This effectively gives us an ability to create the pid namespace init as a child of the process (setns-ed to the pid namespace) different to the process which created the pid namespace itself. Original problem: There is a cool set_tid feature in clone3() syscall, it allows you to create process with desired pids on multiple pid namespace levels. Which is useful to restore processes in CRIU for nested pid namespace case. In nested container case we can potentially see this kind of pid/user namespace tree: Process ┌─────────┐ User NS0 ──▶ Pid NS0 ──▶ Pid p0 │ │ │ │ │ ▼ ▼ │ │ User NS1 ──▶ Pid NS1 ──▶ Pid p1 │ │ │ │ │ ... ... │ ... │ │ │ │ │ ▼ ▼ │ │ User NSn ──▶ Pid NSn ──▶ Pid pn │ └─────────┘ So to create the "Process" and set pids {p0, p1, ... pn} for it on all pid namespace levels we can use clone3() syscall set_tid feature, BUT the syscall does not allow you to set pid on pid namespace levels you don't have permission to. So basically you have to be in "User NS0" when creating the "Process" to actually be able to set pids on all levels. It is ok for almost any process, but with pid namespace init this does not work, as currently we can only create pid namespace init and the pid namespace itself simultaneously, so to make "Pid NSn" owned by "User NSn" we have to be in the "User NSn". We can't possibly be in "User NS0" and "User NSn" at the same time, hence the problem. Alternative solution: Yes, for the case of pid namespace init we can use old and gold /proc/sys/kernel/ns_last_pid interface on the levels lower than n. But it is much more complicated and introduces tons of extra code to do. It would be nice to make clone3() set_tid interface also aplicable to this corner case. Implementation: Now when anyone can setns to the pid namespace before the creation of init, and thus multiple processes can fork children to the pid namespace, we enforce that the first process created is always the init, and only allow other processes after the init sets pid_namespace->child_reaper. To avoid possible problems related to cpu/compiler optimizations around ->child_reaper, let's use WRITE_ONCE (additional to task_list lock) everywhere we write it and use READ_ONCE everywhere we read it without explicit lock. Note: we already had READ_ONCE in nsfs_fh_to_dentry(). Signed-off-by: Pavel Tikhomirov -- v2: Use *_ONCE for ->child_reaper accesses atomicity, and avoid taking task_list lock for reading it. Rebase to master, and thus remove now excess pidns_ready variable. Note: I didn't find anything in copy_process() around setting the ->child_reaper which can influence the pid namespace, so it looks like the pid namespace is fully setup at the point when init sets ->child_reaper to receive more processes. Thus tasklist lock looks excess in pidns_for_children_get()'s ->child_reaper check and it should be safe not to have it in the corresponding checks in alloc_pid(). --- kernel/exit.c | 2 +- kernel/fork.c | 2 +- kernel/pid.c | 5 +++-- kernel/pid_namespace.c | 9 --------- 4 files changed, 5 insertions(+), 13 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 8a87021211ae..567fc3b7b0f9 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -608,7 +608,7 @@ static struct task_struct *find_child_reaper(struct task_struct *father, reaper = find_alive_thread(father); if (reaper) { - pid_ns->child_reaper = reaper; + WRITE_ONCE(pid_ns->child_reaper, reaper); return reaper; } diff --git a/kernel/fork.c b/kernel/fork.c index e832da9d15a4..27d0cdbca67e 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2423,7 +2423,7 @@ __latent_entropy struct task_struct *copy_process( init_task_pid(p, PIDTYPE_SID, task_session(current)); if (is_child_reaper(pid)) { - ns_of_pid(pid)->child_reaper = p; + WRITE_ONCE(ns_of_pid(pid)->child_reaper, p); p->signal->flags |= SIGNAL_UNKILLABLE; } p->signal->shared_pending.signal = delayed.signal; diff --git a/kernel/pid.c b/kernel/pid.c index 3b96571d0fe6..e6116e131d8d 100644 --- a/kernel/pid.c +++ b/kernel/pid.c @@ -219,7 +219,7 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid, * Also fail if a PID != 1 is requested and * no PID 1 exists. */ - if (tid != 1 && !tmp->child_reaper) + if (tid != 1 && !READ_ONCE(tmp->child_reaper)) goto out_abort; retval = -EPERM; if (!checkpoint_restore_ns_capable(tmp->user_ns)) @@ -247,8 +247,9 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *arg_set_tid, * alreay in use. Return EEXIST in that case. */ if (nr == -ENOSPC) - nr = -EEXIST; + } else if (!READ_ONCE(tmp->child_reaper) && idr_get_cursor(&tmp->idr) != 0) { + nr = -EINVAL; } else { int pid_min = 1; /* diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c index e48f5de41361..d36afc58ee1d 100644 --- a/kernel/pid_namespace.c +++ b/kernel/pid_namespace.c @@ -369,15 +369,6 @@ static struct ns_common *pidns_for_children_get(struct task_struct *task) } task_unlock(task); - if (ns) { - read_lock(&tasklist_lock); - if (!ns->child_reaper) { - put_pid_ns(ns); - ns = NULL; - } - read_unlock(&tasklist_lock); - } - return ns ? &ns->ns : NULL; } -- 2.53.0 First testcase "pidns_init_via_setns" checks that a process can become Pid 1 (init) in a new Pid namespace created via unshare() and joined via setns(). Second testcase "pidns_init_via_setns_set_tid" checks that during this process we can use clone3() + set_tid and set the pid in both the new and old pid namespaces (owned by different user namespaces). Signed-off-by: Pavel Tikhomirov --- .../selftests/pid_namespace/.gitignore | 1 + .../testing/selftests/pid_namespace/Makefile | 2 +- .../pid_namespace/pidns_init_via_setns.c | 238 ++++++++++++++++++ 3 files changed, 240 insertions(+), 1 deletion(-) create mode 100644 tools/testing/selftests/pid_namespace/pidns_init_via_setns.c diff --git a/tools/testing/selftests/pid_namespace/.gitignore b/tools/testing/selftests/pid_namespace/.gitignore index 5118f0f3edf4..c647c6eb3367 100644 --- a/tools/testing/selftests/pid_namespace/.gitignore +++ b/tools/testing/selftests/pid_namespace/.gitignore @@ -1,2 +1,3 @@ pid_max +pidns_init_via_setns regression_enomem diff --git a/tools/testing/selftests/pid_namespace/Makefile b/tools/testing/selftests/pid_namespace/Makefile index b972f55d07ae..b01a924ac04b 100644 --- a/tools/testing/selftests/pid_namespace/Makefile +++ b/tools/testing/selftests/pid_namespace/Makefile @@ -1,7 +1,7 @@ # SPDX-License-Identifier: GPL-2.0 CFLAGS += -g $(KHDR_INCLUDES) -TEST_GEN_PROGS = regression_enomem pid_max +TEST_GEN_PROGS = regression_enomem pid_max pidns_init_via_setns LOCAL_HDRS += $(selfdir)/pidfd/pidfd.h diff --git a/tools/testing/selftests/pid_namespace/pidns_init_via_setns.c b/tools/testing/selftests/pid_namespace/pidns_init_via_setns.c new file mode 100644 index 000000000000..7e4c610291d3 --- /dev/null +++ b/tools/testing/selftests/pid_namespace/pidns_init_via_setns.c @@ -0,0 +1,238 @@ +// SPDX-License-Identifier: GPL-2.0 +#define _GNU_SOURCE +#include +#include +#include +#include +#include + +#include "kselftest_harness.h" +#include "../pidfd/pidfd.h" + +/* + * Test that a process can become PID 1 (init) in a new PID namespace + * created via unshare() and joined via setns(). + * + * Flow: + * 1. Parent creates a pipe for synchronization. + * 2. Parent forks a child. + * 3. Parent calls unshare(CLONE_NEWPID) to create a new PID namespace. + * 4. Parent signals the child via the pipe. + * 5. Child opens parent's /proc//ns/pid_for_children and calls + * setns(fd, CLONE_NEWPID) to join the new namespace. + * 6. Child forks a grandchild. + * 7. Grandchild verifies getpid() == 1. + */ +TEST(pidns_init_via_setns) +{ + pid_t child, parent_pid; + int pipe_fd[2]; + char buf; + + parent_pid = getpid(); + + ASSERT_EQ(0, pipe(pipe_fd)); + + child = fork(); + ASSERT_GE(child, 0); + + if (child == 0) { + char path[256]; + int nsfd; + pid_t grandchild; + + close(pipe_fd[1]); + + /* Wait for parent to complete unshare */ + ASSERT_EQ(1, read_nointr(pipe_fd[0], &buf, 1)); + close(pipe_fd[0]); + + snprintf(path, sizeof(path), + "/proc/%d/ns/pid_for_children", parent_pid); + nsfd = open(path, O_RDONLY); + ASSERT_GE(nsfd, 0); + + ASSERT_EQ(0, setns(nsfd, CLONE_NEWPID)); + close(nsfd); + + grandchild = fork(); + ASSERT_GE(grandchild, 0); + + if (grandchild == 0) { + /* Should be init (PID 1) in the new