mingw: safeguard better against backslashes in file names

In 224c7d70fa1 (mingw: only test index entries for backslashes, not tree entries, 2019-12-31), we relaxed the check for backslashes in tree entries to check only index entries.

However, the code change was incorrect: it was added to add_index_entry_with_check(), not to add_index_entry(), so under certain circumstances it was possible to side-step the protection.

Besides, the description of that commit purported that all index entries would be checked when in fact they were only checked when being added to the index (there are code paths that do not do that, constructing "transient" index entries).

In any case, it was pointed out in one insightful review at
https://github.com/git-for-windows/git/pull/2437#issuecomment-566771835
that it would be a much better idea to teach verify_path() to perform the check for a backslash. This is safer, even if it comes with two notable drawbacks:

- verify_path() cannot say _what_ is wrong with the path, therefore the user will no longer be told that there was a backslash in the path, only that the path was invalid.

- The git apply command also calls the verify_path() function, and might have been able to handle Windows-style paths (i.e. with backslashes instead of forward slashes). This will no longer be possible unless the user (temporarily) sets core.protectNTFS=false.

Note that git add <windows-path> will _still_ work because normalize_path_copy_len() will convert the backslashes to forward slashes before hitting the code path that creates an index entry.

The clear advantage is that verify_path()'s purpose is to check the validity of the file name, therefore we naturally tap into all the code paths that need safeguarding, also implicitly into future code paths.

The benefits of that approach outweigh the downsides, so let's move the check from add_index_entry_with_check() to verify_path().

Signed-off-by Johannes Schindelin <[email protected]>

---
mingw: safeguard better against backslashes in file names

I investigated again, and I think that there are code paths involving make_transient_cache_entry() that might be vulnerable again after my recent change in 224c7d70fa1 (mingw: only test index entries for backslashes, not tree entries, 2019-12-31).

This version should help with keeping Git for Windows' users safe.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-690%2Fdscho%2Fonly-error-on-backslash-in-index-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-690/dscho/only-error-on-backslash-in-index-v1
Pull-Request: https://github.com/git/git/pull/690

 read-cache.c               | 12 ++++++------
 t/t7415-submodule-names.sh |  2 +-
 2 files changed, 7 insertions(+), 7 deletions(-)

read-cache.c +6 -6

index 737916ebd9..aa427c5c17 100644
@@ -959,7 +959,7 @@ static int verify_dotfile(const char *rest, unsigned mode)
 
 int verify_path(const char *path, unsigned mode)
 {
-	char c;
+	char c = 0;
 
 	if (has_dos_drive_prefix(path))
 		return 0;

@@ -974,6 +974,7 @@ int verify_path(const char *path, unsigned mode)
 		if (is_dir_sep(c)) {
 inside:
 			if (protect_hfs) {
+
 				if (is_hfs_dotgit(path))
 					return 0;
 				if (S_ISLNK(mode)) {

@@ -982,6 +983,10 @@ int verify_path(const char *path, unsigned mode)
 				}
 			}
 			if (protect_ntfs) {
+#ifdef GIT_WINDOWS_NATIVE
+				if (c == '\\')
+					return 0;
+#endif
 				if (is_ntfs_dotgit(path))
 					return 0;
 				if (S_ISLNK(mode)) {

@@ -1278,11 +1283,6 @@ static int add_index_entry_with_check(struct index_state *istate, struct cache_e
 	int skip_df_check = option & ADD_CACHE_SKIP_DFCHECK;
 	int new_only = option & ADD_CACHE_NEW_ONLY;
 
-#ifdef GIT_WINDOWS_NATIVE
-	if (protect_ntfs && strchr(ce->name, '\\'))
-		return error(_("filename in tree entry contains backslash: '%s'"), ce->name);
-#endif
-
 	if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
 		cache_tree_invalidate_path(istate, ce->name);
 

t/t7415-submodule-names.sh +1 -1

index 7ae0dc8ff4..f70368bc2e 100755
@@ -209,7 +209,7 @@ test_expect_success MINGW 'prevent git~1 squatting on Windows' '
 		hash="$(echo x | git hash-object -w --stdin)" &&
 		test_must_fail git update-index --add \
 			--cacheinfo 160000,$rev,d\\a 2>err &&
-		test_i18ngrep backslash err &&
+		test_i18ngrep "Invalid path" err &&
 		git -c core.protectNTFS=false update-index --add \
 			--cacheinfo 100644,$modules,.gitmodules \
 			--cacheinfo 160000,$rev,c \

base-commit: 7a6a90c6ec48fc78c83d7090d6c1b95d8f3739c0 --
gitgitgadget