By default, a patch that affects outside the working area is
rejected as a mistake; Git itself never creates such a patch
unless the user bends backwards and specifies nonstandard
prefix to "git diff" and friends.

When `git apply` is used without either `--index` or `--cached`
option as a "better GNU patch", the user can pass `--allow-uplevel`
option to override this safety check.  This cannot be used to escape
outside the working tree when using `--index` or `--cached` to apply
the patch to the index.

The new test was stolen from Jeff King with slight enhancements.

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

 * Meant to apply on top of the previous one, but these two are
   about separate and orthogonal issues.

 Documentation/git-apply.txt |  14 ++++-
 builtin/apply.c             |  26 +++++++++
 t/t4139-apply-escape.sh     | 137 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 176 insertions(+), 1 deletion(-)
 create mode 100755 t/t4139-apply-escape.sh

diff --git a/Documentation/git-apply.txt b/Documentation/git-apply.txt
index f605327..20c3a6f 100644
--- a/Documentation/git-apply.txt
+++ b/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] [--allow-uplevel] [<patch>...]
 
 DESCRIPTION
 -----------
@@ -229,6 +229,18 @@ 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`.
 
+--allow-uplevel::
+       By default, a patch that affects outside the working area is
+       rejected as a mistake; Git itself never creates such a patch
+       unless the user bends backwards and specifies nonstandard
+       prefix to "git diff" and friends.
++
+When `git apply` is used without either `--index` or `--cached`
+option as a "better GNU patch", the user can pass `--allow-uplevel`
+option to override this safety check.  This cannot be used to escape
+outside the working tree when using `--index` or `--cached` to apply
+the patch to the index.
+
 Configuration
 -------------
 
diff --git a/builtin/apply.c b/builtin/apply.c
index dcb44fb..ce5a594 100644
--- a/builtin/apply.c
+++ b/builtin/apply.c
@@ -50,6 +50,7 @@ static int apply_verbosely;
 static int allow_overlap;
 static int no_add;
 static int threeway;
+static int allow_uplevel;
 static const char *fake_ancestor;
 static int line_termination = '\n';
 static unsigned int p_context = UINT_MAX;
@@ -3523,6 +3524,23 @@ symlink_found:
 
 }
 
+static void die_on_uplevel_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.
@@ -3614,6 +3632,9 @@ static int check_patch(struct patch *patch)
                return error(_("affected file '%s' is beyond a symbolic link"),
                             patch->new_name);
 
+       if (!allow_uplevel)
+               die_on_uplevel_path(patch);
+
        if (apply_data(patch, &st, ce) < 0)
                return error(_("%s: patch does not apply"), name);
        patch->rejected = 0;
@@ -4423,6 +4444,8 @@ 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, "allow-uplevel", &allow_uplevel,
+                       N_("accept a patch to touch outside the current 
directory")),
                OPT_BOOL(0, "apply", &force_apply,
                        N_("also apply the patch (use with 
--stat/--summary/--check)")),
                OPT_BOOL('3', "3way", &threeway,
@@ -4495,6 +4518,9 @@ int cmd_apply(int argc, const char **argv, const char 
*prefix_)
                        die(_("--cached outside a repository"));
                check_index = 1;
        }
+       if (check_index)
+               allow_uplevel = 0;
+
        for (i = 0; i < argc; i++) {
                const char *arg = argv[i];
                int fd;
diff --git a/t/t4139-apply-escape.sh b/t/t4139-apply-escape.sh
new file mode 100755
index 0000000..39de838
--- /dev/null
+++ b/t/t4139-apply-escape.sh
@@ -0,0 +1,137 @@
+#!/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 add file containing ..' '
+       mkpatch_add ../foo >patch &&
+       test_must_fail git apply patch &&
+       test_path_is_missing ../foo
+'
+
+test_expect_success 'can add file containing .. with --allow-uplevel' '
+       mkpatch_add ../foo >patch &&
+       git apply --allow-uplevel patch &&
+       test_path_is_file ../foo
+'
+
+test_expect_success  'cannot add file containing .. (index)' '
+       mkpatch_add ../foo >patch &&
+       test_must_fail git apply --index patch &&
+       test_path_is_missing ../foo
+'
+
+test_expect_success  'cannot add file containing .. with --allow-uplevel 
(index)' '
+       mkpatch_add ../foo >patch &&
+       test_must_fail git apply --index --allow-uplevel patch &&
+       test_path_is_missing ../foo
+'
+
+test_expect_success 'cannot del file containing ..' '
+       mkpatch_del ../foo >patch &&
+       test_must_fail git apply patch &&
+       test_path_is_file ../foo
+'
+
+test_expect_success 'can del file containing .. with --allow-uplevel' '
+       mkpatch_del ../foo >patch &&
+       git apply --allow-uplevel patch &&
+       test_path_is_missing ../foo
+'
+
+test_expect_success 'cannot del file containing .. (index)' '
+       mkpatch_del ../foo >patch &&
+       test_must_fail git apply --index patch &&
+       test_path_is_file ../foo
+'
+
+test_expect_success 'symlink escape via ..' '
+       {
+               mkpatch_symlink tmp .. &&
+               mkpatch_add tmp/foo ../foo
+       } >patch &&
+       test_must_fail git apply patch &&
+       test_path_is_missing ../foo
+'
+
+test_expect_success 'symlink escape via .. (index)' '
+       {
+               mkpatch_symlink tmp .. &&
+               mkpatch_add tmp/foo ../foo
+       } >patch &&
+       test_must_fail git apply --index patch &&
+       test_path_is_missing ../foo
+'
+
+test_expect_success '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 ../foo
+'
+
+test_expect_success '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 ../foo
+'
+
+test_done
-- 
2.3.0-rc2-158-g17413e7

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