linux-kernel - Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation

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Message-ID: <20210506120526.GA1124@lst.de>
Date:   Thu, 6 May 2021 14:05:26 +0200
From:   Christoph Hellwig <hch@....de>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Andy Lutomirski' <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Christoph Hellwig <hch@....de>,
        Mark Rutland <mark.rutland@....com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit
 uaccess speculation

On Thu, May 06, 2021 at 08:36:08AM +0000, David Laight wrote:
> 	uaddr = access_ok(uaddr, size)

access_ok as a public API is not interesting.  There are very few
valid uses cases for ever calling access_ok outside the usual
uaccess helper.  So leave access_ok alone, there is not point in
touching all the callers except for removing most of them.  If OTOH
we can micro-optimize get_user and put_user by using a different
variant of access_ok that seems fair game and actually useful.