From 3fd37226216620c1a468afa999739d5016fbc349 Mon Sep 17 00:00:00 2001 From: Kirill Tkhai Date: Fri, 12 May 2017 19:11:31 +0300 Subject: [PATCH] pid_ns: Fix race between setns'ed fork() and zap_pid_ns_processes() Imagine we have a pid namespace and a task from its parent's pid_ns, which made setns() to the pid namespace. The task is doing fork(), while the pid namespace's child reaper is dying. We have the race between them: Task from parent pid_ns Child reaper copy_process() .. alloc_pid() .. .. zap_pid_ns_processes() .. disable_pid_allocation() .. read_lock(&tasklist_lock) .. iterate over pids in pid_ns .. kill tasks linked to pids .. read_unlock(&tasklist_lock) write_lock_irq(&tasklist_lock); .. attach_pid(p, PIDTYPE_PID); .. .. .. So, just created task p won't receive SIGKILL signal, and the pid namespace will be in contradictory state. Only manual kill will help there, but does the userspace care about this? I suppose, the most users just inject a task into a pid namespace and wait a SIGCHLD from it. The patch fixes the problem. It simply checks for (pid_ns->nr_hashed & PIDNS_HASH_ADDING) in copy_process(). We do it under the tasklist_lock, and can't skip PIDNS_HASH_ADDING as noted by Oleg: "zap_pid_ns_processes() does disable_pid_allocation() and then takes tasklist_lock to kill the whole namespace. Given that copy_process() checks PIDNS_HASH_ADDING under write_lock(tasklist) they can't race; if copy_process() takes this lock first, the new child will be killed, otherwise copy_process() can't miss the change in ->nr_hashed." If allocation is disabled, we just return -ENOMEM like it's made for such cases in alloc_pid(). v2: Do not move disable_pid_allocation(), do not introduce a new variable in copy_process() and simplify the patch as suggested by Oleg Nesterov. Account the problem with double irq enabling found by Eric W. Biederman. Fixes: c876ad768215 ("pidns: Stop pid allocation when init dies") Signed-off-by: Kirill Tkhai CC: Andrew Morton CC: Ingo Molnar CC: Peter Zijlstra CC: Oleg Nesterov CC: Mike Rapoport CC: Michal Hocko CC: Andy Lutomirski CC: "Eric W. Biederman" CC: Andrei Vagin CC: Cyrill Gorcunov CC: Serge Hallyn Cc: stable@vger.kernel.org Acked-by: Oleg Nesterov Signed-off-by: Eric W. Biederman --- kernel/fork.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/kernel/fork.c b/kernel/fork.c index 06d759ab4c62..aa1076c5e4a9 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1845,11 +1845,13 @@ static __latent_entropy struct task_struct *copy_process( */ recalc_sigpending(); if (signal_pending(current)) { - spin_unlock(¤t->sighand->siglock); - write_unlock_irq(&tasklist_lock); retval = -ERESTARTNOINTR; goto bad_fork_cancel_cgroup; } + if (unlikely(!(ns_of_pid(pid)->nr_hashed & PIDNS_HASH_ADDING))) { + retval = -ENOMEM; + goto bad_fork_cancel_cgroup; + } if (likely(p->pid)) { ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace); @@ -1907,6 +1909,8 @@ static __latent_entropy struct task_struct *copy_process( return p; bad_fork_cancel_cgroup: + spin_unlock(¤t->sighand->siglock); + write_unlock_irq(&tasklist_lock); cgroup_cancel_fork(p); bad_fork_free_pid: cgroup_threadgroup_change_end(current); -- 2.20.1