Ubuntu Manpage: pkeys - overview of Memory Protection Keys

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, 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.

EXAMPLES

       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);
       }

NAME

DESCRIPTION

EXAMPLES

SEE ALSO