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NAME | DESCRIPTION | EXAMPLES | SEE ALSO | COLOPHON |
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pkeys(7) Miscellaneous Information 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, and 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 <err.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
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)
err(EXIT_FAILURE, "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(0, 0);
if (pkey == -1)
err(EXIT_FAILURE, "pkey_alloc");
/*
* Disable access to any memory with "pkey" set,
* even though there is none right now.
*/
status = pkey_set(pkey, PKEY_DISABLE_ACCESS);
if (status)
err(EXIT_FAILURE, "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)
err(EXIT_FAILURE, "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)
err(EXIT_FAILURE, "pkey_free");
exit(EXIT_SUCCESS);
}
pkey_alloc(2), pkey_free(2), pkey_mprotect(2), sigaction(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 pkeys(7)
Pages that refer to this page: mprotect(2), pkey_alloc(2), sigaction(2), proc_pid_smaps(5), systemd.exec(5)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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