Get x86 SMEP CPU flag from user mode

1
votes

I am writing a usermode tool that watches for changes to CPU flags such as WP from CR0, and SMEP from CR4. I can read the WP bit using smsw %rax but there is no similar opcode for reading CR4 (or its SMEP bit).

Is there a way to get the state of SMEP from usermode ?

1
assemblylinux-kernelx86intel
Unlikely. The only reason why SMSW isn't a privileged instruction is for compatibility with the 80286. – Ross Ridge

1 Answers

6
votes

The Linux kernel never changes the SMEP bit after booting. See How can i enable/disable kernel kaslr, smep and smap. By default, it's enabled if it's supported on the processor starting with kernel 3.0. So if you want to check whether SMEP is enabled, do the following:

  • Check whether SMEP is supported by executing cpuid with leaf 0x7. The processor supports SMEP if EBX[0x7] is 1.
  • If SMEP is supported, check whether nosmep is specified in the kernel command line argument list. This can be done using either the sysctl system call or something like cat /proc/cmdline | grep nosmep.

If it's supported on the processor and nosmep is not specified, then it's enabled. Otherwise, it's disabled or not supported.