[PATCH 7/8] pkeys: add details of system call use to Documentation/

From: Dave Hansen
Date: Tue May 31 2016 - 11:29:14 EST

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxx>
Cc: linux-api@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: x86@xxxxxxxxxx
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx
Cc: akpm@xxxxxxxxxxxxxxxxxxxx

b/Documentation/x86/protection-keys.txt | 63 ++++++++++++++++++++++++++++++++
1 file changed, 63 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 2016-05-31 08:27:50.554178908 -0700
+++ b/Documentation/x86/protection-keys.txt 2016-05-31 08:27:50.558179089 -0700
@@ -18,6 +18,69 @@ even though there is theoretically space
permissions are enforced on data access only and have no effect on
instruction fetches.

+=========================== Syscalls ===========================
+There are 5 system calls which directly interact with pkeys:
+ int pkey_alloc(unsigned long flags, unsigned long init_access_rights)
+ int pkey_free(int pkey);
+ int sys_pkey_mprotect(unsigned long start, size_t len,
+ unsigned long prot, int pkey);
+ unsigned long pkey_get(int pkey);
+ int pkey_set(int pkey, unsigned long access_rights);
+Before a pkey can be used, it must first be allocated with
+pkey_alloc(). An application may either call pkey_set() or the
+WRPKRU instruction directly in order to change access permissions
+to memory covered with a key.
+ int real_prot = PROT_READ|PROT_WRITE;
+ pkey = pkey_alloc(0, PKEY_DENY_WRITE);
+ 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:
+ sys_pkey_mprotect(ptr, size, PROT_READ|PROT_WRITE);
+ something(ptr);
+That should be true whether something() is a direct access to 'ptr'
+ *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