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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 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.
EXAMPLE
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);
}
SEE ALSO
pkey_alloc(2), pkey_free(2), pkey_mprotect(2), sigaction(2)
COLOPHON
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