diff options
Diffstat (limited to 'Documentation/x86')
| -rw-r--r-- | Documentation/x86/protection-keys.txt | 70 | ||||
| -rw-r--r-- | Documentation/x86/x86_64/mm.txt | 6 |
2 files changed, 67 insertions, 9 deletions
diff --git a/Documentation/x86/protection-keys.txt b/Documentation/x86/protection-keys.txt index c281ded1ba16..b64304540821 100644 --- a/Documentation/x86/protection-keys.txt +++ b/Documentation/x86/protection-keys.txt @@ -18,10 +18,68 @@ 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. -=========================== Config Option =========================== +=========================== Syscalls =========================== -This config option adds approximately 1.5kb of text. and 50 bytes of -data to the executable. A workload which does large O_DIRECT reads -of holes in XFS files was run to exercise get_user_pages_fast(). No -performance delta was observed with the config option -enabled or disabled. +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_DENY_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_DENY_WRITE + *ptr = foo; // assign something + pkey_set(pkey, PKEY_DENY_WRITE); // set PKEY_DENY_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. diff --git a/Documentation/x86/x86_64/mm.txt b/Documentation/x86/x86_64/mm.txt index 8c7dd5957ae1..5724092db811 100644 --- a/Documentation/x86/x86_64/mm.txt +++ b/Documentation/x86/x86_64/mm.txt @@ -12,13 +12,13 @@ ffffc90000000000 - ffffe8ffffffffff (=45 bits) vmalloc/ioremap space ffffe90000000000 - ffffe9ffffffffff (=40 bits) hole ffffea0000000000 - ffffeaffffffffff (=40 bits) virtual memory map (1TB) ... unused hole ... -ffffec0000000000 - fffffc0000000000 (=44 bits) kasan shadow memory (16TB) +ffffec0000000000 - fffffbffffffffff (=44 bits) kasan shadow memory (16TB) ... unused hole ... ffffff0000000000 - ffffff7fffffffff (=39 bits) %esp fixup stacks ... unused hole ... -ffffffef00000000 - ffffffff00000000 (=64 GB) EFI region mapping space +ffffffef00000000 - fffffffeffffffff (=64 GB) EFI region mapping space ... unused hole ... -ffffffff80000000 - ffffffffa0000000 (=512 MB) kernel text mapping, from phys 0 +ffffffff80000000 - ffffffff9fffffff (=512 MB) kernel text mapping, from phys 0 ffffffffa0000000 - ffffffffff5fffff (=1526 MB) module mapping space ffffffffff600000 - ffffffffffdfffff (=8 MB) vsyscalls ffffffffffe00000 - ffffffffffffffff (=2 MB) unused hole |