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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Bharata B Rao' <bharata@amd.com>,
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Subject: RE: [RFC PATCH v0 0/6] x86/AMD: Userspace address tagging
Thread-Topic: [RFC PATCH v0 0/6] x86/AMD: Userspace address tagging
Thread-Index: AQHYNHBEWuVw0etF4EmWwhK6DFJ++Ky4rWmQ
Date:   Thu, 10 Mar 2022 14:32:46 +0000
Message-ID: <699fb763ac054833bc8c29c9814c63b2@AcuMS.aculab.com>
References: <20220310111545.10852-1-bharata@amd.com>
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From: Bharata B Rao <bharata@amd.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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