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.TH PKEYS 7 2019-03-06 "Linux" "Linux Programmer's Manual"
.SH NAME
pkeys \- overview of Memory Protection Keys
.SH 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.
.PP
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.
.PP
Protection keys work in conjunction with the existing
.BR PROT_READ /
.BR PROT_WRITE /
.BR PROT_EXEC
permissions passed to system calls such as
.BR mprotect (2)
and
.BR mmap (2),
but always act to further restrict these traditional permission
mechanisms.
.PP
If a process performs an access that violates pkey
restrictions, it receives a
.BR SIGSEGV
signal.
See
.BR sigaction (2)
for details of the information available with that signal.
.PP
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 application use.
The default key is assigned to any memory region for which a
pkey has not been explicitly assigned via
.BR pkey_mprotect (2).
.PP
Protection keys have the potential to add a layer of security and
reliability 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.
.PP
Applications should be very careful to ensure that they do not "leak"
protection keys.
For instance, before calling
.BR pkey_free (2),
the application 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 structure, which could impact security or stability.
The kernel currently allows in-use pkeys to have
.BR 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 necessary checks is left up to applications.
Applications may implement these checks by searching the
.IR /proc/[pid]/smaps
file for memory regions with the pkey assigned.
Further details can be found in
.BR proc (5).
.PP
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
.BR pkey_alloc (2)
and test whether the call succeeds,
instead of attempting to detect support for the
feature in any other way.
.PP
Although unnecessary, hardware support for protection keys may be
enumerated with the
.I 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
.IR /proc/cpuinfo
under the "flags" field.
The string "pku" in this field indicates hardware support for protection
keys and the string "ospke" indicates that the kernel contains and has
enabled protection keys support.
.PP
Applications using threads and protection keys should be especially
careful.
Threads inherit the protection key rights of the parent at the time
of the
.BR clone (2),
system call.
Applications should either ensure that their own permissions are
appropriate for child threads at the time when
.BR clone (2)
is called, or ensure that each child thread can perform its
own initialization of protection key rights.
.\"
.SS 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.
.PP
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.
.\"
.SS Protection Keys system calls
The Linux kernel implements the following pkey-related system calls:
.BR pkey_mprotect (2),
.BR pkey_alloc (2),
and
.BR pkey_free (2).
.PP
The Linux pkey system calls are available only if the kernel was
configured and built with the
.BR CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
option.
.SH EXAMPLE
.PP
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.
.PP
.in +4n
.EX
.RB "$" " ./a.out"
buffer contains: 73
about to read buffer again...
Segmentation fault (core dumped)
.EE
.in
.SS Program source
\&
.EX
#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\en\et"
                 : : "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); \e
                           } 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\en", *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...\en");

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

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

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR pkey_alloc (2),
.BR pkey_free (2),
.BR pkey_mprotect (2),
.BR sigaction (2)