mingw: disallow backslash characters in tree objects' file names

The backslash character is not a valid part of a file name on Windows.
Hence it is dangerous to allow writing files that were unpacked from
tree objects, when the stored file name contains a backslash character:
it will be misinterpreted as directory separator.

This not only causes ambiguity when a tree contains a blob `a\b` and a
tree `a` that contains a blob `b`, but it also can be used as part of an
attack vector to side-step the careful protections against writing into
the `.git/` directory during a clone of a maliciously-crafted
repository.

Let's prevent that, addressing CVE-2019-1354.

Note: we guard against backslash characters in tree objects' file names
_only_ on Windows (because on other platforms, even on those where NTFS
volumes can be mounted, the backslash character is _not_ a directory
separator), and _only_ when `core.protectNTFS = true` (because users
might need to generate tree objects for other platforms, of course
without touching the worktree, e.g. using `git update-index
--cacheinfo`).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

1 files changed, 1 insertions, 0 deletions

diff --git a/t/t1450-fsck.sh b/t/t1450-fsck.sh
index cb4b66e29d..33c955f912 100755
--- a/t/t1450-fsck.sh
+++ b/t/t1450-fsck.sh
@@ -419,6 +419,7 @@ while read name path pretty; do
 		(
 			git init $name-$type &&
 			cd $name-$type &&
+			git config core.protectNTFS false &&
 			echo content >file &&
 			git add file &&
 			git commit -m base &&