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pkeys(7)
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 ac‐
cess 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 re‐
ceives a SIGSEGV signal. See sigaction(2) for details of the informa‐
tion 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 be‐
cause the keys have all been allocated, perhaps by a library the appli‐
cation 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 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 en‐
sure 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 de‐
faults. 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 op‐
tion.
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 5.02 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 2019-03-06 PKEYS(7)