Re: get_arg_page() && ptr_size accounting

[Oleg Nesterov <oleg@redhat.com>]

On 09/10, Oleg Nesterov wrote:
>
> On 09/10, Kees Cook wrote:
> >
> > On Mon, Sep 10, 2018 at 9:41 AM, Kees Cook <keescook@chromium.org> wrote:
> > > On Mon, Sep 10, 2018 at 5:29 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> > >> Hi Kees,
> > >>
> > >> I was thinking about backporting the commit 98da7d08850fb8bde
> > >> ("fs/exec.c: account for argv/envp pointers"), but I am not sure
> > >> I understand it...
> >
> > BTW, if you backport that, please get the rest associated with the
> > various Stack Clash related weaknesses:
>
> may be...
>
> > da029c11e6b1 exec: Limit arg stack to at most 75% of _STK_LIM
>
> and I have to admit that I do not understand this patch at all, the
> changelog explains nothing.
>
> Could you explain what this patch actually prevents from? Especially
> now that we have stack_guard_gap?

forgot to mention...

with this patch

	#define MAX_ARG_STRINGS 0x7FFFFFFF

doesn't match the reality. perhaps something like below makes sense just
to make it clear, but this is cosmetic.

Oleg.

--- x/fs/exec.c
+++ x/fs/exec.c
@@ -1789,11 +1789,13 @@ static int __do_execve_file(int fd, stru
 	if (retval)
 		goto out_unmark;
 
-	bprm->argc = count(argv, MAX_ARG_STRINGS);
+	int max_arg_strings = _STK_LIM / 4 * 3 / 2; // actually even less than
+
+	bprm->argc = count(argv, max_arg_strings);
 	if ((retval = bprm->argc) < 0)
 		goto out;
 
-	bprm->envc = count(envp, MAX_ARG_STRINGS);
+	bprm->envc = count(envp, max_arg_strings - bprm->argc);
 	if ((retval = bprm->envc) < 0)
 		goto out;