Merge tag 'cris-for-4.9' of git://git.kernel.org/pub/scm/linux/kernel/git/jesper...

[cascardo/linux.git] / Documentation / x86 / protection-keys.txt

diff --git a/Documentation/x86/protection-keys.txt b/Documentation/x86/protection-keys.txt
index c281ded..b643045 100644 (file)
--- a/Documentation/x86/protection-keys.txt
+++ b/Documentation/x86/protection-keys.txt
@@ -18,10 +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.
 
-=========================== Config Option ===========================
+=========================== Syscalls ===========================
 
-This config option adds approximately 1.5kb of text. and 50 bytes of
-data to the executable.  A workload which does large O_DIRECT reads
-of holes in XFS files was run to exercise get_user_pages_fast().  No
-performance delta was observed with the config option
-enabled or disabled.
+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.