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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>
This commit is contained in:
Dave Hansen 2016-07-29 09:30:20 -07:00 committed by Thomas Gleixner
parent a60f7b69d9
commit c74fe39408
1 changed files with 62 additions and 0 deletions
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62
Documentation/x86/protection-keys.txt
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@ -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