[PATCH] MdeModulePkg/UdfDxe: Fix NULL pointer dereference

[Paulo Alcantara <pcacjr@zytor.com>]

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=704

For root directory, the FID (File Identifier Descriptor) pointer is
accessible through PRIVATE_UDF_FILE_DATA.Root, whereas non-root
directory and regular files, their FIDs are accessible through
PRIVATE_UDF_FILE_DATA.File.

In UdfSetPosition(), the FID was retrieved through
PRIVATE_UDF_FILE_DATA.File, hence when calling it with a root directory,
PRIVATE_UDF_FILE_DATA.File.FileIdentifierDescriptor would be NULL and
then dereferenced.

This patch fixes the NULL pointer dereference by calling _FILE() to
transparently return the correct UDF_FILE_INFO * which points to a valid
FID descriptor of a specific file.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Steven Shi <steven.shi@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Reported-by: Steven Shi <steven.shi@intel.com>
Signed-off-by: Paulo Alcantara <pcacjr@zytor.com>
---
 MdeModulePkg/Universal/Disk/UdfDxe/File.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/MdeModulePkg/Universal/Disk/UdfDxe/File.c b/MdeModulePkg/Universal/Disk/UdfDxe/File.c
index 8b9339567f..a1eb2196df 100644
--- a/MdeModulePkg/Universal/Disk/UdfDxe/File.c
+++ b/MdeModulePkg/Universal/Disk/UdfDxe/File.c
@@ -690,7 +690,8 @@ UdfSetPosition (
 
   PrivFileData = PRIVATE_UDF_FILE_DATA_FROM_THIS (This);
 
-  FileIdentifierDesc = PrivFileData->File.FileIdentifierDesc;
+  FileIdentifierDesc = _FILE (PrivFileData)->FileIdentifierDesc;
+  ASSERT (FileIdentifierDesc != NULL);
   if (IS_FID_DIRECTORY_FILE (FileIdentifierDesc)) {
     //
     // If the file handle is a directory, the _only_ position that may be set is
-- 
2.11.0