Merge tag 'vfs-7.1-rc1.pidfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull clone and pidfs updates from Christian Brauner:
 "Add three new clone3() flags for pidfd-based process lifecycle
  management.

  CLONE_AUTOREAP:

     CLONE_AUTOREAP makes a child process auto-reap on exit without ever
     becoming a zombie. This is a per-process property in contrast to
     the existing auto-reap mechanism via SA_NOCLDWAIT or SIG_IGN for
     SIGCHLD which applies to all children of a given parent.

     Currently the only way to automatically reap children is to set
     SA_NOCLDWAIT or SIG_IGN on SIGCHLD. This is a parent-scoped
     property affecting all children which makes it unsuitable for
     libraries or applications that need selective auto-reaping of
     specific children while still being able to wait() on others.

     CLONE_AUTOREAP stores an autoreap flag in the child's
     signal_struct. When the child exits do_notify_parent() checks this
     flag and causes exit_notify() to transition the task directly to
     EXIT_DEAD. Since the flag lives on the child it survives
     reparenting: if the original parent exits and the child is
     reparented to a subreaper or init the child still auto-reaps when
     it eventually exits. This is cleaner than forcing the subreaper to
     get SIGCHLD and then reaping it. If the parent doesn't care the
     subreaper won't care. If there's a subreaper that would care it
     would be easy enough to add a prctl() that either just turns back
     on SIGCHLD and turns off auto-reaping or a prctl() that just
     notifies the subreaper whenever a child is reparented to it.

     CLONE_AUTOREAP can be combined with CLONE_PIDFD to allow the parent
     to monitor the child's exit via poll() and retrieve exit status via
     PIDFD_GET_INFO. Without CLONE_PIDFD it provides a fire-and-forget
     pattern. No exit signal is delivered so exit_signal must be zero.
     CLONE_THREAD and CLONE_PARENT are rejected: CLONE_THREAD because
     autoreap is a process-level property, and CLONE_PARENT because an
     autoreap child reparented via CLONE_PARENT could become an
     invisible zombie under a parent that never calls wait().

     The flag is not inherited by the autoreap process's own children.
     Each child that should be autoreaped must be explicitly created
     with CLONE_AUTOREAP.

  CLONE_NNP:

     CLONE_NNP sets no_new_privs on the child at clone time. Unlike
     prctl(PR_SET_NO_NEW_PRIVS) which a process sets on itself,
     CLONE_NNP allows the parent to impose no_new_privs on the child at
     creation without affecting the parent's own privileges.
     CLONE_THREAD is rejected because threads share credentials.
     CLONE_NNP is useful on its own for any spawn-and-sandbox pattern
     but was specifically introduced to enable unprivileged usage of
     CLONE_PIDFD_AUTOKILL.

  CLONE_PIDFD_AUTOKILL:

     This flag ties a child's lifetime to the pidfd returned from
     clone3(). When the last reference to the struct file created by
     clone3() is closed the kernel sends SIGKILL to the child. A pidfd
     obtained via pidfd_open() for the same process does not keep the
     child alive and does not trigger autokill - only the specific
     struct file from clone3() has this property. This is useful for
     container runtimes, service managers, and sandboxed subprocess
     execution - any scenario where the child must die if the parent
     crashes or abandons the pidfd or just wants a throwaway helper
     process.

     CLONE_PIDFD_AUTOKILL requires both CLONE_PIDFD and CLONE_AUTOREAP.
     It requires CLONE_PIDFD because the whole point is tying the
     child's lifetime to the pidfd. It requires CLONE_AUTOREAP because a
     killed child with no one to reap it would become a zombie - the
     primary use case is the parent crashing or abandoning the pidfd so
     no one is around to call waitpid(). CLONE_THREAD is rejected
     because autokill targets a process not a thread.

     If CLONE_NNP is specified together with CLONE_PIDFD_AUTOKILL an
     unprivileged user may spawn a process that is autokilled. The child
     cannot escalate privileges via setuid/setgid exec after being
     spawned. If CLONE_PIDFD_AUTOKILL is specified without CLONE_NNP the
     caller must have have CAP_SYS_ADMIN in its user namespace"

* tag 'vfs-7.1-rc1.pidfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  selftests: check pidfd_info->coredump_code correctness
  pidfds: add coredump_code field to pidfd_info
  kselftest/coredump: reintroduce null pointer dereference
  selftests/pidfd: add CLONE_PIDFD_AUTOKILL tests
  selftests/pidfd: add CLONE_NNP tests
  selftests/pidfd: add CLONE_AUTOREAP tests
  pidfd: add CLONE_PIDFD_AUTOKILL
  clone: add CLONE_NNP
  clone: add CLONE_AUTOREAP

3 files changed, 55 insertions, 4 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index a4ec2d1e25ee..55a6906d3014 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2029,6 +2029,41 @@ __latent_entropy struct task_struct *copy_process(
 			return ERR_PTR(-EINVAL);
 	}
 
