[PATCH RFC v3 41/90] pidfd: add pidfd_configure()

[Josef Bacik]

Add the system call that describes the task of an empty pidfd and starts
it. It is modelled on fsconfig(): it takes a command, a key and a value,
and data only flows into the kernel.

    pidfd = pidfd_open(cellfd, PIDFD_EMPTY);
    pidfd_configure(pidfd, PIDFD_SET_STRING, "exec", "/sbin/init", 0);
    pidfd_configure(pidfd, PIDFD_SET_BINARY, "argv", "init\0-v", 8);
    pidfd_configure(pidfd, PIDFD_CMD_START, NULL, NULL, 0);

This patch adds the keys "exec", "argv" and "envp", and PIDFD_CMD_START.
The start command creates the task, applies the configuration in the
context of the task, lets it exec, and waits for the exec to finish. If
anything fails, PIDFD_CMD_START returns the error, and the task is killed
if it already exists. A pidfd can only be started once.

The task gets default signal dispositions and its own session.
CLONE_CLEAR_SIGHAND would keep signals ignored that the caller ignores, so
the handlers are flushed the way they are for a usermode helper. Nobody
has to wait for the task. It is reaped automatically, and the pidfd
reports how it exited.

The pidfd may be kept open for a long time after the start, so the
configuration is freed as soon as it has been used.

The LSM hooks for exec run in the context of the new task, as for any
exec. Seccomp filters apply to the system calls of the new program. But
the work that the start command does is not a system call of the new task,
so the filters are not consulted for it. A filter that denies execve() to
stop a task from running another program must also deny pidfd_configure(),
just as such filters once had to learn about execveat().

Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
---
 include/uapi/linux/pidfd.h |   7 ++
 kernel/embryo.c            | 258 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 265 insertions(+)

diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h
index eb3704687c04..c01bce5e17ad 100644
--- a/include/uapi/linux/pidfd.h
+++ b/include/uapi/linux/pidfd.h
@@ -17,6 +17,13 @@
 #define PIDFD_AUTOKILL O_TRUNC
 #endif
 
+/* pidfd_configure(): @cmd. Data only ever goes in. */
+enum pidfd_configure_cmd {
+	PIDFD_SET_STRING	= 0,	/* Set @key to the string in @value */
+	PIDFD_SET_BINARY	= 1,	/* Set @key to the @aux bytes at @value */
+	PIDFD_SET_FD		= 2,	/* The file to have at number @key, from fd @aux */
+	PIDFD_CMD_START		= 3,	/* Make the task, apply everything and exec */
+};
 /* Flags for pidfd_send_signal(). */
 #define PIDFD_SIGNAL_THREAD		(1UL << 0)
 #define PIDFD_SIGNAL_THREAD_GROUP	(1UL << 1)
diff --git a/kernel/embryo.c b/kernel/embryo.c
index e052c09f9192..f405efb195c7 100644
--- a/kernel/embryo.c
+++ b/kernel/embryo.c
@@ -40,6 +40,10 @@
 #include <net/af_unix.h>
 #include <uapi/linux/pidfd.h>
 
+#define EMBRYO_KEY_MAX		32
+#define EMBRYO_MAX_STRINGS	4096
+#define EMBRYO_MAX_BLOCK	(256 * 1024)
+
 struct embryo_strings {
 	char		*block;		/* NUL terminated strings, back to back */
 	const char	**vec;		/* NULL terminated, points into @block */
@@ -94,6 +98,53 @@ static void embryo_put(struct embryo *e)
 	kfree_rcu(e, rcu);
 }
 
