{
    "mode": "man",
    "parameter": "pkeys",
    "section": "7",
    "url": "https://www.chedong.com/phpMan.php/man/pkeys/7/json",
    "generated": "2026-10-04T07:24:55Z",
    "sections": {
        "NAME": {
            "content": "pkeys - overview of Memory Protection Keys\n",
            "subsections": []
        },
        "DESCRIPTION": {
            "content": "Memory  Protection  Keys  (pkeys) are an extension to existing page-based memory permissions.\nNormal page permissions using page tables require expensive system calls  and  TLB  invalida‐\ntions  when  changing  permissions.   Memory Protection Keys provide a mechanism for changing\nprotections without requiring modification of the page tables on every permission change.\n\nTo use pkeys, software must first \"tag\" a page in the page tables with a  pkey.   After  this\ntag is in place, an application only has to change the contents of a register in order to re‐\nmove write access, or all access to a tagged page.\n\nProtection  keys  work  in conjunction with the existing PROTREAD, PROTWRITE, and PROTEXEC\npermissions passed to system calls such as mprotect(2) and mmap(2), but always act to further\nrestrict these traditional permission mechanisms.\n\nIf a process performs an access that violates pkey restrictions, it receives a  SIGSEGV  sig‐\nnal.  See sigaction(2) for details of the information available with that signal.\n\nTo  use the pkeys feature, the processor must support it, and the kernel must contain support\nfor the feature on a given processor.  As of early 2016 only future Intel x86 processors  are\nsupported, and this hardware supports 16 protection keys in each process.  However, pkey 0 is\nused  as  the  default key, so a maximum of 15 are available for actual application use.  The\ndefault key is assigned to any memory region for which a pkey has  not  been  explicitly  as‐\nsigned via pkeymprotect(2).\n\nProtection  keys  have  the  potential to add a layer of security and reliability to applica‐\ntions.  But they have not been primarily designed as a security feature.  For instance,  WRP‐\nKRU  is  a  completely unprivileged instruction, so pkeys are useless in any case that an at‐\ntacker controls the PKRU register or can execute arbitrary instructions.\n\nApplications should be very careful to ensure that they do not \"leak\" protection  keys.   For\ninstance, before calling pkeyfree(2), the application should be sure that no memory has that\npkey  assigned.   If the application left the freed pkey assigned, a future user of that pkey\nmight inadvertently change the permissions of an unrelated data structure, which could impact\nsecurity or stability.  The kernel currently allows in-use pkeys to have pkeyfree(2)  called\non them because it would have processor or memory performance implications to perform the ad‐\nditional  checks needed to disallow it.  Implementation of the necessary checks is left up to\napplications.  Applications may implement these checks by searching the /proc/pid/smaps  file\nfor memory regions with the pkey assigned.  Further details can be found in proc(5).\n\nAny  application  wanting  to  use protection keys needs to be able to function without them.\nThey might be unavailable because the hardware that the application runs on does not  support\nthem,  the kernel code does not contain support, the kernel support has been disabled, or be‐\ncause the keys have all been allocated, perhaps by a library the application is using.  It is\nrecommended that applications wanting to use protection keys should simply call pkeyalloc(2)\nand test whether the call succeeds, instead of attempting to detect support for  the  feature\nin any other way.\n\nAlthough  unnecessary,  hardware support for protection keys may be enumerated with the cpuid\ninstruction.  Details of how to do this can be found in the Intel Software Developers Manual.\nThe kernel performs this enumeration and exposes the information in /proc/cpuinfo  under  the\n\"flags\" field.  The string \"pku\" in this field indicates hardware support for protection keys\nand  the  string  \"ospke\"  indicates that the kernel contains and has enabled protection keys\nsupport.\n\nApplications using threads and protection keys should be especially careful.  Threads inherit\nthe protection key rights of the parent at the time of the clone(2), system  call.   Applica‐\ntions  should  either  ensure that their own permissions are appropriate for child threads at\nthe time when clone(2) is called, or ensure that each child thread can perform its  own  ini‐\ntialization of protection key rights.\n",
            "subsections": [
                {
                    "name": "Signal Handler Behavior",
                    "content": "Each  time  a signal handler is invoked (including nested signals), the thread is temporarily\ngiven a new, default set of protection key rights that override the rights  from  the  inter‐\nrupted  context.  This means that applications must re-establish their desired protection key\nrights upon entering a signal handler if the desired rights differ from  the  defaults.   The\nrights of any interrupted context are restored when the signal handler returns.\n\nThis  signal  behavior  is  unusual  and is due to the fact that the x86 PKRU register (which\nstores protection key access rights) is managed with the same hardware mechanism (XSAVE) that\nmanages floating-point registers.  The signal behavior is the same as that of  floating-point\nregisters.\n"
