Add documentation updates that capture PowerPC specific changes. Signed-off-by: Thiago Jung Bauermann <bauerman@xxxxxxxxxxxxxxxxxx> Signed-off-by: Ram Pai <linuxram@xxxxxxxxxx> --- Documentation/vm/protection-keys.txt | 125 +++++++++++++++++++++++++++------- 1 files changed, 100 insertions(+), 25 deletions(-) diff --git a/Documentation/vm/protection-keys.txt b/Documentation/vm/protection-keys.txt index b643045..ef5f73b 100644 --- a/Documentation/vm/protection-keys.txt +++ b/Documentation/vm/protection-keys.txt @@ -1,22 +1,45 @@ -Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature -which will be found on future Intel CPUs. - -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. +Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature found on +future Intel CPUs and on PowerPC 5 and higher CPUs. + +Memory Protection Keys provide 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 bits in each page table entry to a "protection key". +There is also a user-accessible register with two separate bits for each +key. Being a CPU register, the user-accessible register is inherently +thread-local, potentially giving each thread a different set of protections +from every other thread. + +On Intel: + + Four previously bits are used the page table entry giving 16 possible keys. + + The user accessible register(PKRU) has a bit each per key to disable + access and to disable write. + + 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. + +On PowerPC: + + Five bits in the page table entry are used giving 32 possible keys. + This support is currently for Hash Page Table mode only. + + The user accessible register(AMR) has a bit each per key to disable + read and write. Access disable can be achieved by disabling + read and write. + + 'mtspr 0xd, mem' reads the AMR register + 'mfspr mem, 0xd' writes into the AMR register. + + Execution can be disabled by allocating a key with execute-disabled + permission. The execute-permissions on the key; however, cannot be + changed through a user accessible register. The CPU will not allow + execution of instruction in pages that are associated with + execute-disabled key. + =========================== Syscalls =========================== @@ -28,9 +51,9 @@ There are 3 system calls which directly interact with pkeys: 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 +pkey_alloc(). An application calls the WRPKRU/AMR instruction directly in order to change access permissions to memory covered -with a key. In this example WRPKRU is wrapped by a C function +with a key. In this example WRPKRU/AMR is wrapped by a C function called pkey_set(). int real_prot = PROT_READ|PROT_WRITE; @@ -52,11 +75,11 @@ is no longer in use: munmap(ptr, PAGE_SIZE); pkey_free(pkey); -(Note: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions. +(Note: pkey_set() is a wrapper for the RDPKRU,WRPKRU or AMR instructions. An example implementation can be found in - tools/testing/selftests/x86/protection_keys.c) + tools/testing/selftests/vm/protection_keys.c) -=========================== Behavior =========================== +=========================== Behavior ================================= The kernel attempts to make protection keys consistent with the behavior of a plain mprotect(). For instance if you do this: @@ -66,7 +89,7 @@ behavior of a plain mprotect(). For instance if you do this: you can expect the same effects with protection keys when doing this: - pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ); + pkey = pkey_alloc(0, PKEY_DISABLE_ACCESS); pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE, pkey); something(ptr); @@ -83,3 +106,55 @@ with a read(): 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. + +========================== sysfs Interface ========================== + +Information about support of protection keys on the system can be +found in the /sys/kernel/mm/protection_keys directory, which +contains the following files: + +- total_keys: Shows the number of keys supported by the hardware. + Not all of those keys may be available for use by a process + because the platform or operating system may reserve some keys + for their own use. + +- usable_keys: Shows the minimum number of keys guaranteed to be + available for use by a process. In other words: total_keys minus + the keys reserved by the platform or operating system. This + number doesn't change to reflect keys that are already being + used by the process reading the file. + + There may be one more key available than what is advertised in + this file because the kernel may use one key for mprotect() + calls setting up memory with execute-only permissions. This file + assumes that this key is being used, but if it is not the + process will have one more key it can use for other purposes. + +- disable_access_supported: Tells whether the system supports keys + which disallow reading from a given page (i.e., the + PKEY_DISABLE_ACCESS flag is supported). + +- disable_write_supported: Tells whether the system supports keys + which disallow writing to a given page (i.e., the + PKEY_DISABLE_WRITE flag is supported). + +- disable_execute_supported: Tells whether the system supports keys + which disallow code execution from a given page (i.e., the + PKEY_DISABLE_EXECUTE flag is supported). + +==================================================================== + Semantic differences + +The following semantic differences exist between x86 and power. + +a) powerpc (PowerPC8 onwards) *also* allows creation of a key with + execute-disabled. + The following is allowed + pkey = pkey_alloc(0, PKEY_DISABLE_EXECUTE); + +b) changes to the key permission bits from a signal handler do not + persist on x86. The PKRU specific fpregs entry need to be + modified for it to persist. On powerpc the key permission can + be modified by programming the AMR register from the signal + handler. The changes persist across signal boundaries. +===================================================================== -- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
