--- /dev/null
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * An 'F' entry keeps its interpreter open for as long as the entry exists,
+ * and the entry only goes away when the binfmt_misc superblock is destroyed.
+ * An interpreter that lives on a mount which in turn keeps that superblock
+ * alive therefore pins the instance that owns it, and nothing can break the
+ * cycle. Check the two ways userspace could arrange for that: an interpreter
+ * on the binfmt_misc instance itself, and one on a filesystem stacked on it.
+ *
+ * Runs unprivileged in a user namespace; binfmt_misc is FS_USERNS_MOUNT.
+ */
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+
+#include "../filesystems/utils.h"
+#include "kselftest_harness.h"
+
+#define MNT "/tmp/binfmt_selfpin"
+#define BACKING "/tmp/binfmt_selfpin_back"
+#define LOWER BACKING "/lower"
+#define MERGED "/tmp/binfmt_selfpin_merged"
+
+#define MAGIC "\\xde\\xad"
+#define RULE(interp) ":selfpin:M::" MAGIC "::" interp ":F"
+/* Not on the instance, and unlike /bin/true it always exists. */
+#define INTERP "/proc/self/exe"
+
+#define OPTS_MAX (3 * PATH_MAX + 64)
+
+static int ensure_dir(const char *path)
+{
+ if (mkdir(path, 0755) && errno != EEXIST)
+ return -1;
+ return 0;
+}
+
+/* Write @rule to this instance's register file, preserving write(2)'s errno. */
+static int register_at(struct __test_metadata *_metadata, const char *rule)
+{
+ int fd, saved;
+ ssize_t n;
+
+ fd = open(MNT "/register", O_WRONLY);
+ ASSERT_GE(fd, 0);
+ n = write(fd, rule, strlen(rule));
+ saved = errno;
+ close(fd);
+ errno = saved;
+ return n < 0 ? -1 : 0;
+}
+
+/*
+ * Mount an overlay over @lower using a private upper/work pair, so the two
+ * mounts this test performs cannot interfere with each other and neither
+ * overlaps the lower layer.
+ */
+static int mount_overlay(const char *lower, int nr)
+{
+ char opts[OPTS_MAX], upper[PATH_MAX], work[PATH_MAX];
+
+ snprintf(upper, sizeof(upper), "%s/upper%d", BACKING, nr);
+ snprintf(work, sizeof(work), "%s/work%d", BACKING, nr);
+ if (mkdir(upper, 0755) || mkdir(work, 0755))
+ return -1;
+
+ snprintf(opts, sizeof(opts), "lowerdir=%s,upperdir=%s,workdir=%s",
+ lower, upper, work);
+ return mount("ovl", MERGED, "overlay", 0, opts);
+}
+
+FIXTURE(selfpin) {
+};
+
+FIXTURE_SETUP(selfpin)
+{
+ /* setup_userns() exits rather than returns if this is not there. */
+ if (access("/proc/self/ns/user", F_OK))
+ SKIP(return, "kernel without user namespaces");
+ ASSERT_EQ(setup_userns(), 0);
+
+ ASSERT_EQ(ensure_dir(MNT), 0);
+ if (mount("binfmt_misc", MNT, "binfmt_misc", 0, NULL)) {
+ int saved = errno;
+
+ /* Teardown doesn't run when setup skips, so clean up here. */
+ rmdir(MNT);
+ SKIP(return, "no binfmt_misc: %s", strerror(saved));
+ }
+}
+
+FIXTURE_TEARDOWN(selfpin)
+{
+ /* The namespaces go with the process; just don't litter /tmp. */
+ umount2(MERGED, MNT_DETACH);
+ umount2(BACKING, MNT_DETACH);
+ umount2(MNT, MNT_DETACH);
+ rmdir(MERGED);
+ rmdir(BACKING);
+ rmdir(MNT);
+}
+
+/*
+ * The instance's own files are regular files the mounter owns, so they can be
+ * made executable. Opening one for exec still has to fail, otherwise the entry
+ * pins the very superblock it lives in.
+ */
+TEST_F(selfpin, interpreter_on_the_instance)
+{
+ ASSERT_EQ(chmod(MNT "/status", 0755), 0);
+
+ ASSERT_NE(register_at(_metadata, RULE(MNT "/status")), 0);
+ EXPECT_EQ(errno, EACCES);
+}
+
+/* Same for an entry file rather than one of the control files. */
+TEST_F(selfpin, interpreter_on_an_entry)
+{
+ ASSERT_EQ(register_at(_metadata, ":victim:M::" MAGIC "::" INTERP ":"), 0);
+ ASSERT_EQ(chmod(MNT "/victim", 0755), 0);
+
+ ASSERT_NE(register_at(_metadata, RULE(MNT "/victim")), 0);
+ EXPECT_EQ(errno, EACCES);
+}
+
+/*
+ * A stacking filesystem holds a private clone of each layer for its whole
+ * lifetime, so an instance used as a layer can be pinned by an interpreter
+ * that does not live on it at all. Refuse to be a layer.
+ */
+TEST_F(selfpin, refuses_to_be_stacked_on)
+{
+ ASSERT_EQ(ensure_dir(BACKING), 0);
+ ASSERT_EQ(mount("tmpfs", BACKING, "tmpfs", 0, NULL), 0);
+ ASSERT_EQ(mkdir(LOWER, 0755), 0);
+ ASSERT_EQ(ensure_dir(MERGED), 0);
+
+ /* Nothing to prove unless overlayfs works here at all. */
+ if (mount_overlay(LOWER, 1)) {
+ if (errno == ENODEV || errno == EPERM)
+ SKIP(return, "no unprivileged overlayfs");
+ SKIP(return, "overlayfs unusable here: %s", strerror(errno));
+ }
+ ASSERT_EQ(umount(MERGED), 0);
+
+ EXPECT_NE(mount_overlay(MNT, 2), 0);
+}
+
+/* An ordinary interpreter still registers with 'F'. */
+TEST_F(selfpin, ordinary_interpreter_still_works)
+{
+ EXPECT_EQ(register_at(_metadata, RULE(INTERP)), 0);
+}
+
+TEST_HARNESS_MAIN
projects / thirdparty / linux.git / commitdiff
