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
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.
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);
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.
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.