powerpc/pkeys: Deny read/write/execute by default

Deny all permissions on all keys, with some exceptions. pkey-0 must
allow all permissions, or else everything comes to a screaching halt.
Execute-only key must allow execute permission.

Fixes: cf43d3b2 ("powerpc: Enable pkey subsystem")
Cc: stable@vger.kernel.org # v4.16+
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/pkeys.c

@@ -124,12 +124,10 @@ int pkey_initialize(void)
 
 	/* register mask is in BE format */
 	pkey_amr_mask = ~0x0ul;
-	pkey_iamr_mask = ~0x0ul;
+	pkey_amr_mask &= ~(0x3ul << pkeyshift(0));
 
-	for (i = 0; i < (pkeys_total - os_reserved); i++) {
-		pkey_amr_mask &= ~(0x3ul << pkeyshift(i));
-		pkey_iamr_mask &= ~(0x1ul << pkeyshift(i));
-	}
+	pkey_iamr_mask = ~0x0ul;
+	pkey_iamr_mask &= ~(0x3ul << pkeyshift(0));
 
 	pkey_uamor_mask = ~0x0ul;
 	pkey_uamor_mask &= ~(0x3ul << pkeyshift(0));