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.
