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Date: Tue, 27 Jun 2017 03:11:58 -0700
From: Ram Pai <linuxram@...ibm.com>
To: linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
linux-arch@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org
Cc: benh@...nel.crashing.org, paulus@...ba.org, mpe@...erman.id.au,
khandual@...ux.vnet.ibm.com, aneesh.kumar@...ux.vnet.ibm.com,
bsingharora@...il.com, dave.hansen@...el.com, hbabu@...ibm.com,
linuxram@...ibm.com, arnd@...db.de, akpm@...ux-foundation.org,
corbet@....net, mingo@...hat.com
Subject: [RFC v4 16/17] Documentation: PowerPC specific updates to memory protection keys
Add documentation updates that capture PowerPC specific changes.
Signed-off-by: Ram Pai <linuxram@...ibm.com>
---
Documentation/vm/protection-keys.txt | 89 ++++++++++++++++++++++++++++--------
1 file changed, 69 insertions(+), 20 deletions(-)
diff --git a/Documentation/vm/protection-keys.txt b/Documentation/vm/protection-keys.txt
index b643045..889f32e 100644
--- a/Documentation/vm/protection-keys.txt
+++ b/Documentation/vm/protection-keys.txt
@@ -1,21 +1,46 @@
-Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature
-which will be found on future Intel CPUs.
+Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature found in
+new generation of intel CPUs and on PowerPC 7 and higher 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
+protections, but without requiring modification of the page tables when an
+application changes protection domains.
+
+
+On Intel:
+
+ 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.
+
+
+On PowerPC:
+
+ It works by dedicating 5 hash-page table entry bits to a "protection key",
+ giving 32 possible keys.
+
+ There is a user-accessible register (AMR) with two separate bits;
+ Access Disable and Write Disable, for each key. Being a CPU
+ register, AMR is inherently thread-local, potentially giving each
+ thread a different set of protections from every other thread. NOTE:
+ Disabling read permission does not disable write and vice-versa.
+
+ The feature is available on 64-bit HPTE mode only.
+ 'mtspr 0xd, mem' reads the AMR register
+ 'mfspr mem, 0xd' writes into the AMR register.
+
+
+
+Permissions are enforced on data access only and have no effect on
instruction fetches.
=========================== Syscalls ===========================
@@ -28,9 +53,9 @@ 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 +77,11 @@ 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)
-=========================== Behavior ===========================
+=========================== Behavior =================================
The kernel attempts to make protection keys consistent with the
behavior of a plain mprotect(). For instance if you do this:
@@ -83,3 +108,27 @@ 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.
+
+
+====================================================================
+ Semantic differences
+
+The following semantic differences exist between x86 and power.
+
+a) powerpc allows creation of a key with execute-disabled. The following
+ is allowed on powerpc.
+ pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_READ |
+ PKEY_DISABLE_EXECUTE);
+ x86 disallows PKEY_DISABLE_EXECUTE during key creation.
+
+b) x86, PKEY_DISABLE_READ disables read and write on the key.
+ Powerpc, PKEY_DISABLE_READ just disables read.
+ PKEY_DISABLE_WRITE has to be specified explicitly to disable write.
+
+c) changing the permission bits of a key from a signal handler does not
+ persist on x86. The PKRU specific fpregs entry needs to be modified
+ for it to persist. On powerpc the permission bits of the key can be
+ modified by programming the AMR register from the signal handler.
+ The changes persists across signal boundaries.
+
+=====================================================================
--
1.8.3.1
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