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, 6 insertions, 0 deletions

diff --git a/tree-walk.c b/tree-walk.c
index d459feda23..54ff959d7f 100644
--- a/tree-walk.c
+++ b/tree-walk.c
@@ -41,6 +41,12 @@ static int decode_tree_entry(struct tree_desc *desc, const char *buf, unsigned l
 		strbuf_addstr(err, _("empty filename in tree entry"));
 		return -1;
 	}
+#ifdef GIT_WINDOWS_NATIVE
+	if (protect_ntfs && strchr(path, '\\')) {
+		strbuf_addf(err, _("filename in tree entry contains backslash: '%s'"), path);
+		return -1;
+	}
+#endif
 	len = strlen(path) + 1;
 
 	/* Initialize the descriptor entry */