Messages in this thread Patch in this message | | | Subject | [PATCH v2] pid_ns: Fix race between setns'ed fork() and zap_pid_ns_processes() | From | Kirill Tkhai <> | Date | Fri, 12 May 2017 19:11:31 +0300 |
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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.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com> CC: Andrew Morton <akpm@linux-foundation.org> CC: Ingo Molnar <mingo@kernel.org> CC: Peter Zijlstra <peterz@infradead.org> CC: Oleg Nesterov <oleg@redhat.com> CC: Mike Rapoport <rppt@linux.vnet.ibm.com> CC: Michal Hocko <mhocko@suse.com> CC: Andy Lutomirski <luto@kernel.org> CC: "Eric W. Biederman" <ebiederm@xmission.com> CC: Andrei Vagin <avagin@openvz.org> CC: Cyrill Gorcunov <gorcunov@openvz.org> CC: Serge Hallyn <serge@hallyn.com> --- kernel/fork.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c index bfd91b180778..d2cade9b6ad8 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);
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