[RFC-2 PATCH 2/4] fork: add CLONE_PIDFD via anonymous inode

[Christian Brauner <christian@brauner.io>]

This patchset makes it possible to retrieve pid file descriptors at process
creation time by introducing a new flag CLONE_PIDFD. As spotted by Linus,
there is exactly one bit left.

In this version of CLONE_PIDFD anonymous inode file descriptors are used.
They serve as a simple opaque handle on pids. Logically, this makes it
possible to interpret a pidfd differently, narrowing or widening the scope
of various operations (e.g. signal sending). Thus, a pidfd cannot just
refer to a tgid, but also a tid, or in theory - given appropriate flag
arguments in relevant syscalls - a process group or session.

This patchset uses anonymous file descriptors instead of file descriptors
from /proc/<pid>.
A pidfd in this style comes with additional information in fdinfo: the pid
of the process it refers to in the current pid namespace (Pid: %d).

Even though originally file descriptors to /proc/<pid> were preferred we
discovered the associated complexity while implementing this solution which
prompted us to implement an alternative and put it up for debate. We have
chosen to implement this alternative to illustrate how strikingly simple
this patchset is in comparision to the original approach.
To remove worries about missing metadata access we have written a POC that
illustrates how a combination of CLONE_PIDFD, fdinfo, and
pidfd_send_signal() can be used to gain race-free access to process
metadata through /proc/<pid>. The sample program can easily be translated
into a helper that would be suitable for inclusion in libc so that users
don't have to worry about writing it themselves.

We hope that this ultimately will be the approach the community prefers.

Signed-off-by: Christian Brauner <christian@brauner.io>
Signed-off-by: Jann Horn <jann@thejh.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: David Howells <dhowells@redhat.com>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Cc: Jonathan Kowalski <bl0pbl33p@gmail.com>
Cc: "Dmitry V. Levin" <ldv@altlinux.org>
Cc: Andy Lutomirsky <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
---
 include/linux/pid.h        |  2 +
 include/uapi/linux/sched.h |  1 +
 kernel/fork.c              | 94 ++++++++++++++++++++++++++++++++++++--
 3 files changed, 92 insertions(+), 5 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index b6f4ba16065a..3c8ef5a199ca 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -66,6 +66,8 @@ struct pid
 
 extern struct pid init_struct_pid;
 
+extern const struct file_operations pidfd_fops;
+
 static inline struct pid *get_pid(struct pid *pid)
 {
 	if (pid)
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 22627f80063e..cd9bd14ce56d 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -10,6 +10,7 @@
 #define CLONE_FS	0x00000200	/* set if fs info shared between processes */
 #define CLONE_FILES	0x00000400	/* set if open files shared between processes */
 #define CLONE_SIGHAND	0x00000800	/* set if signal handlers and blocked signals shared */
+#define CLONE_PIDFD	0x00001000	/* create new pid file descriptor */
 #define CLONE_PTRACE	0x00002000	/* set if we want to let tracing continue on the child too */
 #define CLONE_VFORK	0x00004000	/* set if the parent wants the child to wake it up on mm_release */
 #define CLONE_PARENT	0x00008000	/* set if we want to have the same parent as the cloner */
diff --git a/kernel/fork.c b/kernel/fork.c
index 9dcd18aa210b..5716ea8c32e5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -11,6 +11,7 @@
  * management can be a bitch. See 'mm/memory.c': 'copy_page_range()'
  */
 
+#include <linux/anon_inodes.h>
 #include <linux/slab.h>
 #include <linux/sched/autogroup.h>
 #include <linux/sched/mm.h>
@@ -21,8 +22,10 @@
 #include <linux/sched/task.h>
 #include <linux/sched/task_stack.h>
 #include <linux/sched/cputime.h>
+#include <linux/seq_file.h>
 #include <linux/rtmutex.h>
 #include <linux/init.h>
+#include <linux/fsnotify.h>
 #include <linux/unistd.h>
 #include <linux/module.h>
 #include <linux/vmalloc.h>
@@ -1662,6 +1665,64 @@ static inline void rcu_copy_process(struct task_struct *p)
 #endif /* #ifdef CONFIG_TASKS_RCU */
 }
 
