linux-kernel - Re: [PATCH v2] x86/asm: Replace __force

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Date:   Wed, 2 Sep 2020 12:08:28 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Will Deacon <will@...nel.org>, nadav.amit@...il.com,
        Nathan Chancellor <natechancellor@...il.com>
Subject: Re: [PATCH v2] x86/asm: Replace __force_order with memory clobber

On Wed, Sep 02, 2020 at 11:33:46AM -0400, Arvind Sankar wrote:
> Fix this by:
> - Using a memory clobber for the write functions to additionally prevent
>   caching/reordering memory accesses across CRn writes.
> - Using a dummy input operand with an arbitrary constant address for the
>   read functions, instead of a global variable. This will prevent reads
>   from being reordered across writes, while allowing memory loads to be
>   cached/reordered across CRn reads, which should be safe.
> 

Any thoughts on whether FORCE_ORDER is worth it just for CRn? MSRs don't
use it, Nadav pointed out that PKRU doesn't use it (PKRU doesn't have a
memory clobber on write either). I would guess that most of the volatile
asm has not been written with the assumption that the compiler might
decide to reorder it, so protecting just CRn access doesn't mitigate the
impact of this bug.

Thanks.