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From:   Andy Lutomirski <luto@kernel.org>
Date:   Thu, 7 Mar 2019 12:02:13 -0800
Message-ID: <CALCETrUY6L_Fwd9CZzo2eZL8HT2sBSHFiD-Bp-HCPPFBxkzcdA@mail.gmail.com>
Subject: Re: [PATCH v2 10/20] x86: avoid W^X being broken during modules loading
To:     Borislav Petkov <bp@alien8.de>
Cc:     "H. Peter Anvin" <hpa@zytor.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Damian Tometzki <linux_dti@icloud.com>,
        linux-integrity <linux-integrity@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux-MM <linux-mm@kvack.org>, Will Deacon <will.deacon@arm.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Kristen Carlson Accardi <kristen@linux.intel.com>,
        "Dock, Deneen T" <deneen.t.dock@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>
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On Thu, Mar 7, 2019 at 9:06 AM Borislav Petkov <bp@alien8.de> wrote:
>
> On Thu, Mar 07, 2019 at 08:53:34AM -0800, hpa@zytor.com wrote:
> > If we *do*, what is the issue here? Although boot_cpu_has() isn't
> > slow (it should in general be possible to reduce to one testb
> > instruction followed by a conditional jump) it seems that "avoiding an
> > alternatives slot" *should* be a *very* weak reason, and seems to me
> > to look like papering over some other problem.
>
> Forget the current thread: this is simply trying to document when to use
> static_cpu_has() and when to use boot_cpu_has(). I get asked about it at
> least once a month.
>
> And then it is replacing clear slow paths using static_cpu_has() with
> boot_cpu_has() because there's purely no need to patch there. And having
> a RIP-relative MOV and a JMP is good enough for slow paths.
>

Should we maybe rename these functions?  static_cpu_has() is at least
reasonably obvious.  But cpu_feature_enabled() is different for
reasons I've never understood, and boot_cpu_has() is IMO terribly
named.  It's not about the boot cpu -- it's about doing the same thing
but with less bloat and less performance.

(And can we maybe collapse cpu_feature_enabled() and static_cpu_has()
into the same function?)

--Andy