apply: reject input that touches outside the working area

By default, a patch that affects outside the working area (either a
Git controlled working tree, or the current working directory when
"git apply" is used as a replacement of GNU patch) is rejected as a
mistake (or a mischief).  Git itself does not create such a patch,
unless the user bends over backwards and specifies a non-standard
prefix to "git diff" and friends.

When `git apply` is used as a "better GNU patch", the user can pass
the `--unsafe-paths` option to override this safety check. This
option has no effect when `--index` or `--cached` is in use.

The new test was stolen from Jeff King with slight enhancements.
Note that a few new tests for touching outside the working area by
following a symbolic link are still expected to fail at this step,
but will be fixed in later steps.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-apply.txt

@@ -16,7 +16,7 @@ SYNOPSIS
 	  [--ignore-space-change | --ignore-whitespace ]
 	  [--whitespace=(nowarn|warn|fix|error|error-all)]
 	  [--exclude=<path>] [--include=<path>] [--directory=<root>]
-	  [--verbose] [<patch>...]
+	  [--verbose] [--unsafe-paths] [<patch>...]
 
 DESCRIPTION
 -----------
@@ -229,6 +229,16 @@ For example, a patch that talks about updating `a/git-gui.sh` to `b/git-gui.sh`
 can be applied to the file in the working tree `modules/git-gui/git-gui.sh` by
 running `git apply --directory=modules/git-gui`.
 
+--unsafe-paths::
+	By default, a patch that affects outside the working area
+	(either a Git controlled working tree, or the current working
+	directory when "git apply" is used as a replacement of GNU
+	patch) is rejected as a mistake (or a mischief).
++
+When `git apply` is used as a "better GNU patch", the user can pass
+the `--unsafe-paths` option to override this safety check.  This option
+has no effect when `--index` or `--cached` is in use.
+
 Configuration
 -------------
 

builtin/apply.c

@@ -50,6 +50,7 @@ static int apply_verbosely;
 static int allow_overlap;
 static int no_add;
 static int threeway;
+static int unsafe_paths;
 static const char *fake_ancestor;
 static int line_termination = '\n';
 static unsigned int p_context = UINT_MAX;
@@ -3483,6 +3484,23 @@ static int check_to_create(const char *new_name, int ok_if_exists)
 	return 0;
 }
 
+static void die_on_unsafe_path(struct patch *patch)
+{
+	const char *old_name = NULL;
+	const char *new_name = NULL;
+	if (patch->is_delete)
+		old_name = patch->old_name;
+	else if (!patch->is_new && !patch->is_copy)
+		old_name = patch->old_name;
+	if (!patch->is_delete)
+		new_name = patch->new_name;
+
+	if (old_name && !verify_path(old_name))
+		die(_("invalid path '%s'"), old_name);
+	if (new_name && !verify_path(new_name))
+		die(_("invalid path '%s'"), new_name);
+}
+
 /*
  * Check and apply the patch in-core; leave the result in patch->result
  * for the caller to write it out to the final destination.
@@ -3570,6 +3588,9 @@ static int check_patch(struct patch *patch)
 		}
 	}
 
+	if (!unsafe_paths)
+		die_on_unsafe_path(patch);
+
 	if (apply_data(patch, &st, ce) < 0)
 		return error(_("%s: patch does not apply"), name);
 	patch->rejected = 0;
@@ -4379,6 +4400,8 @@ int cmd_apply(int argc, const char **argv, const char *prefix_)
 			N_("make sure the patch is applicable to the current index")),
 		OPT_BOOL(0, "cached", &cached,
 			N_("apply a patch without touching the working tree")),
+		OPT_BOOL(0, "unsafe-paths", &unsafe_paths,
+			N_("accept a patch that touches outside the working area")),
 		OPT_BOOL(0, "apply", &force_apply,
 			N_("also apply the patch (use with --stat/--summary/--check)")),
 		OPT_BOOL('3', "3way", &threeway,
@@ -4451,6 +4474,9 @@ int cmd_apply(int argc, const char **argv, const char *prefix_)
 			die(_("--cached outside a repository"));
 		check_index = 1;
 	}
+	if (check_index)
+		unsafe_paths = 0;
+
 	for (i = 0; i < argc; i++) {
 		const char *arg = argv[i];
 		int fd;

