efi/libstub/tpm: Initialize pointer variables to zero for mixed mode

As reported by Jeremy Cline, running the new TPM libstub code in mixed
mode (i.e., 64-bit kernel on 32-bit UEFI) results in hangs when invoking
the TCG2 protocol, or when accessing the log_tbl pool allocation.

The reason turns out to be that in both cases, the 64-bit pointer
variables are not fully initialized by the 32-bit EFI code, and so
we should take care to zero initialize these variables beforehand,
or we'll end up dereferencing bogus pointers.

Reported-by: Jeremy Cline <jeremy@jcline.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: hdegoede@redhat.com
Cc: jarkko.sakkinen@linux.intel.com
Cc: javierm@redhat.com
Cc: linux-efi@vger.kernel.org
Cc: tweek@google.com
Link: http://lkml.kernel.org/r/20180313140922.17266-2-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

drivers/firmware/efi/libstub/tpm.c

@@ -68,11 +68,11 @@ void efi_retrieve_tpm2_eventlog_1_2(efi_system_table_t *sys_table_arg)
 	efi_guid_t linux_eventlog_guid = LINUX_EFI_TPM_EVENT_LOG_GUID;
 	efi_status_t status;
 	efi_physical_addr_t log_location, log_last_entry;
-	struct linux_efi_tpm_eventlog *log_tbl;
+	struct linux_efi_tpm_eventlog *log_tbl = NULL;
 	unsigned long first_entry_addr, last_entry_addr;
 	size_t log_size, last_entry_size;
 	efi_bool_t truncated;
-	void *tcg2_protocol;
+	void *tcg2_protocol = NULL;
 
 	status = efi_call_early(locate_protocol, &tcg2_guid, NULL,
 				&tcg2_protocol);