本文将从motivation、usage demo和硬件实现来介绍PKU(Memory Protection Keys for Userspace)。

motivation

Memory Protection Keys (pkeys) are an extension to existing page-based memory permissions. Normal page permissions using page tables require expensive system calls and TLB invalidations when changing permissions. Memory Protection Keys provide a mechanism for changing protections without requiring modification of the page tables on every permission change.

usage demo

To use pkeys, software must first “tag” a page in the page tables with a pkey. After this tag is in place, an application only has to change the contents of a register in order to remove write access, or all access to a tagged page.

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#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <stdio.h>
#include <sys/mman.h>

static inline void
wrpkru(unsigned int pkru)
{
    unsigned int eax = pkru;
    unsigned int ecx = 0;
    unsigned int edx = 0;

    asm volatile(".byte 0x0f,0x01,0xef\n\t"
                 : : "a" (eax), "c" (ecx), "d" (edx));
}

int
pkey_set(int pkey, unsigned long rights, unsigned long flags)
{
    unsigned int pkru = (rights << (2 * pkey));
    return wrpkru(pkru);
}

int
pkey_mprotect(void *ptr, size_t size, unsigned long orig_prot,
              unsigned long pkey)
{
    return syscall(SYS_pkey_mprotect, ptr, size, orig_prot, pkey);
}

int
pkey_alloc(void)
{
    return syscall(SYS_pkey_alloc, 0, 0);
}

int
pkey_free(unsigned long pkey)
{
    return syscall(SYS_pkey_free, pkey);
}

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                           } while (0)

int
main(void)
{
    int status;
    int pkey;
    int *buffer;

    /*
     *Allocate one page of memory
     */
    buffer = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE,
                  MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
    if (buffer == MAP_FAILED)
        errExit("mmap");

    /*
     * Put some random data into the page (still OK to touch)
     */
    *buffer = __LINE__;
    printf("buffer contains: %d\n", *buffer);

    /*
     * Allocate a protection key:
     */
    pkey = pkey_alloc();
    if (pkey == -1)
        errExit("pkey_alloc");

    /*
     * Disable access to any memory with "pkey" set,
     * even though there is none right now
     */
    status = pkey_set(pkey, PKEY_DISABLE_ACCESS, 0);
    if (status)
        errExit("pkey_set");

    /*
     * Set the protection key on "buffer".
     * Note that it is still read/write as far as mprotect() is
     * concerned and the previous pkey_set() overrides it.
     */
    status = pkey_mprotect(buffer, getpagesize(),
                           PROT_READ | PROT_WRITE, pkey);
    if (status == -1)
        errExit("pkey_mprotect");

    printf("about to read buffer again...\n");

    /*
     * This will crash, because we have disallowed access
     */
    printf("buffer contains: %d\n", *buffer);

    status = pkey_free(pkey);
    if (status == -1)
        errExit("pkey_free");

    exit(EXIT_SUCCESS);
}

The program below allocates a page of memory with read and write permissions. It then writes some data to the memory and successfully reads it back. After that, it attempts to allocate a protection key and disallows access to the page by using the WRPKRU instruction. It then tries to access the page, which we now expect to cause a fatal signal to the application.

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$ ./a.out
 buffer contains: 73
 about to read buffer again...
 Segmentation fault (core dumped)

硬件实现

PKU的底层实现依赖于protection key rights register for user pages (PKRU) 。

参考资料：

  1. pkeys - overview of Memory Protection Keys
  2. protection-keys.txt
  3. x86: Memory Protection Keys
  4. Intel SDM