.. SPDX-License-Identifier: GPL-2.0
  
  ======================
  Memory Protection Keys
  ======================

  Memory Protection Keys for Userspace (PKU aka PKEYs) is a feature

  which is found on Intel's Skylake (and later) "Scalable Processor"
  Server CPUs. It will be available in future non-server Intel parts
  and future AMD processors.

  
  For anyone wishing to test or use this feature, it is available in
  Amazon's EC2 C5 instances and is known to work there using an Ubuntu
  17.04 image.

  
  Memory Protection Keys provides a mechanism for enforcing page-based
  protections, but without requiring modification of the page tables
  when an application changes protection domains.  It works by
  dedicating 4 previously ignored bits in each page table entry to a
  "protection key", giving 16 possible keys.
  
  There is also a new user-accessible register (PKRU) with two separate
  bits (Access Disable and Write Disable) for each key.  Being a CPU
  register, PKRU is inherently thread-local, potentially giving each
  thread a different set of protections from every other thread.
  
  There are two new instructions (RDPKRU/WRPKRU) for reading and writing
  to the new register.  The feature is only available in 64-bit mode,
  even though there is theoretically space in the PAE PTEs.  These
  permissions are enforced on data access only and have no effect on
  instruction fetches.

  Syscalls
  ========

  There are 3 system calls which directly interact with pkeys::

  
  	int pkey_alloc(unsigned long flags, unsigned long init_access_rights)
  	int pkey_free(int pkey);
  	int pkey_mprotect(unsigned long start, size_t len,
  			  unsigned long prot, int pkey);
  
  Before a pkey can be used, it must first be allocated with
  pkey_alloc().  An application calls the WRPKRU instruction
  directly in order to change access permissions to memory covered
  with a key.  In this example WRPKRU is wrapped by a C function
  called pkey_set().

  ::

  
  	int real_prot = PROT_READ|PROT_WRITE;

  	pkey = pkey_alloc(0, PKEY_DISABLE_WRITE);

  	ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
  	ret = pkey_mprotect(ptr, PAGE_SIZE, real_prot, pkey);
  	... application runs here
  
  Now, if the application needs to update the data at 'ptr', it can

  gain access, do the update, then remove its write access::

  	pkey_set(pkey, 0); // clear PKEY_DISABLE_WRITE

  	*ptr = foo; // assign something

  	pkey_set(pkey, PKEY_DISABLE_WRITE); // set PKEY_DISABLE_WRITE again

  
  Now when it frees the memory, it will also free the pkey since it

  is no longer in use::

  
  	munmap(ptr, PAGE_SIZE);
  	pkey_free(pkey);

  .. note:: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
            An example implementation can be found in
            tools/testing/selftests/x86/protection_keys.c.

  Behavior
  ========

  
  The kernel attempts to make protection keys consistent with the

  behavior of a plain mprotect().  For instance if you do this::

  
  	mprotect(ptr, size, PROT_NONE);
  	something(ptr);

  you can expect the same effects with protection keys when doing this::

  
  	pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ);
  	pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE, pkey);
  	something(ptr);
  
  That should be true whether something() is a direct access to 'ptr'

  like::

  
  	*ptr = foo;
  
  or when the kernel does the access on the application's behalf like

  with a read()::

  
  	read(fd, ptr, 1);
  
  The kernel will send a SIGSEGV in both cases, but si_code will be set
  to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when
  the plain mprotect() permissions are violated.