily found.
