Hello:
Git patch-id is often used as convenient way to represent patches based
on their content. It accomplishes this by attempting to normalize a
patch before producing a hash of the result -- most notably, by trimming
a lot of whitespace.
Unfortunately, this does not work well with syntactically-significant
whitespace languages, notably Python and Make. For example, the
following two patches produce identical patch-id's, but one of them is
actually malicious.
Benign:
diff --git a/file1.py b/file1.py
index e574c49..6aa1937 100644
--- a/file1.py
+++ b/file1.py
@@ -1,3 +1,13 @@
#!/usr/bin/python
+def is_logged_in(cookie):
+ if cookie:
+ print('User is logged in')
+ return True
+
+ return False
+
+if is_logged_in(True):
+ print('You are logged in')
+
print('Hello!')
Malicious ("return True" is unindented, which results in is_logged_in()
always returning "True"):
diff --git a/file1.py b/file1.py
index e574c49..6aa1937 100644
--- a/file1.py
+++ b/file1.py
@@ -1,3 +1,13 @@
#!/usr/bin/python
+def is_logged_in(cookie):
+ if cookie:
+ print('User is logged in')
+ return True
+
+ return False
+
+if is_logged_in(True):
+ print('You are logged in')
+
print('Hello!')
This mostly becomes a problem if we try to build any kind of patch
indexing/retrieval systems that rely on patch-id to identify patches.
While this is not a high-impact problem by any means, it's not a
theoretical concern: git-format-patch includes functionality to provide
patch dependencies via prerequisite-patch-id trailers [1]. An automated
system attempting to auto-fetch dependencies can potentially retrieve
and apply the malicious version of the patch.
I'm not sure what the solution here is, since changing git-patch-id to
not discard whitespace is obviously going to defeat its entire purpose
of "not ever changing". I mostly wanted to share my findings in case
someone has thoughts on how to best approach this.
-K
[1] https://git-scm.com/docs/git-format-patch#_base_tree_information