+static int pidfd_release(struct inode *inode, struct file *file)
+{
+	struct pid *pid = file->private_data;
+	file->private_data = NULL;
+	put_pid(pid);
+	return 0;
+}
+
+#ifdef CONFIG_PROC_FS
+static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
+{
+	struct pid_namespace *ns = proc_pid_ns(file_inode(m->file));
+	struct pid *pid = f->private_data;
+
+	seq_put_decimal_ull(m, "Pid:\t", pid_nr_ns(pid, ns));
+	seq_putc(m, '\n');
+}
+#endif
+
+const struct file_operations pidfd_fops = {
+	.release = pidfd_release,
+#ifdef CONFIG_PROC_FS
+	.show_fdinfo = pidfd_show_fdinfo,
+#endif
+};
+
+static int pidfd_create_cloexec(struct pid *pid, struct file **file)
+{
+	unsigned int flags = O_RDWR | O_CLOEXEC;
+	int error, fd;
+	struct file *f;
+
+	error = __alloc_fd(current->files, 1, rlimit(RLIMIT_NOFILE), flags);
+	if (error < 0)
+		return error;
+	fd = error;
+
+	f = anon_inode_getfile("pidfd", &pidfd_fops, get_pid(pid), flags);
+	if (IS_ERR(f)) {
+		put_pid(pid);
+		error = PTR_ERR(f);
+		goto err_put_unused_fd;
+	}
+
+	*file = f;
+	return fd;
+
+err_put_unused_fd:
+	put_unused_fd(fd);
+	return error;
+}
+
+static inline void pidfd_put_cloexec(struct pid *pid, int fd, struct file *file)
+{
+        put_unused_fd(fd);
+	fput(file);
+}
+
 /*
  * This creates a new process as a copy of the old one,
  * but does not actually start it yet.
@@ -1678,11 +1739,12 @@ static __latent_entropy struct task_struct *copy_process(
 					struct pid *pid,
 					int trace,
 					unsigned long tls,
-					int node)
+					int node, int *pidfd)
 {
 	int retval;
 	struct task_struct *p;
 	struct multiprocess_signals delayed;
+	struct file *pidfdf = NULL;
 
 	/*
 	 * Don't allow sharing the root directory with processes in a different
@@ -1936,6 +1998,18 @@ static __latent_entropy struct task_struct *copy_process(
 		}
 	}
 
+	/*
+	 * This has to happen after we've potentially unshared the file
+	 * descriptor table (so that the pidfd doesn't leak into the child if
+	 * the fd table isn't shared).
+	 */
+	if (clone_flags & CLONE_PIDFD) {
+		retval = pidfd_create_cloexec(pid, &pidfdf);
+		if (retval < 0)
+			goto bad_fork_free_pid;
+		*pidfd = retval;
+	}
+
 #ifdef CONFIG_BLOCK
 	p->plug = NULL;
 #endif
@@ -1996,7 +2070,7 @@ static __latent_entropy struct task_struct *copy_process(
 	 */
 	retval = cgroup_can_fork(p);
 	if (retval)
-		goto bad_fork_free_pid;
+		goto bad_fork_put_pidfd;
 
 	/*
 	 * From this point on we must avoid any synchronous user-space
@@ -2097,6 +2171,9 @@ static __latent_entropy struct task_struct *copy_process(
 	syscall_tracepoint_update(p);
 	write_unlock_irq(&tasklist_lock);
 
+	if (clone_flags & CLONE_PIDFD)
+		fd_install(*pidfd, pidfdf);
+
 	proc_fork_connector(p);
 	cgroup_post_fork(p);
 	cgroup_threadgroup_change_end(current);
@@ -2111,6 +2188,9 @@ static __latent_entropy struct task_struct *copy_process(
 	spin_unlock(&current->sighand->siglock);
 	write_unlock_irq(&tasklist_lock);
 	cgroup_cancel_fork(p);
+bad_fork_put_pidfd:
+	if (clone_flags & CLONE_PIDFD)
+		pidfd_put_cloexec(pid, *pidfd, pidfdf);
 bad_fork_free_pid:
 	cgroup_threadgroup_change_end(current);
 	if (pid != &init_struct_pid)
@@ -2177,7 +2257,7 @@ struct task_struct *fork_idle(int cpu)
 {
 	struct task_struct *task;
 	task = copy_process(CLONE_VM, 0, 0, NULL, &init_struct_pid, 0, 0,
-			    cpu_to_node(cpu));
+			    cpu_to_node(cpu), NULL);
 	if (!IS_ERR(task)) {
 		init_idle_pids(task);
 		init_idle(task, cpu);
@@ -2202,7 +2282,7 @@ long _do_fork(unsigned long clone_flags,
 	struct completion vfork;
 	struct pid *pid;
 	struct task_struct *p;
-	int trace = 0;
+	int pidfd, trace = 0;
 	long nr;
 
 	/*
@@ -2224,7 +2304,7 @@ long _do_fork(unsigned long clone_flags,
 	}
 
 	p = copy_process(clone_flags, stack_start, stack_size,
-			 child_tidptr, NULL, trace, tls, NUMA_NO_NODE);
+			 child_tidptr, NULL, trace, tls, NUMA_NO_NODE, &pidfd);
 	add_latent_entropy();
 
 	if (IS_ERR(p))
@@ -2260,6 +2340,10 @@ long _do_fork(unsigned long clone_flags,
 	}
 
 	put_pid(pid);
+
+	if (clone_flags & CLONE_PIDFD)
+		nr = pidfd;
+
 	return nr;
 }
 
-- 
2.21.0