powerpc/rtas: Serialize firmware activation sequences

Use rtas_ibm_activate_firmware_lock to prevent interleaving call
sequences of the ibm,activate-firmware RTAS function, which typically
requires multiple calls to complete the update. While the spec does
not specifically prohibit interleaved sequences, there's almost
certainly no advantage to allowing them.
Reviewed-by: "Aneesh Kumar K.V (IBM)" <aneesh.kumar@kernel.org>
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231212-papr-sys_rtas-vs-lockdown-v6-7-e9eafd0c8c6c@linux.ibm.com

arch/powerpc/kernel/rtas.c

@@ -1734,10 +1734,14 @@ void rtas_activate_firmware(void)
 		return;
 	}
 
+	mutex_lock(&rtas_ibm_activate_firmware_lock);
+
 	do {
 		fwrc = rtas_call(token, 0, 1, NULL);
 	} while (rtas_busy_delay(fwrc));
 
+	mutex_unlock(&rtas_ibm_activate_firmware_lock);
+
 	if (fwrc)
 		pr_err("ibm,activate-firmware failed (%i)\n", fwrc);
 }