Merge branch 'ps/apply-beyond-symlink' into maint-2.30

Fix a vulnerability (CVE-2023-23946) that allows crafted input to trick
`git apply` into writing files outside of the working tree.

* ps/apply-beyond-symlink:
  dir-iterator: prevent top-level symlinks without FOLLOW_SYMLINKS

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

apply.c

@@ -4400,6 +4400,33 @@ static int create_one_file(struct apply_state *state,
 	if (state->cached)
 		return 0;
 
+	/*
+	 * We already try to detect whether files are beyond a symlink in our
+	 * up-front checks. But in the case where symlinks are created by any
+	 * of the intermediate hunks it can happen that our up-front checks
+	 * didn't yet see the symlink, but at the point of arriving here there
+	 * in fact is one. We thus repeat the check for symlinks here.
+	 *
+	 * Note that this does not make the up-front check obsolete as the
+	 * failure mode is different:
+	 *
+	 * - The up-front checks cause us to abort before we have written
+	 *   anything into the working directory. So when we exit this way the
+	 *   working directory remains clean.
+	 *
+	 * - The checks here happen in the middle of the action where we have
+	 *   already started to apply the patch. The end result will be a dirty
+	 *   working directory.
+	 *
+	 * Ideally, we should update the up-front checks to catch what would
+	 * happen when we apply the patch before we damage the working tree.
+	 * We have all the information necessary to do so.  But for now, as a
+	 * part of embargoed security work, having this check would serve as a
+	 * reasonable first step.
+	 */
+	if (path_is_beyond_symlink(state, path))
+		return error(_("affected file '%s' is beyond a symbolic link"), path);
+
 	res = try_create_file(state, path, mode, buf, size);
 	if (res < 0)
 		return -1;

t/t4115-apply-symlink.sh

@@ -44,4 +44,85 @@ test_expect_success 'apply --index symlink patch' '
 
 '
 
+test_expect_success 'symlink setup' '
+	ln -s .git symlink &&
+	git add symlink &&
+	git commit -m "add symlink"
+'
+
+test_expect_success SYMLINKS 'symlink escape when creating new files' '
+	test_when_finished "git reset --hard && git clean -dfx" &&
+
+	cat >patch <<-EOF &&
+	diff --git a/symlink b/renamed-symlink
+	similarity index 100%
+	rename from symlink
+	rename to renamed-symlink
+	--
+	diff --git /dev/null b/renamed-symlink/create-me
+	new file mode 100644
+	index 0000000..039727e
+	--- /dev/null
+	+++ b/renamed-symlink/create-me
+	@@ -0,0 +1,1 @@
+	+busted
+	EOF
+
+	test_must_fail git apply patch 2>stderr &&
+	cat >expected_stderr <<-EOF &&
+	error: affected file ${SQ}renamed-symlink/create-me${SQ} is beyond a symbolic link
+	EOF
+	test_cmp expected_stderr stderr &&
+	! test_path_exists .git/create-me
+'
+
+test_expect_success SYMLINKS 'symlink escape when modifying file' '
+	test_when_finished "git reset --hard && git clean -dfx" &&
+	touch .git/modify-me &&
+
+	cat >patch <<-EOF &&
+	diff --git a/symlink b/renamed-symlink
+	similarity index 100%
+	rename from symlink
+	rename to renamed-symlink
+	--
+	diff --git a/renamed-symlink/modify-me b/renamed-symlink/modify-me
+	index 1111111..2222222 100644
+	--- a/renamed-symlink/modify-me
+	+++ b/renamed-symlink/modify-me
+	@@ -0,0 +1,1 @@
+	+busted
+	EOF
+
+	test_must_fail git apply patch 2>stderr &&
+	cat >expected_stderr <<-EOF &&
+	error: renamed-symlink/modify-me: No such file or directory
+	EOF
+	test_cmp expected_stderr stderr &&
+	test_must_be_empty .git/modify-me
+'
+
+test_expect_success SYMLINKS 'symlink escape when deleting file' '
+	test_when_finished "git reset --hard && git clean -dfx && rm .git/delete-me" &&
+	touch .git/delete-me &&
+
+	cat >patch <<-EOF &&
+	diff --git a/symlink b/renamed-symlink
+	similarity index 100%
+	rename from symlink
+	rename to renamed-symlink
+	--
+	diff --git a/renamed-symlink/delete-me b/renamed-symlink/delete-me
+	deleted file mode 100644
+	index 1111111..0000000 100644
+	EOF
+
+	test_must_fail git apply patch 2>stderr &&
+	cat >expected_stderr <<-EOF &&
+	error: renamed-symlink/delete-me: No such file or directory
+	EOF
+	test_cmp expected_stderr stderr &&
+	test_path_is_file .git/delete-me
+'
+
 test_done