linux-kernel - Re: [PATCH v11 09/12] x86: Enable User-Mode Instruction Prevention at runtime

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Message-ID: <20171108095259.a33qh5n77knc3kxr@gmail.com>
Date:   Wed, 8 Nov 2017 10:52:59 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Huang Rui <ray.huang@....com>, Jiri Slaby <jslaby@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Chen Yucong <slaoub@...il.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, ricardo.neri@...el.com,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH v11 09/12] x86: Enable User-Mode Instruction Prevention
 at runtime


* Ricardo Neri <ricardo.neri-calderon@...ux.intel.com> wrote:

> User-Mode Instruction Prevention (UMIP) is enabled by setting/clearing a
> bit in %cr4.
> 
> It makes sense to enable UMIP at some point while booting, before user
> spaces come up. Like SMAP and SMEP, is not critical to have it enabled
> very early during boot. This is because UMIP is relevant only when there is
> a user space to be protected from. Given these similarities, UMIP can be
> enabled along with SMAP and SMEP.
> 
> At the moment, UMIP is disabled by default at build time. It can be enabled
> at build time by selecting CONFIG_X86_INTEL_UMIP. If enabled at build time,
> it can be disabled at run time by adding clearcpuid=514 to the kernel
> parameters.

> +config X86_INTEL_UMIP
> +	def_bool n
> +	depends on CPU_SUP_INTEL
> +	prompt "Intel User Mode Instruction Prevention" if EXPERT
> +	---help---
> +	  The User Mode Instruction Prevention (UMIP) is a security
> +	  feature in newer Intel processors. If enabled, a general
> +	  protection fault is issued if the instructions SGDT, SLDT,
> +	  SIDT, SMSW and STR are executed in user mode.

Ok, I really like this latest series.

One small request: could you please make the feature default-y, which is what we 
do for new hardware features that don't have a significant runtime cost. There's 
no ABI breakage expected, right?

Another request: could you please extend the Kconfig description to _explain_ to 
the user why this feature is useful - in a short sentence or so. I.e. point out 
that these instructions expose information about hardware state that is not really 
necessary for the vast majority of applications.

Plus:

> +       cr4_set_bits(X86_CR4_UMIP);

Please also inform admins that it's enabled, via something like:

	pr_info("x86/cpu: Activated the Intel User Mode Instruction Prevention (UMIP) CPU feature\n");

... or so.

Please do these changes in a separate add-on patch #13 on top of this series, as 
I've already started testing these bits.

Thanks,

	Ingo