[edk2-platforms][PATCH v2 03/11] Ext4Pkg: Fix global buffer overflow in Ext4ReadDir

["Savva Mitrofanov" <savvamtr@gmail.com>]

Directory entry structure can contain name_len bigger than size of "."
or "..", that's why CompareMem in such cases leads to global buffer
overflow. So there are two problems. The first is that statement doesn't
check cases when name_len != 0 but > 2 and the second is that we passing
big Length to CompareMem routine.
The correct way here is to check that name_len <= 2 and check for
null-terminator presence

Cc: Marvin Häuser <mhaeuser@posteo.de>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Vitaly Cheptsov <vit9696@protonmail.com>
Fixes: e55f0527dde48a5f139c1b8f35acc4e6b59dd794
Signed-off-by: Savva Mitrofanov <savvamtr@gmail.com>
---
 Features/Ext4Pkg/Ext4Dxe/Directory.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/Features/Ext4Pkg/Ext4Dxe/Directory.c b/Features/Ext4Pkg/Ext4Dxe/Directory.c
index 8b8fce568e43..ffc0e8043076 100644
--- a/Features/Ext4Pkg/Ext4Dxe/Directory.c
+++ b/Features/Ext4Pkg/Ext4Dxe/Directory.c
@@ -491,11 +491,9 @@ Ext4ReadDir (
 
     // Entry.name_len may be 0 if it's a nameless entry, like an unused entry
     // or a checksum at the end of the directory block.
-    // memcmp (and CompareMem) return 0 when the passed length is 0.
-
-    IsDotOrDotDot = Entry.name_len != 0 &&
-                    (CompareMem (Entry.name, ".", Entry.name_len) == 0 ||
-                     CompareMem (Entry.name, "..", Entry.name_len) == 0);
+    IsDotOrDotDot = Entry.name_len <= 2 &&
+                    ((Entry.name[0] == '.') &&
+                     (Entry.name[1] == '.' || Entry.name[1] == '\0'));
 
     // When inode = 0, it's unused.
     ShouldSkip = Entry.inode == 0 || IsDotOrDotDot;
-- 
2.38.1