linux-kernel - RE: [RFC PATCH v0 0/6] x86/AMD: Userspace address tagging

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Message-ID: <699fb763ac054833bc8c29c9814c63b2@AcuMS.aculab.com>
Date:   Thu, 10 Mar 2022 14:32:46 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Bharata B Rao' <bharata@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "ananth.narayan@....com" <ananth.narayan@....com>
Subject: RE: [RFC PATCH v0 0/6] x86/AMD: Userspace address tagging

From: Bharata B Rao <bharata@....com>
> Sent: 10 March 2022 11:16
> 
> This patchset makes use of Upper Address Ignore (UAI) feature available
> on upcoming AMD processors to provide user address tagging support for x86/AMD.
> 
> UAI allows software to store a tag in the upper 7 bits of a logical
> address [63:57]. When enabled, the processor will suppress the
> traditional canonical address checks on the addresses. More information
> about UAI can be found in section 5.10 of 'AMD64 Architecture
> Programmer's Manual, Vol 2: System Programming' which is available from
> 
> https://bugzilla.kernel.org/attachment.cgi?id=300549

Is that really allowing bit 63 to be used?
That is normally the user-kernel bit.
I can't help feeling that will just badly break things.

Otherwise the best thing is just to change access_ok()
to only reject addresses with the top bit set.
Then you shouldn't need any extra tests in the fast-path
of access_ok().

	David

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