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Subject: Re: [PATCH 33/33] x86, pkeys: execute-only support
To: Kees Cook <keescook@google.com>
References: <20160212210152.9CAD15B0@viggo.jf.intel.com>
 <20160212210240.CB4BB5CA@viggo.jf.intel.com>
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Cc: LKML <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>,
        "x86@kernel.org" <x86@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>
From: Dave Hansen <dave@sr71.net>
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Date: Wed, 17 Feb 2016 13:33:20 -0800
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On 02/17/2016 01:27 PM, Kees Cook wrote:
> Is there a way to detect this feature's availability without userspace
> having to set up a segv handler and attempting to read a
> PROT_EXEC-only region? (i.e. cpu flag for protection keys, or a way to
> check the protection to see if PROT_READ got added automatically,
> etc?)

You can kinda do it with /proc/$pid/(s)maps.  Here's smaps, for instance:

> 00401000-00402000 --xp 00001000 08:14 4897479                            /root/pkeys/pkey-xonly
> Size:                  4 kB
> Rss:                   4 kB
...
> KernelPageSize:        4 kB
> MMUPageSize:           4 kB
> Locked:                0 kB
> ProtectionKey:        15
> VmFlags: ex mr mw me dw 

You can see "--x" and the ProtectionKey itself being nonzero.  That's a
reasonable indication.  There's also the "OSPKE" cpuid bit which only
shows up when the kernel has enabled protection keys.  This is
_separate_ from the bit that says whether the processor support pkeys.

I check them in test code like this:

> static inline void __cpuid(unsigned int *eax, unsigned int *ebx,
>                                 unsigned int *ecx, unsigned int *edx)
> {
>         /* ecx is often an input as well as an output. */
>         asm volatile(
>                 "cpuid;"
>                 : "=a" (*eax),
>                   "=b" (*ebx),
>                   "=c" (*ecx),
>                   "=d" (*edx)
>                 : "0" (*eax), "2" (*ecx));
> }
> 
> /* Intel-defined CPU features, CPUID level 0x00000007:0 (ecx) */
> #define X86_FEATURE_PKU        (1<<3) /* Protection Keys for Userspace */
> #define X86_FEATURE_OSPKE      (1<<4) /* OS Protection Keys Enable */
> 
> static inline int cpu_has_pku(void)
> {
>         unsigned int eax;
>         unsigned int ebx;
>         unsigned int ecx;
>         unsigned int edx;
>         eax = 0x7;
>         ecx = 0x0;
>         __cpuid(&eax, &ebx, &ecx, &edx);
> 
>         if (!(ecx & X86_FEATURE_PKU)) {
>                 dprintf2("cpu does not have PKU\n");
>                 return 0;
>         }
>         if (!(ecx & X86_FEATURE_OSPKE)) {
>                 dprintf2("cpu does not have OSPKE\n");
>                 return 0;
>         }
>         return 1;
> }