linux-kernel - Re: [PATCH v2] x86/paravirt: Drop {read,write}

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <602B4D96-E2A9-45BE-8247-4E3481ED5402@vmware.com>
Date:   Mon, 15 Jul 2019 18:17:59 +0000
From:   Nadav Amit <namit@...are.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
CC:     LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Feng Tang <feng.tang@...el.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>
Subject: Re: [PATCH v2] x86/paravirt: Drop {read,write}_cr8() hooks

> On Jul 15, 2019, at 8:16 AM, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
> 
> There is a lot of infrastructure for functionality which is used
> exclusively in __{save,restore}_processor_state() on the suspend/resume
> path.
> 
> cr8 is an alias of APIC_TASKPRI, and APIC_TASKPRI is saved/restored by
> lapic_{suspend,resume}().  Saving and restoring cr8 independently of the
> rest of the Local APIC state isn't a clever thing to be doing.
> 
> Delete the suspend/resume cr8 handling, which shrinks the size of struct
> saved_context, and allows for the removal of both PVOPS.

I think removing the interface for CR8 writes is also good to avoid
potential correctness issues, as the SDM says (10.8.6.1 "Interaction of Task
Priorities between CR8 and APIC”):

"Operating software should implement either direct APIC TPR updates or CR8
style TPR updates but not mix them. Software can use a serializing
instruction (for example, CPUID) to serialize updates between MOV CR8 and
stores to the APIC.”

And native_write_cr8() did not even issue a serializing instruction.