[PATCH 3/3] pids: Make it possible to clone tasks with given pids

[Pavel Emelyanov <xemul@parallels.com>]

When restoring a task (or a set of tasks) we need to recreate them
with exactly the same pid(s) as they had before. Thus we need the
ability to create a task with specified pid. The proposal is to reuse
the already free CLONE_STOPPED clone flag, introduce the new one
called CLONE_CHILD_USEPIDS and point to the desired pids with the
child_tidptr.

The child_tidptr points to an array of pids for current namespace and
its ancestors. When 0 is met in this array the pid number for the
corresponding namespace is generated, rather than set.

For security reasons after a regular clone/fork is done in a namespace
further cloning with predefined pid is not allowed.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

---
 include/linux/pid.h   |    2 +-
 include/linux/sched.h |    3 +-
 kernel/fork.c         |    4 ++-
 kernel/pid.c          |   50 +++++++++++++++++++++++++++++++++++++++++++++++-
 4 files changed, 53 insertions(+), 6 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index b152d44..95aa618 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -119,7 +119,7 @@ extern struct pid *find_get_pid(int nr);
 extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 int next_pidmap(struct pid_namespace *pid_ns, unsigned int last);
 
-extern struct pid *alloc_pid(struct pid_namespace *ns);
+extern struct pid *alloc_pid(struct pid_namespace *ns, int __user *want_pids);
 extern void free_pid(struct pid *pid);
 
 /*
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 41d0237..5472c4e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -21,8 +21,7 @@
 #define CLONE_DETACHED		0x00400000	/* Unused, ignored */
 #define CLONE_UNTRACED		0x00800000	/* set if the tracing process can't force CLONE_PTRACE on this clone */
 #define CLONE_CHILD_SETTID	0x01000000	/* set the TID in the child */
-/* 0x02000000 was previously the unused CLONE_STOPPED (Start in stopped state)
-   and is now available for re-use. */
+#define CLONE_CHILD_USEPIDS	0x02000000	/* use the pids given by user */
 #define CLONE_NEWUTS		0x04000000	/* New utsname group? */
 #define CLONE_NEWIPC		0x08000000	/* New ipcs */
 #define CLONE_NEWUSER		0x10000000	/* New user namespace */
diff --git a/kernel/fork.c b/kernel/fork.c
index 45a5f54..26c67ff 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1253,7 +1253,9 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 		goto bad_fork_cleanup_io;
 
 	if (pid != &init_struct_pid) {
-		pid = alloc_pid(p->nsproxy->pid_ns);
+		pid = alloc_pid(p->nsproxy->pid_ns,
+				(clone_flags & CLONE_CHILD_USEPIDS) ?
+					child_tidptr : NULL);
 		if (IS_ERR(pid)) {
 			retval = PTR_ERR(pid);
 			goto bad_fork_cleanup_io;
diff --git a/kernel/pid.c b/kernel/pid.c
index 86bf7d2..fc7d35c 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -224,6 +224,38 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
 	return -ENOMEM;
 }
 
+static int set_pidmap(struct pid_namespace *pid_ns, int pid)
+{
+	int offset;
+	struct pidmap *map;
+
+	/*
+	 * When creating a new pid namespace we must make its init
+	 * have pid == 1 in it.
+	 */
+	if (pid_ns->child_reaper == NULL)
+		return 0;
+
+	/*
+	 * Don't allow to create a task with a pid which has recently
+	 * belonged to some other (dead already) task. Only init (of
+	 * a freshly created namespace) and his clones can do this.
+	 */
+	if (pid_ns->last_pid != 1)
+		return -EPERM;
+
+	map = &pid_ns->pidmap[pid/BITS_PER_PAGE];
+	offset = pid & BITS_PER_PAGE_MASK;
+
+	if (alloc_pidmap_page(map))
+		return -ENOMEM;
+
+	if (test_and_set_bit(offset, map->page))
+		return -EBUSY;
+
+	return pid;
+}
+
 int next_pidmap(struct pid_namespace *pid_ns, unsigned int last)
 {
 	int offset;
@@ -284,7 +316,7 @@ void free_pid(struct pid *pid)
 	call_rcu(&pid->rcu, delayed_put_pid);
 }
 
-struct pid *alloc_pid(struct pid_namespace *ns)
+struct pid *alloc_pid(struct pid_namespace *ns, int __user *want_pids)
 {
 	struct pid *pid;
 	enum pid_type type;
@@ -298,7 +330,21 @@ struct pid *alloc_pid(struct pid_namespace *ns)
 
 	tmp = ns;
 	for (i = ns->level; i >= 0; i--) {
-		nr = alloc_pidmap(tmp);
+		nr = 0;
+		if (unlikely(want_pids != NULL)) {
+			if (get_user(nr, want_pids)) {
+				nr = -EFAULT;
+				goto out_free;
+			}
+
+			if (nr != 0) {
+				want_pids++;
+				nr = set_pidmap(tmp, nr);
+			} else
+				want_pids = NULL; /* optimize above */
+		}
+		if (nr == 0)
+			nr = alloc_pidmap(tmp);
 		if (nr < 0)
 			goto out_free;
 
-- 
1.5.5.6