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

From: Ingo Molnar <mingo@kernel.org>
To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Brian Gerst <brgerst@gmail.com>,
	Chris Metcalf <cmetcalf@mellanox.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Huang Rui <ray.huang@amd.com>, Jiri Slaby <jslaby@suse.cz>,
	Jonathan Corbet <corbet@lwn.net>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Paul Gortmaker <paul.gortmaker@windriver.com>,
	Vlastimil Babka <vbabka@suse.cz>, Chen Yucong <slaoub@gmail.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Shuah Khan <shuah@kernel.org>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	ricardo.neri@intel.com, Tony Luck <tony.luck@intel.com>,
	Fenghua Yu <fenghua.yu@intel.com>
Subject: Re: [PATCH v11 09/12] x86: Enable User-Mode Instruction Prevention at runtime
Date: Wed, 8 Nov 2017 10:52:59 +0100	[thread overview]
Message-ID: <20171108095259.a33qh5n77knc3kxr@gmail.com> (raw)
In-Reply-To: <1509935277-22138-10-git-send-email-ricardo.neri-calderon@linux.intel.com>

* Ricardo Neri <ricardo.neri-calderon@linux.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