namespace */ + if (getpid() != 1) + _exit(1); + _exit(0); + } + + ASSERT_EQ(0, wait_for_pid(grandchild)); + _exit(0); + } + + close(pipe_fd[0]); + + if (geteuid()) + ASSERT_EQ(0, unshare(CLONE_NEWUSER)); + + ASSERT_EQ(0, unshare(CLONE_NEWPID)); + + /* Signal child that the new PID namespace is ready */ + buf = 0; + ASSERT_EQ(1, write_nointr(pipe_fd[1], &buf, 1)); + close(pipe_fd[1]); + + ASSERT_EQ(0, wait_for_pid(child)); +} + +/* + * Similar to pidns_init_via_setns, but: + * 1. Parent enters a new PID namespace right from the start to be able to + * later freely use pid 1001 in it. + * 2. After forking child, parent also calls unshare(CLONE_NEWUSER) + * before unshare(CLONE_NEWPID) so that new old and new pid namespaces have + * different user namespace owners. + * 3. Child uses clone3() with set_tid={1, 1001} instead of fork() and + * grandchild checks that it gets desired pids . + * + * Flow: + * 1. Test process creates a new PID namespace and forks a wrapper + * (PID 1 in the outer namespace). + * 2. Wrapper forks a child. + * 3. Wrapper calls unshare(CLONE_NEWUSER) + unshare(CLONE_NEWPID) + * to create an inner PID namespace. + * 4. Wrapper signals the child via pipe. + * 5. Child opens wrapper's /proc//ns/pid_for_children and calls + * setns(fd, CLONE_NEWPID) to join the inner namespace. + * 6. Child calls clone3() with set_tid={1, 1001}. + * 7. Grandchild verifies its NSpid ends with "1001 1". + */ + +pid_t set_tid[] = {1, 1001}; + +static int pidns_init_via_setns_set_tid_grandchild(struct __test_metadata *_metadata) +{ + char *line = NULL; + size_t len = 0; + int found = 0; + FILE *gf; + + gf = fopen("/proc/self/status", "r"); + ASSERT_NE(gf, NULL); + + while (getline(&line, &len, gf) != -1) { + if (strncmp(line, "NSpid:", 6) != 0) + continue; + + for (int i = 0; i < 2; i++) { + char *last = strrchr(line, '\t'); + pid_t pid; + + ASSERT_NE(last, NULL); + ASSERT_EQ(sscanf(last, "%d", &pid), 1); + ASSERT_EQ(pid, set_tid[i]); + *last = '\0'; + } + + found = true; + break; + } + + free(line); + fclose(gf); + ASSERT_TRUE(found); + return 0; +} + +static int pidns_init_via_setns_set_tid_child(struct __test_metadata *_metadata, + pid_t parent_pid, int pipe_fd[2]) +{ + struct __clone_args args = { + .exit_signal = SIGCHLD, + .set_tid = ptr_to_u64(set_tid), + .set_tid_size = 2, + }; + pid_t grandchild; + char path[256]; + char buf; + int nsfd; + + close(pipe_fd[1]); + + ASSERT_EQ(1, read_nointr(pipe_fd[0], &buf, 1)); + close(pipe_fd[0]); + + snprintf(path, sizeof(path), + "/proc/%d/ns/pid_for_children", parent_pid); + nsfd = open(path, O_RDONLY); + ASSERT_GE(nsfd, 0); + + ASSERT_EQ(0, setns(nsfd, CLONE_NEWPID)); + close(nsfd); + + grandchild = sys_clone3(&args, sizeof(args)); + ASSERT_GE(grandchild, 0); + + if (grandchild == 0) + _exit(pidns_init_via_setns_set_tid_grandchild(_metadata)); + + ASSERT_EQ(0, wait_for_pid(grandchild)); + return 0; +} + +static int pidns_init_via_setns_set_tid_wrapper(struct __test_metadata *_metadata) +{ + int pipe_fd[2]; + pid_t child, parent_pid; + char buf; + FILE *f; + + /* + * We are PID 1 inside the new namespace, but /proc is + * mounted from the host. Read our host-visible PID so + * the child can reach our pid_for_children via /proc. + */ + f = fopen("/proc/self/stat", "r"); + ASSERT_NE(f, NULL); + ASSERT_EQ(fscanf(f, "%d", &parent_pid), 1); + ASSERT_EQ(0, pipe(pipe_fd)); + + child = fork(); + ASSERT_GE(child, 0); + + if (child == 0) + _exit(pidns_init_via_setns_set_tid_child(_metadata, parent_pid, pipe_fd)); + + close(pipe_fd[0]); + + ASSERT_EQ(0, unshare(CLONE_NEWUSER)); + ASSERT_EQ(0, unshare(CLONE_NEWPID)); + + buf = 0; + ASSERT_EQ(1, write_nointr(pipe_fd[1], &buf, 1)); + close(pipe_fd[1]); + + ASSERT_EQ(0, wait_for_pid(child)); + + fclose(f); + return 0; +} + +TEST(pidns_init_via_setns_set_tid) +{ + pid_t wrapper; + + if (geteuid()) + ASSERT_EQ(0, unshare(CLONE_NEWUSER)); + + ASSERT_EQ(0, unshare(CLONE_NEWPID)); + + wrapper = fork(); + ASSERT_GE(wrapper, 0); + + if (wrapper == 0) + _exit(pidns_init_via_setns_set_tid_wrapper(_metadata)); + + ASSERT_EQ(0, wait_for_pid(wrapper)); +} + +TEST_HARNESS_MAIN -- 2.53.0