efi/tpm: Don't access event->count when it isn't mapped

Some machines generate a lot of event log entries.  When we're
iterating over them, the code removes the old mapping and adds a
new one, so once we cross the page boundary we're unmapping the page
with the count on it.  Hilarity ensues.

This patch keeps the info from the header in local variables so we don't
need to access that page again or keep track of if it's mapped.

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: 44038bc ("tpm: Abstract crypto agile event size calculations")
Link: https://lkml.kernel.org/r/[email protected]
[ Minor edits. ]
Signed-off-by: Ingo Molnar <[email protected]>

Author: vathpela

include/linux/tpm_eventlog.h

@@ -170,6 +170,7 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
 	u16 halg;
 	int i;
 	int j;
+	u32 count, event_type;
 
 	marker = event;
 	marker_start = marker;
@@ -190,16 +191,22 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
 	}
 
 	event = (struct tcg_pcr_event2_head *)mapping;
+	/*
+	 * The loop below will unmap these fields if the log is larger than
+	 * one page, so save them here for reference:
+	 */
+	count = READ_ONCE(event->count);
+	event_type = READ_ONCE(event->event_type);
 
 	efispecid = (struct tcg_efi_specid_event_head *)event_header->event;
 
 	/* Check if event is malformed. */
-	if (event->count > efispecid->num_algs) {
+	if (count > efispecid->num_algs) {
 		size = 0;
 		goto out;
 	}
 
-	for (i = 0; i < event->count; i++) {
+	for (i = 0; i < count; i++) {
 		halg_size = sizeof(event->digests[i].alg_id);
 
 		/* Map the digest's algorithm identifier */
@@ -256,8 +263,9 @@ static inline int __calc_tpm2_event_size(struct tcg_pcr_event2_head *event,
 		+ event_field->event_size;
 	size = marker - marker_start;
 
-	if ((event->event_type == 0) && (event_field->event_size == 0))
+	if (event_type == 0 && event_field->event_size == 0)
 		size = 0;
+
 out:
 	if (do_mapping)
 		TPM_MEMUNMAP(mapping, mapping_size);