+/* The task is done being an embryo, one way or the other. */
+static void embryo_detach(struct embryo *e)
+{
+	WRITE_ONCE(current->embryo, NULL);
+	embryo_put(e);
+}
+
+static int embryo_exec(struct embryo *e)
+{
+	static const char * const no_strings[] = { NULL };
+	const char *argv0[] = { e->exec, NULL };
+
+	return kernel_execve(e->exec, e->argv.vec ?: argv0,
+			     e->envp.vec ?: no_strings);
+}
+
+static int embryo_main(void *arg)
+{
+	struct embryo *e = arg;
+	int ret;
+
+	/* CLONE_CLEAR_SIGHAND leaves what the starter ignores ignored. */
+	flush_signal_handlers(current, 1);
+
+	/* The starter's session, process group and terminal are not ours. */
+	ksys_setsid();
+	set_personality(PER_LINUX);
+
+	/* @started is set, so nobody changes anything any more. */
+	ret = embryo_exec(e);
+	embryo_drop_config(e);
+	/*
+	 * A signal to the embryo is no signal to whoever started it. Whatever
+	 * interrupted the exec, the embryo is about to be killed.
+	 */
+	if (ret && (fatal_signal_pending(current) || ret <= -ERESTARTSYS))
+		ret = -ESRCH;
+	e->result = ret;
+	complete_all(&e->done);
+	embryo_detach(e);
+	if (ret)
+		do_group_exit(SIGKILL);
+
+	/* Off to user mode. */
+	return 0;
+}
+
 /**
  * embryo_fork_finish - done creating an embryo, one way or the other
  * @args: the arguments of the clone, with @args->embryo set
@@ -242,3 +293,210 @@ void embryo_pidfd_release(struct file *file)
 		free_pid_struct(e->pid);
 	embryo_put(e);
 }
+
+static struct embryo *embryo_from_file(struct file *file)
+{
+	struct pid *pid;
+
+	pid = pidfd_pid(file);
+	if (IS_ERR(pid))
+		return ERR_CAST(pid);
+	/* Only the file pidfd_open() returned will do. Nothing else ever has this. */
+	if (!(file->f_flags & PIDFD_EMPTY))
+		return ERR_PTR(-EPERM);
+	return file->private_data;
+}
+
+static int embryo_set_strings(struct embryo_strings *s, const void __user *ublock,
+			      size_t len)
+{
+	struct embryo_strings new = {};
+	unsigned int nr = 0, i;
+	size_t off;
+
+	if (!len || len > EMBRYO_MAX_BLOCK)
+		return -E2BIG;
+
+	new.block = vmemdup_user(ublock, len);
+	if (IS_ERR(new.block))
+		return PTR_ERR(new.block);
+	if (new.block[len - 1]) {
+		kvfree(new.block);
+		return -EINVAL;
+	}
+
+	for (off = 0; off < len; off += strlen(new.block + off) + 1)
+		nr++;
+	if (nr > EMBRYO_MAX_STRINGS) {
+		kvfree(new.block);
+		return -E2BIG;
+	}
+
+	new.vec = kvmalloc_array(nr + 1, sizeof(*new.vec), GFP_KERNEL_ACCOUNT);
+	if (!new.vec) {
+		kvfree(new.block);
+		return -ENOMEM;
+	}
+	for (off = 0, i = 0; off < len; off += strlen(new.block + off) + 1)
+		new.vec[i++] = new.block + off;
+	new.vec[i] = NULL;
+
+	embryo_strings_free(s);
+	*s = new;
+	return 0;
+}
+
+static int embryo_start(struct embryo *e)
+{
+	struct kernel_clone_args args = {
+		/*
+		 * CLONE_VM only says "do not copy the mm": there is none,
+		 * copy_mm() sees to that. Nobody has to wait for the task, the
+		 * pidfd tells. The rest is what every task of a cell gets.
+		 */
+		.flags		= CLONE_VM | CLONE_UNTRACED | CLONE_AUTOREAP |
+				  CLONE_INTO_CGROUP | CLONE_NNP | CLONE_CLEAR_SIGHAND,
+		.fn		= embryo_main,
+		.fn_arg		= e,
+		.name		= "embryo",
+		.pid		= e->pid,
+		.cell		= e->cell,
+		.embryo		= e,
+	};
+	pid_t nr;
+	int ret;
+
+	/* Until embryo_fork_finish(). */
+	ret = cell_fork_prepare(e->cell);
+	if (ret)
+		return ret;
+
+	scoped_guard(mutex, &e->lock) {
+		ret = e->started ? -EBUSY : e->exec ? 0 : -EINVAL;
