Re: [PATCH 33/33] x86, pkeys: execute-only support
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From: Kees Cook <hidden>
Date: 2016-02-17 21:36:17
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On Wed, Feb 17, 2016 at 1:33 PM, Dave Hansen [off-list ref] wrote:
On 02/17/2016 01:27 PM, Kees Cook wrote:quoted
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:quoted
00401000-00402000 --xp 00001000 08:14 4897479 /root/pkeys/pkey-xonly Size: 4 kB Rss: 4 kB...quoted
KernelPageSize: 4 kB MMUPageSize: 4 kB Locked: 0 kB ProtectionKey: 15 VmFlags: ex mr mw me dw
Ah-ha, perfect. Thanks!
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:quoted
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; }
Great, thanks for the example! -Kees -- Kees Cook Chrome OS & Brillo Security -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>