[PATCH 12/21] selftests/filesystems: check that an automount below an overlay layer is refused

[Christian Brauner <brauner@kernel.org>]

Make sure that we don't automount on top of internal things.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 .../selftests/filesystems/overlayfs/.gitignore     |   1 +
 .../selftests/filesystems/overlayfs/Makefile       |   1 +
 .../filesystems/overlayfs/automount_in_layer.c     | 174 +++++++++++++++++++++
 3 files changed, 176 insertions(+)

diff --git a/tools/testing/selftests/filesystems/overlayfs/.gitignore b/tools/testing/selftests/filesystems/overlayfs/.gitignore
index 077f7a128168..b343cc430051 100644
--- a/tools/testing/selftests/filesystems/overlayfs/.gitignore
+++ b/tools/testing/selftests/filesystems/overlayfs/.gitignore
@@ -2,3 +2,4 @@
 dev_in_maps
 set_layers_via_fds
 idmapped_mounts
+automount_in_layer
diff --git a/tools/testing/selftests/filesystems/overlayfs/Makefile b/tools/testing/selftests/filesystems/overlayfs/Makefile
index b3185f684add..382a59bda5e7 100644
--- a/tools/testing/selftests/filesystems/overlayfs/Makefile
+++ b/tools/testing/selftests/filesystems/overlayfs/Makefile
@@ -9,6 +9,7 @@ LOCAL_HDRS += ../wrappers.h log.h
 TEST_GEN_PROGS := dev_in_maps
 TEST_GEN_PROGS += set_layers_via_fds
 TEST_GEN_PROGS += idmapped_mounts
+TEST_GEN_PROGS += automount_in_layer
 
