powerpc/pkeys: Detach execute_only key on !PROT_EXEC

Disassociate the exec_key from a VMA if the VMA permission is not
PROT_EXEC anymore. Otherwise the exec_only key continues to be
associated with the vma, causing unexpected behavior.

The problem was reported on x86 by Shakeel Butt, which is also
applicable on powerpc.

Fixes: 5586cf61 ("powerpc: introduce execute-only pkey")
Cc: stable@vger.kernel.org # v4.16+
Reported-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/pkeys.c

@@ -383,9 +383,9 @@ int __arch_override_mprotect_pkey(struct vm_area_struct *vma, int prot,
 {
 	/*
 	 * If the currently associated pkey is execute-only, but the requested
-	 * protection requires read or write, move it back to the default pkey.
+	 * protection is not execute-only, move it back to the default pkey.
 	 */
-	if (vma_is_pkey_exec_only(vma) && (prot & (PROT_READ | PROT_WRITE)))
+	if (vma_is_pkey_exec_only(vma) && (prot != PROT_EXEC))
 		return 0;
 
 	/*