pkeys

PKEYS(7)                   Linux Programmer's Manual                  PKEYS(7)

NAME
       pkeys - overview of Memory Protection Keys

DESCRIPTION
       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.

       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.

       Protection  keys  work  in  conjunction  with  the  existing PROT_READ/
       PROT_WRITE/ PROT_EXEC permissions passed to system calls such as  mpro-
       tect(2)  and  mmap(2),  but always act to further restrict these tradi-
       tional permission mechanisms.

       If a process performs an access that  violates  pkey  restrictions,  it
       receives  a SIGSEGV signal.  See sigaction(2) for details of the infor-
       mation available with that signal.

       To use the pkeys feature, the processor must support it, and the kernel
       must contain support for the feature on a given processor.  As of early
       2016 only future Intel x86 processors are supported, and this  hardware
       supports  16  protection keys in each process.  However, pkey 0 is used
       as the default key, so a maximum of 15 are available for actual  appli-
       cation use.  The default key is assigned to any memory region for which
       a pkey has not been explicitly assigned via pkey_mprotect(2).

       Protection keys have the potential to add a layer of security and reli-
       ability  to applications.  But they have not been primarily designed as
       a security feature.  For instance, WRPKRU is a completely  unprivileged
       instruction, so pkeys are useless in any case that an attacker controls
       the PKRU register or can execute arbitrary instructions.

       Applications should be very careful to ensure that they do  not  "leak"
       protection keys.  For instance, before calling pkey_free(2), the appli-
       cation should be sure that no memory has that pkey  assigned.   If  the
       application  left  the  freed pkey assigned, a future user of that pkey
       might inadvertently change the permissions of an unrelated data  struc-
       ture,  which  could impact security or stability.  The kernel currently
       allows in-use pkeys to have pkey_free(2)  called  on  them  because  it
       would  have processor or memory performance implications to perform the
       additional checks needed to disallow it.  Implementation of the  neces-
       sary  checks  is  left  up to applications.  Applications may implement
       these checks by searching the /proc/[pid]/smaps file for memory regions
       with the pkey assigned.  Further details can be found in proc(5).

       Any  application  wanting  to  use  protection keys needs to be able to
       function without them.  They might be unavailable because the  hardware
       that  the  application  runs  on does not support them, the kernel code
       does not contain support, the kernel  support  has  been  disabled,  or
       because  the  keys  have  all  been allocated, perhaps by a library the
       application is using.  It is recommended that applications  wanting  to
       use  protection  keys should simply call pkey_alloc(2) and test whether
       the call succeeds, instead of attempting to detect support for the fea-
       ture in any other way.

       Although  unnecessary, hardware support for protection keys may be enu-
       merated with the cpuid instruction.  Details of how to do this  can  be
       found  in  the  Intel  Software Developers Manual.  The kernel performs
       this enumeration and exposes the information in /proc/cpuinfo under the
       "flags"  field.  The string "pku" in this field indicates hardware sup-
       port for protection keys and the string "ospke" indicates that the ker-
       nel contains and has enabled protection keys support.

       Applications  using  threads  and  protection keys should be especially
       careful.  Threads inherit the protection key rights of  the  parent  at
       the  time  of  the  clone(2),  system call.  Applications should either
       ensure that their own permissions are appropriate for child threads  at
       the  time when clone(2) is called, or ensure that each child thread can
       perform its own initialization of protection key rights.

   Signal Handler Behavior
       Each time a signal handler is invoked (including nested  signals),  the
       thread is temporarily given a new, default set of protection key rights
       that override the rights from the interrupted context.  This means that
       applications must re-establish their desired protection key rights upon
       entering a signal  handler  if  the  desired  rights  differ  from  the
       defaults.   The rights of any interrupted context are restored when the
       signal handler returns.

       This signal behavior is unusual and is due to the  fact  that  the  x86
       PKRU  register  (which  stores protection key access rights) is managed
       with the same hardware mechanism (XSAVE)  that  manages  floating-point
       registers.   The  signal behavior is the same as that of floating-point
       registers.

   Protection Keys system calls
       The Linux kernel implements the following  pkey-related  system  calls:
       pkey_mprotect(2), pkey_alloc(2), and pkey_free(2).

       The  Linux  pkey system calls are available only if the kernel was con-
       figured  and  built  with  the  CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
       option.

EXAMPLE
       The  program  below allocates a page of memory with read and write per-
       missions.  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.

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

   Program source

       #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);
       }

SEE ALSO
       pkey_alloc(2), pkey_free(2), pkey_mprotect(2), sigaction(2)

COLOPHON
       This page is part of release 4.15 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                          PKEYS(7)
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