                },
                {
                    "name": "Protection Keys system calls",
                    "content": "The  Linux  kernel  implements  the  following  pkey-related  system calls: pkeymprotect(2),\npkeyalloc(2), and pkeyfree(2).\n\nThe Linux pkey system calls are available only if the kernel was configured  and  built  with\nthe CONFIGX86INTELMEMORYPROTECTIONKEYS option.\n"
                }
            ]
        },
        "EXAMPLES": {
            "content": "The program below allocates a page of memory with read and write permissions.  It then writes\nsome  data to the memory and successfully reads it back.  After that, it attempts to allocate\na protection key and disallows access to the page by using the WRPKRU instruction.   It  then\ntries to access the page, which we now expect to cause a fatal signal to the application.\n\n$ ./a.out\nbuffer contains: 73\nabout to read buffer again...\nSegmentation fault (core dumped)\n",
            "subsections": [
                {
                    "name": "Program source",
                    "content": "#define GNUSOURCE\n#include <err.h>\n#include <unistd.h>\n#include <stdio.h>\n#include <stdlib.h>\n#include <sys/mman.h>\n\nint\nmain(void)\n{\nint status;\nint pkey;\nint *buffer;\n\n/*\n* Allocate one page of memory.\n*/\nbuffer = mmap(NULL, getpagesize(), PROTREAD | PROTWRITE,\nMAPANONYMOUS | MAPPRIVATE, -1, 0);\nif (buffer == MAPFAILED)\nerr(EXITFAILURE, \"mmap\");\n\n/*\n* Put some random data into the page (still OK to touch).\n*/\n*buffer = LINE;\nprintf(\"buffer contains: %d\\n\", *buffer);\n\n/*\n* Allocate a protection key:\n*/\npkey = pkeyalloc(0, 0);\nif (pkey == -1)\nerr(EXITFAILURE, \"pkeyalloc\");\n\n/*\n* Disable access to any memory with \"pkey\" set,\n* even though there is none right now.\n*/\nstatus = pkeyset(pkey, PKEYDISABLEACCESS);\nif (status)\nerr(EXITFAILURE, \"pkeyset\");\n\n/*\n* Set the protection key on \"buffer\".\n* Note that it is still read/write as far as mprotect() is\n* concerned and the previous pkeyset() overrides it.\n*/\nstatus = pkeymprotect(buffer, getpagesize(),\nPROTREAD | PROTWRITE, pkey);\nif (status == -1)\nerr(EXITFAILURE, \"pkeymprotect\");\n\nprintf(\"about to read buffer again...\\n\");\n\n/*\n* This will crash, because we have disallowed access.\n*/\nprintf(\"buffer contains: %d\\n\", *buffer);\n\nstatus = pkeyfree(pkey);\nif (status == -1)\nerr(EXITFAILURE, \"pkeyfree\");\n\nexit(EXITSUCCESS);\n}\n"
                }
            ]
        },
        "SEE ALSO": {
            "content": "pkeyalloc(2), pkeyfree(2), pkeymprotect(2), sigaction(2)\n\nLinux man-pages 6.7                          2023-10-31                                     pkeys(7)",
            "subsections": []
        }
    },
    "summary": "pkeys - overview of Memory Protection Keys",
    "flags": [],
    "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)",
        "#define GNUSOURCE",
        "#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(), PROTREAD | PROTWRITE,",
        "MAPANONYMOUS | MAPPRIVATE, -1, 0);",
        "if (buffer == MAPFAILED)",
        "err(EXITFAILURE, \"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 = pkeyalloc(0, 0);",
        "if (pkey == -1)",
        "err(EXITFAILURE, \"pkeyalloc\");",
        "/*",
        "* Disable access to any memory with \"pkey\" set,",
        "* even though there is none right now.",
        "*/",
        "status = pkeyset(pkey, PKEYDISABLEACCESS);",
        "if (status)",
        "err(EXITFAILURE, \"pkeyset\");",
        "/*",
        "* Set the protection key on \"buffer\".",
        "* Note that it is still read/write as far as mprotect() is",
        "* concerned and the previous pkeyset() overrides it.",
        "*/",
        "status = pkeymprotect(buffer, getpagesize(),",
        "PROTREAD | PROTWRITE, pkey);",
        "if (status == -1)",
        "err(EXITFAILURE, \"pkeymprotect\");",
        "printf(\"about to read buffer again...\\n\");",
        "/*",
        "* This will crash, because we have disallowed access.",
        "*/",
        "printf(\"buffer contains: %d\\n\", *buffer);",
        "status = pkeyfree(pkey);",
        "if (status == -1)",
        "err(EXITFAILURE, \"pkeyfree\");",
        "exit(EXITSUCCESS);"
    ],
    "see_also": [
        {
            "name": "pkeyalloc",
            "section": "2",
            "url": "https://www.chedong.com/phpMan.php/man/pkeyalloc/2/json"
        },
        {
            "name": "pkeyfree",
            "section": "2",
            "url": "https://www.chedong.com/phpMan.php/man/pkeyfree/2/json"
        },
        {
            "name": "pkeymprotect",
            "section": "2",
            "url": "https://www.chedong.com/phpMan.php/man/pkeymprotect/2/json"
        },
        {
            "name": "sigaction",
            "section": "2",
            "url": "https://www.chedong.com/phpMan.php/man/sigaction/2/json"
        }
    ]
}