linux-kernel - Re: [PATCHv3 5/8] x86/uaccess: Provide untagged_addr() and remove tags before address check

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Message-ID: <Yqr5FmoBB5LZRhU5@hirez.programming.kicks-ass.net>
Date:   Thu, 16 Jun 2022 11:34:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "Lutomirski, Andy" <luto@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "kcc@...gle.com" <kcc@...gle.com>,
        "andreyknvl@...il.com" <andreyknvl@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "ryabinin.a.a@...il.com" <ryabinin.a.a@...il.com>,
        "glider@...gle.com" <glider@...gle.com>
Subject: Re: [PATCHv3 5/8] x86/uaccess: Provide untagged_addr() and remove
 tags before address check

On Mon, Jun 13, 2022 at 05:36:43PM +0000, Edgecombe, Rick P wrote:

> Is this special kernel address handling only needed because
> copy_to_kernel_nofault(), etc call the user helpers?

It is to make absolutely sure we don't need to go audit everything, if
code is correct without untag_pointer() it will still be correct with it
on.

Also avoids future bugs due to being robust in general.