 include ../../lib.mk
 
diff --git a/tools/testing/selftests/filesystems/overlayfs/automount_in_layer.c b/tools/testing/selftests/filesystems/overlayfs/automount_in_layer.c
new file mode 100644
index 000000000000..0476c4582bdf
--- /dev/null
+++ b/tools/testing/selftests/filesystems/overlayfs/automount_in_layer.c
@@ -0,0 +1,174 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * A layer of an overlay is a private clone of the mount it was given and
+ * belongs to no mount namespace. fanotify hands out descriptors on it. An
+ * automount triggered through one has no namespace to go into: the open has
+ * to fail, not oops with namespace_sem held.
+ */
+#define _GNU_SOURCE
+#include <dirent.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <poll.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <linux/magic.h>
+#include <sys/fanotify.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <sys/vfs.h>
+
+#include "../../kselftest_harness.h"
+
+#define DIR_LEN		64
+#define PATH_LEN	192
+
+static bool have_fs(const char *name)
+{
+	char line[128];
+	bool found = false;
+	FILE *f;
+
+	f = fopen("/proc/filesystems", "re");
+	if (!f)
+		return false;
+	while (fgets(line, sizeof(line), f)) {
+		char *nl = strchr(line, '\n');
+		char *tab = strchr(line, '\t');
+
+		if (nl)
+			*nl = 0;
+		if (tab && !strcmp(tab + 1, name))
+			found = true;
+	}
+	fclose(f);
+	return found;
+}
+
+static int mnt_id_of(int fd)
+{
+	char path[64], buf[4096], *p;
+	ssize_t n;
+	int info;
+
+	snprintf(path, sizeof(path), "/proc/self/fdinfo/%d", fd);
+	info = open(path, O_RDONLY | O_CLOEXEC);
+	if (info < 0)
+		return -1;
+	n = read(info, buf, sizeof(buf) - 1);
+	close(info);
+	if (n <= 0)
+		return -1;
+	buf[n] = 0;
+	p = strstr(buf, "mnt_id:");
+	return p ? atoi(p + strlen("mnt_id:")) : -1;
+}
+
+static bool on_debugfs(int fd)
+{
+	struct statfs sf;
+
+	return !fstatfs(fd, &sf) && sf.f_type == DEBUGFS_MAGIC;
+}
+
+FIXTURE(layer) {
+	char base[DIR_LEN];
+	int fan;
+	int evfd;	/* on the layer clone of the lower debugfs */
+};
+
+FIXTURE_SETUP(layer)
+{
+	char lower[PATH_LEN], other[PATH_LEN], ovl[PATH_LEN], opts[2 * PATH_LEN + 16];
+	struct fanotify_event_metadata *ev;
+	struct pollfd pfd;
+	char buf[4096];
+	int lfd, lower_id;
+	ssize_t n;
+	DIR *d;
+
+	self->fan = -1;
+	self->evfd = -1;
+	if (geteuid())
+		SKIP(return, "test requires root");
+	if (!have_fs("debugfs") || !have_fs("tracefs"))
+		SKIP(return, "test requires debugfs with the tracefs automount");
+	if (!have_fs("overlay"))
+		SKIP(return, "test requires overlayfs");
+
+	snprintf(self->base, sizeof(self->base), "/tmp/layer.XXXXXX");
+	ASSERT_NE(mkdtemp(self->base), NULL);
+	ASSERT_EQ(unshare(CLONE_NEWNS), 0);
+	ASSERT_EQ(mount("", "/", NULL, MS_REC | MS_PRIVATE, NULL), 0);
+	ASSERT_EQ(mount("tmpfs", self->base, "tmpfs", 0, "mode=0755"), 0);
+	snprintf(lower, sizeof(lower), "%s/lower", self->base);
+	snprintf(other, sizeof(other), "%s/other", self->base);
+	snprintf(ovl, sizeof(ovl), "%s/ovl", self->base);
+	ASSERT_EQ(mkdir(lower, 0755), 0);
+	ASSERT_EQ(mkdir(other, 0755), 0);
+	ASSERT_EQ(mkdir(ovl, 0755), 0);
+	ASSERT_EQ(mount("debugfs", lower, "debugfs", 0, NULL), 0);
+	snprintf(opts, sizeof(opts), "lowerdir=%s:%s", lower, other);
+	ASSERT_EQ(mount("overlay", ovl, "overlay", MS_RDONLY, opts), 0);
+
+	self->fan = fanotify_init(FAN_CLASS_NOTIF | FAN_NONBLOCK | FAN_CLOEXEC,
+				  O_RDONLY | O_CLOEXEC);
+	ASSERT_GE(self->fan, 0);
+	ASSERT_EQ(fanotify_mark(self->fan, FAN_MARK_ADD | FAN_MARK_FILESYSTEM,
+				FAN_OPEN | FAN_ONDIR, AT_FDCWD, lower), 0);
+	lfd = open(lower, O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+	ASSERT_GE(lfd, 0);
+	lower_id = mnt_id_of(lfd);
+	close(lfd);
+
+	/* the overlay opens its lower directory through the layer clone */
+	d = opendir(ovl);
+	ASSERT_NE(d, NULL);
+	closedir(d);
+	pfd.fd = self->fan;
+	pfd.events = POLLIN;
+	ASSERT_EQ(poll(&pfd, 1, 5000), 1);
+	n = read(self->fan, buf, sizeof(buf));
+	ASSERT_GT(n, 0);
+	for (ev = (void *)buf; FAN_EVENT_OK(ev, n); ev = FAN_EVENT_NEXT(ev, n)) {
+		if (ev->fd < 0)
+			continue;
+		if (self->evfd < 0 && on_debugfs(ev->fd) &&
+		    mnt_id_of(ev->fd) != lower_id)
+			self->evfd = ev->fd;
+		else
+			close(ev->fd);
+	}
+	ASSERT_GE(self->evfd, 0)
+		TH_LOG("no event on the layer clone");
+}
+
+FIXTURE_TEARDOWN(layer)
+{
+	if (self->evfd >= 0)
+		close(self->evfd);
+	if (self->fan >= 0)
+		close(self->fan);
+	umount2(self->base, MNT_DETACH);
+	rmdir(self->base);
+}
+
+TEST_F(layer, automount_below_the_clone_is_refused)
+{
+	struct stat st;
+	int fd;
+
+	/* the automount point is there */
+	ASSERT_EQ(fstatat(self->evfd, "tracing", &st, AT_NO_AUTOMOUNT), 0);
+	/* the clone is in no namespace, so the automount has nowhere to go */
+	fd = openat(self->evfd, "tracing", O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+	EXPECT_LT(fd, 0);
+	EXPECT_EQ(errno, EINVAL);
+	if (fd >= 0)
+		close(fd);
+}
+
+TEST_HARNESS_MAIN

-- 
2.53.0