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NAME | DESCRIPTION | EXAMPLE | SEE ALSO | COLOPHON |
PKEYS(7) Linux Programmer's Manual PKEYS(7)
pkeys - overview of Memory Protection Keys
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
mprotect(2) and mmap(2), but always act to further restrict these
traditional permission mechanisms.
If a process performs an access that violates pkey restrictions, it
receives a SIGSEGV signal. See sigaction(2) for details of the
information 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 application 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
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.
Applications should be very careful to ensure that they do not "leak"
protection keys. For instance, before calling 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 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 /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 feature in any other way.
Although unnecessary, hardware support for protection keys may be
enumerated 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 support for protection keys and the string
"ospke" indicates that the kernel 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
configured and built with the CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
option.
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.
$ ./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);
}
pkey_alloc(2), pkey_free(2), pkey_mprotect(2), sigaction(2)
This page is part of release 4.12 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 2016-12-12 PKEYS(7)
Pages that refer to this page: mprotect(2), pkey_alloc(2), sigaction(2), proc(5)