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Date: Fri, 9 Sep 2016 04:13:17 -0700
From: tip-bot for Dave Hansen <tipbot@...or.com>
To: linux-tip-commits@...r.kernel.org
Cc: dave@...1.net, linux-kernel@...r.kernel.org, tglx@...utronix.de,
hpa@...or.com, dave.hansen@...ux.intel.com, mingo@...nel.org
Subject: [tip:mm/pkeys] pkeys: Add details of system call use to
Documentation/
Commit-ID: c74fe3940848c6afea83bfbda64a9baf9da547c8
Gitweb: http://git.kernel.org/tip/c74fe3940848c6afea83bfbda64a9baf9da547c8
Author: Dave Hansen <dave.hansen@...ux.intel.com>
AuthorDate: Fri, 29 Jul 2016 09:30:20 -0700
Committer: Thomas Gleixner <tglx@...utronix.de>
CommitDate: Fri, 9 Sep 2016 13:02:28 +0200
pkeys: Add details of system call use to Documentation/
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.hansen@...ux.intel.com>
Cc: linux-arch@...r.kernel.org
Cc: Dave Hansen <dave@...1.net>
Cc: mgorman@...hsingularity.net
Cc: arnd@...db.de
Cc: linux-api@...r.kernel.org
Cc: linux-mm@...ck.org
Cc: luto@...nel.org
Cc: akpm@...ux-foundation.org
Cc: torvalds@...ux-foundation.org
Link: http://lkml.kernel.org/r/20160729163020.59350E33@viggo.jf.intel.com
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
Documentation/x86/protection-keys.txt | 62 +++++++++++++++++++++++++++++++++++
1 file changed, 62 insertions(+)
diff --git a/Documentation/x86/protection-keys.txt b/Documentation/x86/protection-keys.txt
index c281ded..6da7689 100644
--- a/Documentation/x86/protection-keys.txt
+++ b/Documentation/x86/protection-keys.txt
@@ -18,6 +18,68 @@ 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 2 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);
+
+=========================== 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.
+
=========================== Config Option ===========================
This config option adds approximately 1.5kb of text. and 50 bytes of
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