firmware: dmi: Stop decoding on broken entry

If a DMI table entry is shorter than 4 bytes, it is invalid. Due to
how DMI table parsing works, it is impossible to safely recover from
such an error, so we have to stop decoding the table.

Signed-off-by: Jean Delvare <[email protected]>
Link: https://lore.kernel.org/linux-kernel/Zh2K3-HLXOesT_vZ@liuwe-devbox-debian-v2/T/
Reviewed-by: Michael Kelley <[email protected]>

Author: jdelvare

drivers/firmware/dmi_scan.c

@@ -101,6 +101,17 @@ static void dmi_decode_table(u8 *buf,
 	       (data - buf + sizeof(struct dmi_header)) <= dmi_len) {
 		const struct dmi_header *dm = (const struct dmi_header *)data;
 
+		/*
+		 * If a short entry is found (less than 4 bytes), not only it
+		 * is invalid, but we cannot reliably locate the next entry.
+		 */
+		if (dm->length < sizeof(struct dmi_header)) {
+			pr_warn(FW_BUG
+				"Corrupted DMI table, offset %zd (only %d entries processed)\n",
+				data - buf, i);
+			break;
+		}
+
 		/*
 		 *  We want to know the total length (formatted area and
 		 *  strings) before decoding to make sure we won't run off the