From: Dave Hansen <dave.han...@linux.intel.com> This spells out all of the pkey-related system calls that we have and provides some example code fragments to demonstrate how we expect them to be used.
Signed-off-by: Dave Hansen <dave.han...@linux.intel.com> Cc: linux-...@vger.kernel.org Cc: linux...@kvack.org Cc: x...@kernel.org Cc: torva...@linux-foundation.org Cc: a...@linux-foundation.org --- b/Documentation/x86/protection-keys.txt | 63 ++++++++++++++++++++++++++++++++ 1 file changed, 63 insertions(+) diff -puN Documentation/x86/protection-keys.txt~pkeys-98-syscall-docs Documentation/x86/protection-keys.txt --- a/Documentation/x86/protection-keys.txt~pkeys-98-syscall-docs 2016-02-22 17:09:25.814409138 -0800 +++ b/Documentation/x86/protection-keys.txt 2016-02-22 17:09:25.818409320 -0800 @@ -19,6 +19,69 @@ even though there is theoretically space permissions are enforced on data access only and have no effect on instruction fetches. +=========================== Syscalls =========================== + +There are 5 system calls which directly interact with pkeys: + + int pkey_alloc(unsigned long flags, unsigned long init_access_rights) + int pkey_free(int pkey); + int sys_pkey_mprotect(unsigned long start, size_t len, + unsigned long prot, int pkey); + unsigned long pkey_get(int pkey); + int pkey_set(int pkey, unsigned long access_rights); + +Before a pkey can be used, it must first be allocated with +pkey_alloc(). An application may either call pkey_set() or the +WRPKRU instruction directly in order to change access permissions +to memory covered with a key. + + 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); + +=========================== 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: + + sys_pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ); + sys_pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE); + 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. + =========================== Config Option =========================== This config option adds approximately 1.5kb of text. and 50 bytes of _
