efi/tpm: Don't traverse an event log with no events

When there are no entries to put into the final event log, some machines
will return the template they would have populated anyway.  In this case
the nr_events field is 0, but the rest of the log is just garbage.

This patch stops us from trying to iterate the table with
__calc_tpm2_event_size() when the number of events in the table is 0.

Tested-by: Lyude Paul <[email protected]>
Signed-off-by: Peter Jones <[email protected]>
Signed-off-by: Jarkko Sakkinen <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
Reviewed-by: Jarkko Sakkinen <[email protected]>
Acked-by: Matthew Garrett <[email protected]>
Acked-by: Ard Biesheuvel <[email protected]>
Cc: Ben Dooks <[email protected]>
Cc: Dave Young <[email protected]>
Cc: Jerry Snitselaar <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Lukas Wunner <[email protected]>
Cc: Octavian Purdila <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Scott Talbert <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Fixes: c46f340 ("tpm: Reserve the TPM final events table")
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: vathpela

drivers/firmware/efi/tpm.c

@@ -75,11 +75,16 @@ int __init efi_tpm_eventlog_init(void)
 		goto out;
 	}
 
-	tbl_size = tpm2_calc_event_log_size((void *)efi.tpm_final_log
-					    + sizeof(final_tbl->version)
-					    + sizeof(final_tbl->nr_events),
-					    final_tbl->nr_events,
-					    log_tbl->log);
+	tbl_size = 0;
+	if (final_tbl->nr_events != 0) {
+		void *events = (void *)efi.tpm_final_log
+				+ sizeof(final_tbl->version)
+				+ sizeof(final_tbl->nr_events);
+
+		tbl_size = tpm2_calc_event_log_size(events,
+						    final_tbl->nr_events,
+						    log_tbl->log);
+	}
 	memblock_reserve((unsigned long)final_tbl,
 			 tbl_size + sizeof(*final_tbl));
 	early_memunmap(final_tbl, sizeof(*final_tbl));