+	if (clone_flags & CLONE_AUTOREAP) {
+		if (clone_flags & CLONE_THREAD)
+			return ERR_PTR(-EINVAL);
+		if (clone_flags & CLONE_PARENT)
+			return ERR_PTR(-EINVAL);
+		if (args->exit_signal)
+			return ERR_PTR(-EINVAL);
+	}
+
+	if ((clone_flags & CLONE_PARENT) && current->signal->autoreap)
+		return ERR_PTR(-EINVAL);
+
+	if (clone_flags & CLONE_NNP) {
+		if (clone_flags & CLONE_THREAD)
+			return ERR_PTR(-EINVAL);
+	}
+
+	if (clone_flags & CLONE_PIDFD_AUTOKILL) {
+		if (!(clone_flags & CLONE_PIDFD))
+			return ERR_PTR(-EINVAL);
+		if (!(clone_flags & CLONE_AUTOREAP))
+			return ERR_PTR(-EINVAL);
+		if (clone_flags & CLONE_THREAD)
+			return ERR_PTR(-EINVAL);
+		/*
+		 * Without CLONE_NNP the child could escalate privileges
+		 * after being spawned, so require CAP_SYS_ADMIN.
+		 * With CLONE_NNP the child can't gain new privileges,
+		 * so allow unprivileged usage.
+		 */
+		if (!(clone_flags & CLONE_NNP) &&
+		    !ns_capable(current_user_ns(), CAP_SYS_ADMIN))
+			return ERR_PTR(-EPERM);
+	}
+
 	/*
 	 * Force any signals received before this point to be delivered
 	 * before the fork happens.  Collect up signals sent to multiple
@@ -2251,13 +2286,18 @@ __latent_entropy struct task_struct *copy_process(
 	 * if the fd table isn't shared).
 	 */
 	if (clone_flags & CLONE_PIDFD) {
-		int flags = (clone_flags & CLONE_THREAD) ? PIDFD_THREAD : 0;
+		unsigned flags = PIDFD_STALE;
+
+		if (clone_flags & CLONE_THREAD)
+			flags |= PIDFD_THREAD;
+		if (clone_flags & CLONE_PIDFD_AUTOKILL)
+			flags |= PIDFD_AUTOKILL;
 
 		/*
 		 * Note that no task has been attached to @pid yet indicate
 		 * that via CLONE_PIDFD.
 		 */
-		retval = pidfd_prepare(pid, flags | PIDFD_STALE, &pidfile);
+		retval = pidfd_prepare(pid, flags, &pidfile);
 		if (retval < 0)
 			goto bad_fork_free_pid;
 		pidfd = retval;
@@ -2413,6 +2453,9 @@ __latent_entropy struct task_struct *copy_process(
 	 */
 	copy_seccomp(p);
 
+	if (clone_flags & CLONE_NNP)
+		task_set_no_new_privs(p);
+
 	init_task_pid_links(p);
 	if (likely(p->pid)) {
 		ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace);
@@ -2436,6 +2479,8 @@ __latent_entropy struct task_struct *copy_process(
 			 */
 			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
 							 p->real_parent->signal->is_child_subreaper;
+			if (clone_flags & CLONE_AUTOREAP)
+				p->signal->autoreap = 1;
 			list_add_tail(&p->sibling, &p->real_parent->children);
 			list_add_tail_rcu(&p->tasks, &init_task.tasks);
 			attach_pid(p, PIDTYPE_TGID);
@@ -2897,7 +2942,8 @@ static bool clone3_args_valid(struct kernel_clone_args *kargs)
 {
 	/* Verify that no unknown flags are passed along. */
 	if (kargs->flags &
-	    ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND | CLONE_INTO_CGROUP))
+	    ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND | CLONE_INTO_CGROUP |
+	      CLONE_AUTOREAP | CLONE_NNP | CLONE_PIDFD_AUTOKILL))
 		return false;
 
 	/*
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 392ec2f75f01..68c17daef8d4 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -549,7 +549,8 @@ static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
 	if (!dead && thread_group_empty(p)) {
 		if (!same_thread_group(p->real_parent, tracer))
 			dead = do_notify_parent(p, p->exit_signal);
-		else if (ignoring_children(tracer->sighand)) {
+		else if (ignoring_children(tracer->sighand) ||
+			 p->signal->autoreap) {
 			__wake_up_parent(p, tracer);
 			dead = true;
 		}
diff --git a/kernel/signal.c b/kernel/signal.c
index d65d0fe24bfb..e61f39fa8c8a 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2251,6 +2251,10 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
 		if (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN)
 			sig = 0;
 	}
+	if (!tsk->ptrace && tsk->signal->autoreap) {
+		autoreap = true;
+		sig = 0;
+	}
 	/*
 	 * Send with __send_signal as si_pid and si_uid are in the
 	 * parent's namespaces.