powerpc/mm/pkeys: Make pkey access check work on execute_only_key

Jan reported that LTP mmap03 was getting stuck in a page fault loop
after commit c46241a370 ("powerpc/pkeys: Check vma before returning
key fault error to the user"), as well as a minimised reproducer:

  #include <fcntl.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <unistd.h>
  #include <sys/mman.h>

  int main(int ac, char **av)
  {
  	int page_sz = getpagesize();
  	int fildes;
  	char *addr;

  	fildes = open("tempfile", O_WRONLY | O_CREAT, 0666);
  	write(fildes, &fildes, sizeof(fildes));
  	close(fildes);

  	fildes = open("tempfile", O_RDONLY);
  	unlink("tempfile");

  	addr = mmap(0, page_sz, PROT_EXEC, MAP_FILE | MAP_PRIVATE, fildes, 0);

  	printf("%d\n", *addr);
  	return 0;
  }

And noticed that access_pkey_error() in page fault handler now always
seem to return false:

  __do_page_fault
    access_pkey_error(is_pkey: 1, is_exec: 0, is_write: 0)
      arch_vma_access_permitted
	pkey_access_permitted
	  if (!is_pkey_enabled(pkey))
	    return true
      return false

pkey_access_permitted() should not check if the pkey is available in
UAMOR (using is_pkey_enabled()). The kernel needs to do that check
only when allocating keys. This also makes sure the execute_only_key
which is marked as non-manageable via UAMOR is handled correctly in
pkey_access_permitted(), and fixes the bug.

Fixes: c46241a370 ("powerpc/pkeys: Check vma before returning key fault error to the user")
Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
[mpe: Include bug report details etc. in the change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200627070147.297535-1-aneesh.kumar@linux.ibm.com

arch/powerpc/mm/book3s64/pkeys.c

@@ -353,9 +353,6 @@ static bool pkey_access_permitted(int pkey, bool write, bool execute)
 	int pkey_shift;
 	u64 amr;
 
-	if (!is_pkey_enabled(pkey))
-		return true;
-
 	pkey_shift = pkeyshift(pkey);
 	if (execute && !(read_iamr() & (IAMR_EX_BIT << pkey_shift)))
 		return true;