Re: [PATCH] user_ns: use correct check for single-threadedness

[Kirill A. Shutemov]

On Tue, Jul 28, 2015 at 02:35:04PM -0700, Andrew Morton wrote:
> On Tue, 28 Jul 2015 10:15:00 -0700 Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 
> > From: Ricky Zhou <rickyz@xxxxxxxxxxxx>
> > 
> > Checking mm_users > 1 does not mean a process is multithreaded. For
> > example, reading /proc/PID/maps temporarily increments mm_users, allowing
> > other processes to (accidentally) interfere with unshare() calls.
> > 
> > This fixes observed failures of unshare(CLONE_NEWUSER) incorrectly
> > returning EINVAL if another processes happened to be simultaneously
> > reading the maps file.
> 
> Yikes.  current_is_single_threaded() is expensive.  Are we sure this
> isn't going to kill someone's workload?

It's expensive only if mm_users > 1. We will go to for_each_process() only
if somebody outside of the process grabs mm_users references (like reading
/proc/PID/maps). Or if it called it from multithreaded application.

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov
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