linux-libre/Documentation/x86/protection-keys.txt

Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature
which will be found on future Intel CPUs.

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_DENY_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_DENY_WRITE
	*ptr = foo; // assign something
	pkey_set(pkey, PKEY_DENY_WRITE); // set PKEY_DENY_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.