firmware: dmi: Fix unlikely out-of-bounds read in save_mem_devices

Before reading the Extended Size field, we should ensure it fits in
the DMI record. There is already a record length check but it does
not cover that field.

It would take a seriously corrupted DMI table to hit that bug, so no
need to worry, but we should still fix it.
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: 6deae96b ("firmware, DMI: Add function to look up a handle and return DIMM size")
Cc: Tony Luck <tony.luck@intel.com>
Cc: Borislav Petkov <bp@suse.de>

drivers/firmware/dmi_scan.c

@@ -408,7 +408,7 @@ static void __init save_mem_devices(const struct dmi_header *dm, void *v)
 		bytes = ~0ull;
 	else if (size & 0x8000)
 		bytes = (u64)(size & 0x7fff) << 10;
-	else if (size != 0x7fff)
+	else if (size != 0x7fff || dm->length < 0x20)
 		bytes = (u64)size << 20;
 	else
 		bytes = (u64)get_unaligned((u32 *)&d[0x1C]) << 20;