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authorDave Hansen <dave.hansen@linux.intel.com>2016-07-29 09:30:20 -0700
committerThomas Gleixner <tglx@linutronix.de>2016-09-09 13:02:28 +0200
commitc74fe3940848c6afea83bfbda64a9baf9da547c8 (patch)
treef2b4047c9e1da051ec79f775adf4e56cd5bc30c6 /Documentation/x86
parenta60f7b69d92c0142c80a30d669a76b617b7f6879 (diff)
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@linux.intel.com> Cc: linux-arch@vger.kernel.org Cc: Dave Hansen <dave@sr71.net> Cc: mgorman@techsingularity.net Cc: arnd@arndb.de Cc: linux-api@vger.kernel.org Cc: linux-mm@kvack.org Cc: luto@kernel.org Cc: akpm@linux-foundation.org Cc: torvalds@linux-foundation.org Link: http://lkml.kernel.org/r/20160729163020.59350E33@viggo.jf.intel.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Diffstat (limited to 'Documentation/x86')
-rw-r--r--Documentation/x86/protection-keys.txt62
1 files changed, 62 insertions, 0 deletions
diff --git a/Documentation/x86/protection-keys.txt b/Documentation/x86/protection-keys.txt
index c281ded1ba16..6da7689601d1 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