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

[Andrew Morton]

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?

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