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authorKirill Tkhai <ktkhai@virtuozzo.com>2017-05-12 19:11:31 +0300
committerEric W. Biederman <ebiederm@xmission.com>2017-05-13 17:26:02 -0500
commit3fd37226216620c1a468afa999739d5016fbc349 (patch)
tree6ac4783bd114b147f9a5279086ea05670caa5b64 /kernel/ptrace.c
parentb9a985db98961ae1ba0be169f19df1c567e4ffe0 (diff)
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 <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> Cc: stable@vger.kernel.org Acked-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
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