powerpc/pseries: Fix endianness issue when parsing PLPKS secvar flags

When a user updates a variable through the PLPKS secvar interface, we take
the first 8 bytes of the data written to the update attribute to pass
through to the H_PKS_SIGNED_UPDATE hcall as flags. These bytes are always
written in big-endian format.

Currently, the flags bytes are memcpy()ed into a u64, which is then loaded
into a register to pass as part of the hcall. This means that on LE
systems, the bytes are in the wrong order.

Use be64_to_cpup() instead, to ensure the flags bytes are byteswapped if
necessary.

Reported-by: Stefan Berger <stefanb@linux.ibm.com>
Fixes: ccadf154cb ("powerpc/pseries: Implement secvars for dynamic secure boot")
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230216070903.355091-1-ajd@linux.ibm.com

arch/powerpc/platforms/pseries/plpks-secvar.c

@@ -135,7 +135,8 @@ static int plpks_set_variable(const char *key, u64 key_len, u8 *data,
 		goto err;
 	var.namelen = rc * 2;
 
-	memcpy(&flags, data, sizeof(flags));
+	// Flags are contained in the first 8 bytes of the buffer, and are always big-endian
+	flags = be64_to_cpup((__be64 *)data);
 
 	var.datalen = data_size - sizeof(flags);
 	var.data = data + sizeof(flags);