From 1eecbcdca2bd8d96881cace19ad105dc0f0263f5 Mon Sep 17 00:00:00 2001 From: Mauro Carvalho Chehab Date: Fri, 7 Jun 2019 15:54:31 -0300 Subject: docs: move protection-keys.rst to the core-api book This document is used by multiple architectures: $ echo $(git grep -l pkey_mprotect arch|cut -d'/' -f 2|sort|uniq) alpha arm arm64 ia64 m68k microblaze mips parisc powerpc s390 sh sparc x86 xtensa So, let's move it to the core book and adjust the links to it accordingly. Signed-off-by: Mauro Carvalho Chehab Signed-off-by: Jonathan Corbet --- Documentation/x86/index.rst | 1 - Documentation/x86/protection-keys.rst | 99 ----------------------------------- 2 files changed, 100 deletions(-) delete mode 100644 Documentation/x86/protection-keys.rst (limited to 'Documentation/x86') diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst index ae36fc5fc649..f2de1b2d3ac7 100644 --- a/Documentation/x86/index.rst +++ b/Documentation/x86/index.rst @@ -19,7 +19,6 @@ x86-specific Documentation tlb mtrr pat - protection-keys intel_mpx amd-memory-encryption pti diff --git a/Documentation/x86/protection-keys.rst b/Documentation/x86/protection-keys.rst deleted file mode 100644 index 49d9833af871..000000000000 --- a/Documentation/x86/protection-keys.rst +++ /dev/null @@ -1,99 +0,0 @@ -.. SPDX-License-Identifier: GPL-2.0 - -====================== -Memory Protection Keys -====================== - -Memory Protection Keys for Userspace (PKU aka PKEYs) is a feature -which is found on Intel's Skylake "Scalable Processor" Server CPUs. -It will be avalable in future non-server parts. - -For anyone wishing to test or use this feature, it is available in -Amazon's EC2 C5 instances and is known to work there using an Ubuntu -17.04 image. - -Memory Protection Keys provides a mechanism for enforcing page-based -protections, but without requiring modification of the page tables -when an application changes protection domains. It works by -dedicating 4 previously ignored bits in each page table entry to a -"protection key", giving 16 possible keys. - -There is also a new user-accessible register (PKRU) with two separate -bits (Access Disable and Write Disable) for each key. Being a CPU -register, PKRU is inherently thread-local, potentially giving each -thread a different set of protections from every other thread. - -There are two new instructions (RDPKRU/WRPKRU) for reading and writing -to the new register. The feature is only available in 64-bit mode, -even though there is theoretically space in the PAE PTEs. These -permissions are enforced on data access only and have no effect on -instruction fetches. - -Syscalls -======== - -There are 3 system calls which directly interact with pkeys:: - - int pkey_alloc(unsigned long flags, unsigned long init_access_rights) - int pkey_free(int pkey); - int pkey_mprotect(unsigned long start, size_t len, - unsigned long prot, int pkey); - -Before a pkey can be used, it must first be allocated with -pkey_alloc(). An application calls the WRPKRU instruction -directly in order to change access permissions to memory covered -with a key. In this example WRPKRU is wrapped by a C function -called pkey_set(). -:: - - int real_prot = PROT_READ|PROT_WRITE; - pkey = pkey_alloc(0, PKEY_DISABLE_WRITE); - ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0); - ret = pkey_mprotect(ptr, PAGE_SIZE, real_prot, pkey); - ... application runs here - -Now, if the application needs to update the data at 'ptr', it can -gain access, do the update, then remove its write access:: - - pkey_set(pkey, 0); // clear PKEY_DISABLE_WRITE - *ptr = foo; // assign something - pkey_set(pkey, PKEY_DISABLE_WRITE); // set PKEY_DISABLE_WRITE again - -Now when it frees the memory, it will also free the pkey since it -is no longer in use:: - - munmap(ptr, PAGE_SIZE); - pkey_free(pkey); - -.. note:: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions. - An example implementation can be found in - tools/testing/selftests/x86/protection_keys.c. - -Behavior -======== - -The kernel attempts to make protection keys consistent with the -behavior of a plain mprotect(). For instance if you do this:: - - mprotect(ptr, size, PROT_NONE); - something(ptr); - -you can expect the same effects with protection keys when doing this:: - - pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ); - pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE, pkey); - something(ptr); - -That should be true whether something() is a direct access to 'ptr' -like:: - - *ptr = foo; - -or when the kernel does the access on the application's behalf like -with a read():: - - read(fd, ptr, 1); - -The kernel will send a SIGSEGV in both cases, but si_code will be set -to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when -the plain mprotect() permissions are violated. -- cgit v1.2.3-55-g7522