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Date:   Wed, 5 May 2021 09:48:22 -0500
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     David Laight <David.Laight@aculab.com>,
        Al Viro <viro@zeniv.linux.org.uk>, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Will Deacon <will@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <longman@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Andy Lutomirski <luto@kernel.org>,
        Christoph Hellwig <hch@lst.de>, Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess
 speculation
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On Wed, May 05, 2021 at 03:25:42PM +0100, Mark Rutland wrote:
> On arm64 we needed to have a sequence here because the addr_limit used
> to be variable, but now that we've removed set_fs() and split the
> user/kernel access routines we could simplify that to an AND with an
> immediate mask to force all pointers into the user half of the address
> space. IIUC x86_64 could do the same, and I think that was roughly what
> David was suggesting.

True.  On 64-bit arches it might be as simple as just clearing the
most-significant bit.

-- 
Josh