submodule-config: verify submodule names as paths

Submodule "names" come from the untrusted .gitmodules file,
but we blindly append them to $GIT_DIR/modules to create our
on-disk repo paths. This means you can do bad things by
putting "../" into the name (among other things).

Let's sanity-check these names to avoid building a path that
can be exploited. There are two main decisions:

1. What should the allowed syntax be?

It's tempting to reuse verify_path(), since submodule
names typically come from in-repo paths. But there are
two reasons not to:

a. It's technically more strict than what we need, as
we really care only about breaking out of the
$GIT_DIR/modules/ hierarchy.  E.g., having a
submodule named "foo/.git" isn't actually
dangerous, and it's possible that somebody has
manually given such a funny name.

b. Since we'll eventually use this checking logic in
fsck to prevent downstream repositories, it should
be consistent across platforms. Because
verify_path() relies on is_dir_sep(), it wouldn't
block "foo\..\bar" on a non-Windows machine.

2. Where should we enforce it? These days most of the
.gitmodules reads go through submodule-config.c, so
I've put it there in the reading step. That should
cover all of the C code.

We also construct the name for "git submodule add"
inside the git-submodule.sh script. This is probably
not a big deal for security since the name is coming
from the user anyway, but it would be polite to remind
them if the name they pick is invalid (and we need to
expose the name-checker to the shell anyway for our
test scripts).

This patch issues a warning when reading .gitmodules
and just ignores the related config entry completely.
This will generally end up producing a sensible error,
as it works the same as a .gitmodules file which is
missing a submodule entry (so "submodule update" will
barf, but "git clone --recurse-submodules" will print
an error but not abort the clone.

There is one minor oddity, which is that we print the
warning once per malformed config key (since that's how
the config subsystem gives us the entries). So in the
new test, for example, the user would see three
warnings. That's OK, since the intent is that this case
should never come up outside of malicious repositories
(and then it might even benefit the user to see the
message multiple times).

Credit for finding this vulnerability and the proof of
concept from which the test script was adapted goes to
Etienne Stalmans.

Signed-off-by: Jeff King <peff@peff.net>

diff --git a/builtin/submodule--helper.c b/builtin/submodule--helper.c
index cbb17a90212cb7fadc00b36d16dd770ff93bbef0..b4b4d29d82896e706be44f8509cb39a6e34037fa 100644 (file)
--- a/builtin/submodule--helper.c
+++ b/builtin/submodule--helper.c
@@ -1195,6 +1195,29 @@ static int is_active(int argc, const char **argv, const char *prefix)
        return !is_submodule_initialized(argv[1]);
 }
 
+/*
+ * Exit non-zero if any of the submodule names given on the command line is
+ * invalid. If no names are given, filter stdin to print only valid names
+ * (which is primarily intended for testing).
+ */
+static int check_name(int argc, const char **argv, const char *prefix)
+{
+       if (argc > 1) {
+               while (*++argv) {
+                       if (check_submodule_name(*argv) < 0)
+                               return 1;
+               }
+       } else {
+               struct strbuf buf = STRBUF_INIT;
+               while (strbuf_getline(&buf, stdin) != EOF) {
+                       if (!check_submodule_name(buf.buf))
+                               printf("%s\n", buf.buf);
+               }
+               strbuf_release(&buf);
+       }
+       return 0;
+}
+
 #define SUPPORT_SUPER_PREFIX (1<<0)
 
 struct cmd_struct {
@@ -1216,6 +1239,7 @@ static struct cmd_struct commands[] = {
        {"push-check", push_check, 0},
        {"absorb-git-dirs", absorb_git_dirs, SUPPORT_SUPER_PREFIX},
        {"is-active", is_active, 0},
+       {"check-name", check_name, 0},
 };
 
 int cmd_submodule__helper(int argc, const char **argv, const char *prefix)

diff --git a/git-submodule.sh b/git-submodule.sh
index c0d0e9a4c63495d4522666b6edf984c01c0cfa23..92750b9e2fc54a451f7fe9dad42737861e47dbb7 100755 (executable)
--- a/git-submodule.sh
+++ b/git-submodule.sh
@@ -228,6 +228,11 @@ Use -f if you really want to add it." >&2
                sm_name="$sm_path"
        fi
 
+       if ! git submodule--helper check-name "$sm_name"
+       then
+               die "$(eval_gettext "'$sm_name' is not a valid submodule name")"
+       fi
+
        # perhaps the path exists and is already a git repo, else clone it
        if test -e "$sm_path"
        then

diff --git a/submodule-config.c b/submodule-config.c
index 4f58491ddb0705ab56e238006199d35db19dfb5a..de54351c6f26bc7db4a3f4c6fa8304da6bc9d0c7 100644 (file)
--- a/submodule-config.c
+++ b/submodule-config.c
@@ -163,6 +163,31 @@ static struct submodule *cache_lookup_name(struct submodule_cache *cache,
        return NULL;
 }
 
