efi/tpm: Only set 'efi_tpm_final_log_size' after successful event log parsing

If __calc_tpm2_event_size() fails to parse an event it will return 0,
resulting tpm2_calc_event_log_size() returning -1. Currently there is
no check of this return value, and 'efi_tpm_final_log_size' can end up
being set to this negative value resulting in a crash like this one:

  BUG: unable to handle page fault for address: ffffbc8fc00866ad
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page

  RIP: 0010:memcpy_erms+0x6/0x10
  Call Trace:
   tpm_read_log_efi()
   tpm_bios_log_setup()
   tpm_chip_register()
   tpm_tis_core_init.cold.9+0x28c/0x466
   tpm_tis_plat_probe()
   platform_drv_probe()
   ...

Also __calc_tpm2_event_size() returns a size of 0 when it fails
to parse an event, so update function documentation to reflect this.

The root cause of the issue that caused the failure of event parsing
in this case is resolved by Peter Jone's patchset dealing with large
event logs where crossing over a page boundary causes the page with
the event count to be unmapped.

Signed-off-by: Jerry Snitselaar <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
Cc: Ben Dooks <[email protected]>
Cc: Dave Young <[email protected]>
Cc: Jarkko Sakkinen <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Lukas Wunner <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: Matthew Garrett <[email protected]>
Cc: Octavian Purdila <[email protected]>
Cc: Peter Jones <[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: snits

drivers/firmware/efi/tpm.c

@@ -85,11 +85,18 @@ int __init efi_tpm_eventlog_init(void)
 						    final_tbl->nr_events,
 						    log_tbl->log);
 	}
+
+	if (tbl_size < 0) {
+		pr_err(FW_BUG "Failed to parse event in TPM Final Events Log\n");
+		goto out_calc;
+	}
+
 	memblock_reserve((unsigned long)final_tbl,
 			 tbl_size + sizeof(*final_tbl));
-	early_memunmap(final_tbl, sizeof(*final_tbl));
 	efi_tpm_final_log_size = tbl_size;
 
+out_calc:
+	early_memunmap(final_tbl, sizeof(*final_tbl));
 out:
 	early_memunmap(log_tbl, sizeof(*log_tbl));
 	return ret;

include/linux/tpm_eventlog.h

@@ -152,7 +152,7 @@ struct tcg_algorithm_info {
  * total. Once we've done this we know the offset of the data length field,
  * and can calculate the total size of the event.
  *
- * Return: size of the event on success, <0 on failure
+ * Return: size of the event on success, 0 on failure
  */
 
 static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,