linux-kernel - [PATCH 07/10] pkeys: add details of system call use to Documentation/

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Message-Id: <20160729163020.59350E33@viggo.jf.intel.com>
Date:	Fri, 29 Jul 2016 09:30:20 -0700
From:	Dave Hansen <dave@...1.net>
To:	linux-kernel@...r.kernel.org
Cc:	x86@...nel.org, linux-api@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-mm@...ck.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	luto@...nel.org, mgorman@...hsingularity.net,
	Dave Hansen <dave@...1.net>, dave.hansen@...ux.intel.com,
	arnd@...db.de
Subject: [PATCH 07/10] pkeys: add details of system call use to Documentation/


From: Dave Hansen <dave.hansen@...ux.intel.com>

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-api@...r.kernel.org
Cc: linux-arch@...r.kernel.org
Cc: linux-mm@...ck.org
Cc: x86@...nel.org
Cc: torvalds@...ux-foundation.org
Cc: akpm@...ux-foundation.org
Cc: Arnd Bergmann <arnd@...db.de>
Cc: mgorman@...hsingularity.net
---

 b/Documentation/x86/protection-keys.txt |   62 ++++++++++++++++++++++++++++++++
 1 file changed, 62 insertions(+)

diff -puN Documentation/x86/protection-keys.txt~pkeys-120-syscall-docs Documentation/x86/protection-keys.txt
--- a/Documentation/x86/protection-keys.txt~pkeys-120-syscall-docs	2016-07-29 09:18:58.863582283 -0700
+++ b/Documentation/x86/protection-keys.txt	2016-07-29 09:18:58.866582419 -0700
@@ -18,6 +18,68 @@ even though there is theoretically space
 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
_