+int check_submodule_name(const char *name)
+{
+       /* Disallow empty names */
+       if (!*name)
+               return -1;
+
+       /*
+        * Look for '..' as a path component. Check both '/' and '\\' as
+        * separators rather than is_dir_sep(), because we want the name rules
+        * to be consistent across platforms.
+        */
+       goto in_component; /* always start inside component */
+       while (*name) {
+               char c = *name++;
+               if (c == '/' || c == '\\') {
+in_component:
+                       if (name[0] == '.' && name[1] == '.' &&
+                           (!name[2] || name[2] == '/' || name[2] == '\\'))
+                               return -1;
+               }
+       }
+
+       return 0;
+}
+
 static int name_and_item_from_var(const char *var, struct strbuf *name,
                                  struct strbuf *item)
 {
@@ -174,6 +199,12 @@ static int name_and_item_from_var(const char *var, struct strbuf *name,
                return 0;
 
        strbuf_add(name, subsection, subsection_len);
+       if (check_submodule_name(name->buf) < 0) {
+               warning(_("ignoring suspicious submodule name: %s"), name->buf);
+               strbuf_release(name);
+               return 0;
+       }
+
        strbuf_addstr(item, key);
 
        return 1;

diff --git a/submodule-config.h b/submodule-config.h
index d434ecdb45c074dd386405f20ed73ac1c8b646bd..103cc79dd8aa4e0052b1efe9a9a83db48143d4dc 100644 (file)
--- a/submodule-config.h
+++ b/submodule-config.h
@@ -35,4 +35,11 @@ extern int gitmodule_sha1_from_commit(const unsigned char *commit_sha1,
                                      struct strbuf *rev);
 extern void submodule_free(void);
 
+/*
+ * Returns 0 if the name is syntactically acceptable as a submodule "name"
+ * (e.g., that may be found in the subsection of a .gitmodules file) and -1
+ * otherwise.
+ */
+int check_submodule_name(const char *name);
+
 #endif /* SUBMODULE_CONFIG_H */

diff --git a/t/t7415-submodule-names.sh b/t/t7415-submodule-names.sh
new file mode 100755 (executable)
index 0000000..75fa071
--- /dev/null
+++ b/t/t7415-submodule-names.sh
@@ -0,0 +1,76 @@
+#!/bin/sh
+
+test_description='check handling of .. in submodule names
+
+Exercise the name-checking function on a variety of names, and then give a
+real-world setup that confirms we catch this in practice.
+'
+. ./test-lib.sh
+
+test_expect_success 'check names' '
+       cat >expect <<-\EOF &&
+       valid
+       valid/with/paths
+       EOF
+
+       git submodule--helper check-name >actual <<-\EOF &&
+       valid
+       valid/with/paths
+
+       ../foo
+       /../foo
+       ..\foo
+       \..\foo
+       foo/..
+       foo/../
+       foo\..
+       foo\..\
+       foo/../bar
+       EOF
+
+       test_cmp expect actual
+'
+
+test_expect_success 'create innocent subrepo' '
+       git init innocent &&
+       git -C innocent commit --allow-empty -m foo
+'
+
+test_expect_success 'submodule add refuses invalid names' '
+       test_must_fail \
+               git submodule add --name ../../modules/evil "$PWD/innocent" evil
+'
+
+test_expect_success 'add evil submodule' '
+       git submodule add "$PWD/innocent" evil &&
+
+       mkdir modules &&
+       cp -r .git/modules/evil modules &&
+       write_script modules/evil/hooks/post-checkout <<-\EOF &&
+       echo >&2 "RUNNING POST CHECKOUT"
+       EOF
+
+       git config -f .gitmodules submodule.evil.update checkout &&
+       git config -f .gitmodules --rename-section \
+               submodule.evil submodule.../../modules/evil &&
+       git add modules &&
+       git commit -am evil
+'
+
+# This step seems like it shouldn't be necessary, since the payload is
+# contained entirely in the evil submodule. But due to the vagaries of the
+# submodule code, checking out the evil module will fail unless ".git/modules"
+# exists. Adding another submodule (with a name that sorts before "evil") is an
+# easy way to make sure this is the case in the victim clone.
+test_expect_success 'add other submodule' '
+       git submodule add "$PWD/innocent" another-module &&
+       git add another-module &&
+       git commit -am another
+'
+
+test_expect_success 'clone evil superproject' '
+       git clone --recurse-submodules . victim >output 2>&1 &&
+       ! grep "RUNNING POST CHECKOUT" output
+'
+
+test_done