Input: gtco - bounds check collection indent level

The GTCO tablet input driver configures itself from an HID report sent
via USB during the initial enumeration process. Some debugging messages
are generated during the parsing. A debugging message indentation
counter is not bounds checked, leading to the ability for a specially
crafted HID report to cause '-' and null bytes be written past the end
of the indentation array. As long as the kernel has CONFIG_DYNAMIC_DEBUG
enabled, this code will not be optimized out.  This was discovered
during code review after a previous syzkaller bug was found in this
driver.
Signed-off-by: Grant Hernandez <granthernandez@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/tablet/gtco.c

@@ -78,6 +78,7 @@ Scott Hill shill@gtcocalcomp.com
 
 /* Max size of a single report */
 #define REPORT_MAX_SIZE       10
+#define MAX_COLLECTION_LEVELS  10
 
 
 /* Bitmask whether pen is in range */
@@ -223,8 +224,7 @@ static void parse_hid_report_descriptor(struct gtco *device, char * report,
 	char  maintype = 'x';
 	char  globtype[12];
 	int   indent = 0;
-	char  indentstr[10] = "";
-
+	char  indentstr[MAX_COLLECTION_LEVELS + 1] = { 0 };
 
 	dev_dbg(ddev, "======>>>>>>PARSE<<<<<<======\n");
 
@@ -350,6 +350,13 @@ static void parse_hid_report_descriptor(struct gtco *device, char * report,
 			case TAG_MAIN_COL_START:
 				maintype = 'S';
 
+				if (indent == MAX_COLLECTION_LEVELS) {
+					dev_err(ddev, "Collection level %d would exceed limit of %d\n",
+						indent + 1,
+						MAX_COLLECTION_LEVELS);
+					break;
+				}
+
 				if (data == 0) {
 					dev_dbg(ddev, "======>>>>>> Physical\n");
 					strcpy(globtype, "Physical");
@@ -369,8 +376,15 @@ static void parse_hid_report_descriptor(struct gtco *device, char * report,
 				break;
 
 			case TAG_MAIN_COL_END:
-				dev_dbg(ddev, "<<<<<<======\n");
 				maintype = 'E';
+
+				if (indent == 0) {
+					dev_err(ddev, "Collection level already at zero\n");
+					break;
+				}
+
+				dev_dbg(ddev, "<<<<<<======\n");
+
 				indent--;
 				for (x = 0; x < indent; x++)
 					indentstr[x] = '-';