1 files changed, 103 insertions, 4 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index fbe9dfcd8680..8b03d93ba068 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>
@@ -21,6 +22,7 @@
 #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/unistd.h>
@@ -1670,6 +1672,58 @@ 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
+};
+
+/**
+ * pidfd_create() - Create a new pid file descriptor.
+ *
+ * @pid:  struct pid that the pidfd will reference
+ *
+ * This creates a new pid file descriptor with the O_CLOEXEC flag set.
+ *
+ * Note, that this function can only be called after the fd table has
+ * been unshared to avoid leaking the pidfd to the new process.
+ *
+ * Return: On success, a cloexec pidfd is returned.
+ *         On error, a negative errno number will be returned.
+ */
+static int pidfd_create(struct pid *pid)
+{
+        int fd;
+
+        fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
+                              O_RDWR | O_CLOEXEC);
+        if (fd < 0)
+                put_pid(pid);
+
+        return fd;
+}
+
 /*
  * This creates a new process as a copy of the old one,
  * but does not actually start it yet.
@@ -1682,13 +1736,14 @@ static __latent_entropy struct task_struct *copy_process(
                                         unsigned long clone_flags,
                                         unsigned long stack_start,
                                         unsigned long stack_size,
+                                        int __user *parent_tidptr,
                                         int __user *child_tidptr,
                                         struct pid *pid,
                                         int trace,
                                         unsigned long tls,
                                         int node)
 {
-        int retval;
+        int pidfd = -1, retval;
         struct task_struct *p;
         struct multiprocess_signals delayed;
 
@@ -1738,6 +1793,31 @@ static __latent_entropy struct task_struct *copy_process(
                         return ERR_PTR(-EINVAL);
         }
 
+        if (clone_flags & CLONE_PIDFD) {
+                int reserved;
+
+                /*
+                 * - CLONE_PARENT_SETTID is useless for pidfds and also
+                 *   parent_tidptr is used to return pidfds.
+                 * - CLONE_DETACHED is blocked so that we can potentially
+                 *   reuse it later for CLONE_PIDFD.
+                 * - CLONE_THREAD is blocked until someone really needs it.
+                 */
+                if (clone_flags &
+                    (CLONE_DETACHED | CLONE_PARENT_SETTID | CLONE_THREAD))
+                        return ERR_PTR(-EINVAL);
+
+                /*
+                 * Verify that parent_tidptr is sane so we can potentially
+                 * reuse it later.
+                 */
+                if (get_user(reserved, parent_tidptr))
+                        return ERR_PTR(-EFAULT);
+
+                if (reserved != 0)
+                        return ERR_PTR(-EINVAL);
+        }
+
         /*
          * Force any signals received before this point to be delivered
          * before the fork happens.  Collect up signals sent to multiple
@@ -1944,6 +2024,22 @@ 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(pid);
+                if (retval < 0)
+                        goto bad_fork_free_pid;
+
+                pidfd = retval;
+                retval = put_user(pidfd, parent_tidptr);
+                if (retval)
+                        goto bad_fork_put_pidfd;
+        }
+
 #ifdef CONFIG_BLOCK
         p->plug = NULL;
 #endif
@@ -2004,7 +2100,7 @@ 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
@@ -2119,6 +2215,9 @@ bad_fork_cancel_cgroup:
         spin_unlock(&current->sighand->siglock);
         write_unlock_irq(&tasklist_lock);
         cgroup_cancel_fork(p);
+bad_fork_put_pidfd:
+        if (clone_flags & CLONE_PIDFD)
+                ksys_close(pidfd);
 bad_fork_free_pid:
         cgroup_threadgroup_change_end(current);
         if (pid != &init_struct_pid)
@@ -2184,7 +2283,7 @@ static inline void init_idle_pids(struct task_struct *idle)
 struct task_struct *fork_idle(int cpu)
 {
         struct task_struct *task;
-        task = copy_process(CLONE_VM, 0, 0, NULL, &init_struct_pid, 0, 0,
+        task = copy_process(CLONE_VM, 0, 0, NULL, NULL, &init_struct_pid, 0, 0,
                             cpu_to_node(cpu));
         if (!IS_ERR(task)) {
                 init_idle_pids(task);
@@ -2236,7 +2335,7 @@ long _do_fork(unsigned long clone_flags,
                         trace = 0;
         }
 
-        p = copy_process(clone_flags, stack_start, stack_size,
+        p = copy_process(clone_flags, stack_start, stack_size, parent_tidptr,
                          child_tidptr, NULL, trace, tls, NUMA_NO_NODE);
         add_latent_entropy();