linux-kernel - Re: [PATCH] sched: fix a potential double-fetch bug in sched_copy

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Message-ID: <20190107171101.GI14122@hirez.programming.kicks-ass.net>
Date:   Mon, 7 Jan 2019 18:11:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kangjie Lu <kjlu@....edu>
Cc:     pakki001@....edu, Ingo Molnar <mingo@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: fix a potential double-fetch bug in
 sched_copy_attr

On Tue, Dec 25, 2018 at 04:16:47PM -0600, Kangjie Lu wrote:
> "uattr->size" is copied in from user space and checked. However, it is
> copied in again after the security check. A malicious user may race to
> change it. The fix checks if uattr->size is ever changed after the
> check.
> 
> Signed-off-by: Kangjie Lu <kjlu@....edu>
> ---

> +	/* Sanity check if size was changed in user space */
> +	if (attr->size != size)
> +		return -EINVAL;
> +

What perf_copy_attr() does (from whence we copied this code) is:

	attr->size = size;

Would that not also fix things?