+		if (!ret)
+			e->started = true;
+	}
+	if (ret) {
+		cell_fork_finish(e->cell);
+		return ret;
+	}
+
+	/* For the task. */
+	refcount_inc(&e->ref);
+	nr = kernel_clone(&args);
+	/*
+	 * A signal is pending for the caller, so the fork is to be started over
+	 * once that has been seen to, as any fork is. It has not been tried.
+	 */
+	if (nr == -ERESTARTNOINTR) {
+		scoped_guard(mutex, &e->lock)
+			e->started = false;
+		return nr;
+	}
+	if (nr < 0) {
+		scoped_guard(mutex, &e->lock)
+			embryo_drop_config(e);
+		return nr;
+	}
+
+	/*
+	 * Everything up to the point of no return of the exec is reported
+	 * here. Past that it is a fatal signal, for the pidfd to tell.
+	 */
+	if (wait_for_completion_killable(&e->done))
+		return -EINTR;
+	return e->result;
+}
+
+/**
+ * sys_pidfd_configure - say what the task of an empty pidfd is to be, and start it
+ * @pidfd:  what pidfd_open() returned for PIDFD_EMPTY, and no other pidfd
+ * @cmd:    one of enum pidfd_configure_cmd
+ * @ukey:   the name of what is set, or NULL for PIDFD_CMD_START
+ * @uvalue: a string for PIDFD_SET_STRING, @aux bytes for PIDFD_SET_BINARY
+ * @aux:    a length for PIDFD_SET_BINARY, a file descriptor for PIDFD_SET_FD
+ *
+ * Shaped like fsconfig(). Data only goes in.
+ *
+ * PIDFD_CMD_START makes the task, applies everything in its context, has it exec
+ * and waits for that. Whatever goes wrong is returned here, and the task, if it
+ * got that far, is killed. It can be tried only once. Being started over because
+ * a signal is pending for the caller, as a fork is, does not count.
+ *
+ * Return: 0, -EPERM for any other pidfd, -EBUSY once it was started, -ESRCH if
+ * the task or the cell was killed, -EOPNOTSUPP for an unknown @cmd, -EINVAL for
+ * an unknown key or arguments that do not go with @cmd, or what went wrong.
+ */
+SYSCALL_DEFINE5(pidfd_configure, int, pidfd, unsigned int, cmd,
+		const char __user *, ukey, const void __user *, uvalue, int, aux)
+{
+	char *key __free(kfree) = NULL;
+	char *str __free(kfree) = NULL;
+	struct embryo *e;
+	int ret;
+
+	switch (cmd) {
+	case PIDFD_SET_STRING:
+		if (!ukey || !uvalue || aux)
+			return -EINVAL;
+		break;
+	case PIDFD_SET_BINARY:
+		if (!ukey || !uvalue || aux <= 0)
+			return -EINVAL;
+		break;
+	case PIDFD_CMD_START:
+		if (ukey || uvalue || aux)
+			return -EINVAL;
+		break;
+	default:
+		return -EOPNOTSUPP;
+	}
+
+	if (ukey) {
+		key = strndup_user(ukey, EMBRYO_KEY_MAX);
+		if (IS_ERR(key))
+			return PTR_ERR(no_free_ptr(key));
+	}
+	if (cmd == PIDFD_SET_STRING) {
+		str = strndup_user(uvalue, PATH_MAX);
+		if (IS_ERR(str))
+			return PTR_ERR(no_free_ptr(str));
+	}
+
+	CLASS(fd, f)(pidfd);
+	if (fd_empty(f))
+		return -EBADF;
+	e = embryo_from_file(fd_file(f));
+	if (IS_ERR(e))
+		return PTR_ERR(e);
+
+	if (cmd == PIDFD_CMD_START)
+		return embryo_start(e);
+
+	mutex_lock(&e->lock);
+	ret = -EBUSY;
+	if (e->started)
+		goto out;
+
+	switch (cmd) {
+	case PIDFD_SET_STRING:
+		ret = -EINVAL;
+		if (strcmp(key, "exec"))
+			break;
+		kfree(e->exec);
+		e->exec = no_free_ptr(str);
+		ret = 0;
+		break;
+	case PIDFD_SET_BINARY:
+		if (!strcmp(key, "argv"))
+			ret = embryo_set_strings(&e->argv, uvalue, aux);
+		else if (!strcmp(key, "envp"))
+			ret = embryo_set_strings(&e->envp, uvalue, aux);
+		else
+			ret = -EINVAL;
+		break;
+	}
+out:
+	mutex_unlock(&e->lock);
+	return ret;
+}

-- 
2.55.0