Memory Protection Keys¶

Memory Protection Keys provide a mechanism for enforcing page-based protections, but without requiring modification of the page tables when an application changes protection domains.

Pkeys Userspace (PKU) is a feature which can be found on:

Intel server CPUs, Skylake and later
Intel client CPUs, Tiger Lake (11th Gen Core) and later
Future AMD CPUs
arm64 CPUs implementing the Permission Overlay Extension (FEAT_S1POE)

x86_64¶

Pkeys work by dedicating 4 previously Reserved bits in each page table entry to a “protection key”, giving 16 possible keys.

Protections for each key are defined with a per-CPU user-accessible register (PKRU). Each of these is a 32-bit register storing two bits (Access Disable and Write Disable) for each of 16 keys.

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 instructions (RDPKRU/WRPKRU) for reading and writing to the 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.

arm64¶

Pkeys use 3 bits in each page table entry, to encode a “protection key index”, giving 8 possible keys.

Protections for each key are defined with a per-CPU user-writable system register (POR_EL0). This is a 64-bit register encoding read, write and execute overlay permissions for each protection key index.

Being a CPU register, POR_EL0 is inherently thread-local, potentially giving each thread a different set of protections from every other thread.

Unlike x86_64, the protection key permissions also apply to 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 writes to the architecture specific CPU register directly in order to change access permissions to memory covered with a key. In this example this 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 around writing to the CPU register. Example implementations can be found in tools/testing/selftests/mm/pkey-{arm64,powerpc,x86}.h

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.

Note that kernel accesses from a kthread (such as io_uring) will use a default value for the protection key register and so will not be consistent with userspace’s value of the register or mprotect().