[PATCH v2] hw/i386: Improve bounds checking in OVMF table parsing

[Dov Murik <dovmurik@linux.ibm.com>]

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>

---

v2:
- add error message example to commit description
- replace magic numbers 48 and 50 with size calculations (thanks Phil MD)
---
 hw/i386/pc_sysfw_ovmf.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/hw/i386/pc_sysfw_ovmf.c b/hw/i386/pc_sysfw_ovmf.c
index f4dd92c588..1c9a16e9e6 100644
--- a/hw/i386/pc_sysfw_ovmf.c
+++ b/hw/i386/pc_sysfw_ovmf.c
@@ -24,11 +24,14 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/error-report.h"
 #include "hw/i386/pc.h"
 #include "cpu.h"
 
 #define OVMF_TABLE_FOOTER_GUID "96b582de-1fb2-45f7-baea-a366c55a082d"
 
+static const int bytes_after_table_footer = 32;
+
 static bool ovmf_flash_parsed;
 static uint8_t *ovmf_table;
 static int ovmf_table_len;
@@ -38,6 +41,8 @@ void pc_system_parse_ovmf_flash(uint8_t *flash_ptr, size_t flash_size)
     uint8_t *ptr;
     QemuUUID guid;
     int tot_len;
+    int max_tot_len = flash_size - (bytes_after_table_footer +
+                                    sizeof(guid) + sizeof(uint16_t));
 
     /* should only be called once */
     if (ovmf_flash_parsed) {
@@ -52,12 +57,13 @@ void pc_system_parse_ovmf_flash(uint8_t *flash_ptr, size_t flash_size)
 
     /*
      * if this is OVMF there will be a table footer
-     * guid 48 bytes before the end of the flash file.  If it's
-     * not found, silently abort the flash parsing.
+     * guid 48 bytes before the end of the flash file
+     * (= 32 bytes after the table + 16 bytes the GUID itself).
+     * If it's not found, silently abort the flash parsing.
      */
     qemu_uuid_parse(OVMF_TABLE_FOOTER_GUID, &guid);
     guid = qemu_uuid_bswap(guid); /* guids are LE */
-    ptr = flash_ptr + flash_size - 48;
+    ptr = flash_ptr + flash_size - (bytes_after_table_footer + sizeof(guid));
     if (!qemu_uuid_is_equal((QemuUUID *)ptr, &guid)) {
         return;
     }
@@ -66,7 +72,13 @@ 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 > max_tot_len) {
+        error_report("OVMF table has invalid size %d", tot_len);
+        return;
+    }
+
+    if (tot_len == 0) {
+        /* no entries in the OVMF table */
         return;
     }
 

base-commit: 48033ad678ae2def43bf0d543a2c4c3d2a93feaf
-- 
2.25.1