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Subject[PATCH 6/6] Documentation/vm: PowerPC specific updates to memory protection keys
Date
Add documentation updates that capture PowerPC specific changes.

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
---
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
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