[PATCH v2 1/3] pidns: fix vfork() after unshare(CLONE_NEWPID)

[Oleg Nesterov <oleg@redhat.com>]

8382fcac "pidns: Outlaw thread creation after unshare(CLONE_NEWPID)"
nacks CLONE_VM if the forking process unshared pid_ns, this obviously
breaks vfork:

	int main(void)
	{
		assert(unshare(CLONE_NEWUSER | CLONE_NEWPID) == 0);
		assert(vfork() >= 0);
		_exit(0);
		return 0;
	}

fails without this patch.

Change this check to use CLONE_SIGHAND instead. This also forbids
CLONE_THREAD automatically, and this is what the comment implies.

We could probably even drop CLONE_SIGHAND and use CLONE_THREAD,
but it would be safer to not do this. The current check denies
CLONE_SIGHAND implicitely and there is no reason to change this.

Reported-by: Colin Walters <walters@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
---
 kernel/fork.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index e23bb19..29c9f6b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1173,10 +1173,11 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 		return ERR_PTR(-EINVAL);
 
 	/*
-	 * If the new process will be in a different pid namespace
-	 * don't allow the creation of threads.
+	 * If the new process will be in a different pid namespace don't
+	 * allow it to share a thread group or signal handlers with the
+	 * forking task.
 	 */
-	if ((clone_flags & (CLONE_VM|CLONE_NEWPID)) &&
+	if ((clone_flags & (CLONE_SIGHAND | CLONE_NEWPID)) &&
 	    (task_active_pid_ns(current) != current->nsproxy->pid_ns))
 		return ERR_PTR(-EINVAL);
 
-- 
1.5.5.1