[PATCH] tpm: eventlog: check the payload size before reading tag events

[Guanglei Zhu <zhugl3@xiaopeng.com>]

The EVENT_TAG branch of get_event_name() casts the event payload to
struct tcpa_pc_event and reads from it:

	pc_event = (struct tcpa_pc_event *)event_entry;

	switch (do_endian_conversion(pc_event->event_id)) {
	...
		for (i = 0; i < 20; i++)
			d_len += sprintf(&data[2*i], "%02x",
					pc_event->event_data[i]);

The log iterators only verify that the event header plus the declared
event_size stays within the log buffer; they do not require the
payload of an EVENT_TAG entry to be large enough for the fields the
branch reads. A truncated entry makes the branch read up to 20 bytes
of event_data[] past the end of the entry, and, for the last entry in
the log, past the end of the buffer itself.

Skip entries whose payload cannot even hold the event id, and skip
the 20-byte digest dump when the payload cannot hold the full
tcpa_pc_event header plus the digest, so the branch never reads
beyond the entry it is formatting.

Fixes: 55a82ab3181b ("[PATCH] tpm: add bios measurement log")
Cc: stable@vger.kernel.org
Signed-off-by: Guanglei Zhu <zhugl3@xiaopeng.com>
---
 drivers/char/tpm/eventlog/tpm1.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/char/tpm/eventlog/tpm1.c b/drivers/char/tpm/eventlog/tpm1.c
index 0397e3361020..f93a358a706e 100644
--- a/drivers/char/tpm/eventlog/tpm1.c
+++ b/drivers/char/tpm/eventlog/tpm1.c
@@ -178,6 +178,10 @@ static int get_event_name(char *dest, struct tcpa_event *event,
 		}
 		break;
 	case EVENT_TAG:
+		if (do_endian_conversion(event->event_size) <
+		    sizeof(pc_event->event_id))
+			break;
+
 		pc_event = (struct tcpa_pc_event *)event_entry;
 
 		/* ToDo Row data -> Base64 */
@@ -203,6 +207,9 @@ static int get_event_name(char *dest, struct tcpa_event *event,
 			name = tcpa_pc_event_id_strings[do_endian_conversion
 							(pc_event->event_id)];
 			n_len = strlen(name);
+			if (do_endian_conversion(event->event_size) <
+			    sizeof(struct tcpa_pc_event) + 20)
+				break;
 			for (i = 0; i < 20; i++)
 				d_len += sprintf(&data[2*i], "%02x",
 						pc_event->event_data[i]);
-- 
2.50.1