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Message-Id: <1497671564-20030-12-git-send-email-linuxram@us.ibm.com>
Date: Fri, 16 Jun 2017 20:52:43 -0700
From: Ram Pai <linuxram@...ibm.com>
To: linuxppc-dev@...ts.ozlabs.org, linux-kernel@...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
Subject: [RFC v2 11/12]Documentation: Documentation updates.
The Documentaton file is moved from x86 into the generic area,
since this feature is now supported by more than one archs.
Signed-off-by: Ram Pai <linuxram@...ibm.com>
---
Documentation/vm/protection-keys.txt | 110 ++++++++++++++++++++++++++++++++++
Documentation/x86/protection-keys.txt | 85 --------------------------
2 files changed, 110 insertions(+), 85 deletions(-)
create mode 100644 Documentation/vm/protection-keys.txt
delete mode 100644 Documentation/x86/protection-keys.txt
diff --git a/Documentation/vm/protection-keys.txt b/Documentation/vm/protection-keys.txt
new file mode 100644
index 0000000..b49e6bb
--- /dev/null
+++ b/Documentation/vm/protection-keys.txt
@@ -0,0 +1,110 @@
+Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature
+found in new generation of intel CPUs on PowerPC 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.
+
+
+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 page table entry 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 ===========================
+
+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/protection-keys.txt b/Documentation/x86/protection-keys.txt
deleted file mode 100644
index b643045..0000000
--- a/Documentation/x86/protection-keys.txt
+++ /dev/null
@@ -1,85 +0,0 @@
-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.
-
-=========================== Syscalls ===========================
-
-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.
--
1.8.3.1
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