t/t4139-apply-escape.sh

@@ -0,0 +1,141 @@
+#!/bin/sh
+
+test_description='paths written by git-apply cannot escape the working tree'
+. ./test-lib.sh
+
+# tests will try to write to ../foo, and we do not
+# want them to escape the trash directory when they
+# fail
+test_expect_success 'bump git repo one level down' '
+	mkdir inside &&
+	mv .git inside/ &&
+	cd inside
+'
+
+# $1 = name of file
+# $2 = current path to file (if different)
+mkpatch_add () {
+	rm -f "${2:-$1}" &&
+	cat <<-EOF
+	diff --git a/$1 b/$1
+	new file mode 100644
+	index 0000000..53c74cd
+	--- /dev/null
+	+++ b/$1
+	@@ -0,0 +1 @@
+	+evil
+	EOF
+}
+
+mkpatch_del () {
+	echo evil >"${2:-$1}" &&
+	cat <<-EOF
+	diff --git a/$1 b/$1
+	deleted file mode 100644
+	index 53c74cd..0000000
+	--- a/$1
+	+++ /dev/null
+	@@ -1 +0,0 @@
+	-evil
+	EOF
+}
+
+# $1 = name of file
+# $2 = content of symlink
+mkpatch_symlink () {
+	rm -f "$1" &&
+	cat <<-EOF
+	diff --git a/$1 b/$1
+	new file mode 120000
+	index 0000000..$(printf "%s" "$2" | git hash-object --stdin)
+	--- /dev/null
+	+++ b/$1
+	@@ -0,0 +1 @@
+	+$2
+	\ No newline at end of file
+	EOF
+}
+
+test_expect_success 'cannot create file containing ..' '
+	mkpatch_add ../foo >patch &&
+	test_must_fail git apply patch &&
+	test_path_is_missing ../foo
+'
+
+test_expect_success 'can create file containing .. with --unsafe-paths' '
+	mkpatch_add ../foo >patch &&
+	git apply --unsafe-paths patch &&
+	test_path_is_file ../foo
+'
+
+test_expect_success  'cannot create file containing .. (index)' '
+	mkpatch_add ../foo >patch &&
+	test_must_fail git apply --index patch &&
+	test_path_is_missing ../foo
+'
+
+test_expect_success  'cannot create file containing .. with --unsafe-paths (index)' '
+	mkpatch_add ../foo >patch &&
+	test_must_fail git apply --index --unsafe-paths patch &&
+	test_path_is_missing ../foo
+'
+
+test_expect_success 'cannot delete file containing ..' '
+	mkpatch_del ../foo >patch &&
+	test_must_fail git apply patch &&
+	test_path_is_file ../foo
+'
+
+test_expect_success 'can delete file containing .. with --unsafe-paths' '
+	mkpatch_del ../foo >patch &&
+	git apply --unsafe-paths patch &&
+	test_path_is_missing ../foo
+'
+
+test_expect_success 'cannot delete file containing .. (index)' '
+	mkpatch_del ../foo >patch &&
+	test_must_fail git apply --index patch &&
+	test_path_is_file ../foo
+'
+
+test_expect_failure SYMLINKS 'symlink escape via ..' '
+	{
+		mkpatch_symlink tmp .. &&
+		mkpatch_add tmp/foo ../foo
+	} >patch &&
+	test_must_fail git apply patch &&
+	test_path_is_missing tmp &&
+	test_path_is_missing ../foo
+'
+
+test_expect_failure SYMLINKS 'symlink escape via .. (index)' '
+	{
+		mkpatch_symlink tmp .. &&
+		mkpatch_add tmp/foo ../foo
+	} >patch &&
+	test_must_fail git apply --index patch &&
+	test_path_is_missing tmp &&
+	test_path_is_missing ../foo
+'
+
+test_expect_failure SYMLINKS 'symlink escape via absolute path' '
+	{
+		mkpatch_symlink tmp "$(pwd)" &&
+		mkpatch_add tmp/foo ../foo
+	} >patch &&
+	test_must_fail git apply patch &&
+	test_path_is_missing tmp &&
+	test_path_is_missing ../foo
+'
+
+test_expect_failure SYMLINKS 'symlink escape via absolute path (index)' '
+	{
+		mkpatch_symlink tmp "$(pwd)" &&
+		mkpatch_add tmp/foo ../foo
+	} >patch &&
+	test_must_fail git apply --index patch &&
+	test_path_is_missing tmp &&
+	test_path_is_missing ../foo
+'
+
+test_done