1 .. SPDX-License-Identifier: GPL-2.0 2 3 ====================== 4 Memory Protection Keys 5 ====================== 6 7 Memory Protection Keys provide a mechanism for enforcing page-based 8 protections, but without requiring modification of the page tables when an 9 application changes protection domains. 10 11 Pkeys Userspace (PKU) is a feature which can be found on: 12 * Intel server CPUs, Skylake and later 13 * Intel client CPUs, Tiger Lake (11th Gen Core) and later 14 * Future AMD CPUs 15 16 Pkeys work by dedicating 4 previously Reserved bits in each page table entry to 17 a "protection key", giving 16 possible keys. 18 19 Protections for each key are defined with a per-CPU user-accessible register 20 (PKRU). Each of these is a 32-bit register storing two bits (Access Disable 21 and Write Disable) for each of 16 keys. 22 23 Being a CPU register, PKRU is inherently thread-local, potentially giving each 24 thread a different set of protections from every other thread. 25 26 There are two instructions (RDPKRU/WRPKRU) for reading and writing to the 27 register. The feature is only available in 64-bit mode, even though there is 28 theoretically space in the PAE PTEs. These permissions are enforced on data 29 access only and have no effect on instruction fetches. 30 31 Syscalls 32 ======== 33 34 There are 3 system calls which directly interact with pkeys:: 35 36 int pkey_alloc(unsigned long flags, unsigned long init_access_rights) 37 int pkey_free(int pkey); 38 int pkey_mprotect(unsigned long start, size_t len, 39 unsigned long prot, int pkey); 40 41 Before a pkey can be used, it must first be allocated with 42 pkey_alloc(). An application calls the WRPKRU instruction 43 directly in order to change access permissions to memory covered 44 with a key. In this example WRPKRU is wrapped by a C function 45 called pkey_set(). 46 :: 47 48 int real_prot = PROT_READ|PROT_WRITE; 49 pkey = pkey_alloc(0, PKEY_DISABLE_WRITE); 50 ptr = mmap(NULL, PAGE_SIZE, PROT_NONE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0); 51 ret = pkey_mprotect(ptr, PAGE_SIZE, real_prot, pkey); 52 ... application runs here 53 54 Now, if the application needs to update the data at 'ptr', it can 55 gain access, do the update, then remove its write access:: 56 57 pkey_set(pkey, 0); // clear PKEY_DISABLE_WRITE 58 *ptr = foo; // assign something 59 pkey_set(pkey, PKEY_DISABLE_WRITE); // set PKEY_DISABLE_WRITE again 60 61 Now when it frees the memory, it will also free the pkey since it 62 is no longer in use:: 63 64 munmap(ptr, PAGE_SIZE); 65 pkey_free(pkey); 66 67 .. note:: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions. 68 An example implementation can be found in 69 tools/testing/selftests/x86/protection_keys.c. 70 71 Behavior 72 ======== 73 74 The kernel attempts to make protection keys consistent with the 75 behavior of a plain mprotect(). For instance if you do this:: 76 77 mprotect(ptr, size, PROT_NONE); 78 something(ptr); 79 80 you can expect the same effects with protection keys when doing this:: 81 82 pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ); 83 pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE, pkey); 84 something(ptr); 85 86 That should be true whether something() is a direct access to 'ptr' 87 like:: 88 89 *ptr = foo; 90 91 or when the kernel does the access on the application's behalf like 92 with a read():: 93 94 read(fd, ptr, 1); 95 96 The kernel will send a SIGSEGV in both cases, but si_code will be set 97 to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when 98 the plain mprotect() permissions are violated.
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