hw/i386: Improve bounds checking in OVMF table parsing

When pc_system_parse_ovmf_flash() parses the optional GUIDed table in
the end of the OVMF flash memory area, the table length field is checked
for sizes that are too small, but doesn't error on sizes that are too
big (bigger than the flash content itself).

Add a check for maximal size of the OVMF table, and add an error report
in case the size is invalid.  In such a case, an error like this will be
displayed during launch:

    qemu-system-x86_64: OVMF table has invalid size 4047

and the table parsing is skipped.

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Message-Id: <20220222071906.2632426-2-dovmurik@linux.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

hw/i386/pc_sysfw_ovmf.c

@@ -24,6 +24,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/error-report.h"
 #include "hw/i386/pc.h"
 #include "cpu.h"
 
@@ -66,7 +67,13 @@ void pc_system_parse_ovmf_flash(uint8_t *flash_ptr, size_t flash_size)
     ptr -= sizeof(uint16_t);
     tot_len = le16_to_cpu(*(uint16_t *)ptr) - sizeof(guid) - sizeof(uint16_t);
 
-    if (tot_len <= 0) {
+    if (tot_len < 0 || tot_len > (ptr - flash_ptr)) {
+        error_report("OVMF table has invalid size %d", tot_len);
+        return;
+    }
+
+    if (tot_len == 0) {
+        /* no entries in the OVMF table */
         return;
     }