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author | Kirill Tkhai <ktkhai@virtuozzo.com> | 2017-05-12 19:11:31 +0300 |
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committer | Eric W. Biederman <ebiederm@xmission.com> | 2017-05-13 17:26:02 -0500 |
commit | 3fd37226216620c1a468afa999739d5016fbc349 (patch) | |
tree | 6ac4783bd114b147f9a5279086ea05670caa5b64 /kernel/fork.c | |
parent | b9a985db98961ae1ba0be169f19df1c567e4ffe0 (diff) | |
download | lwn-3fd37226216620c1a468afa999739d5016fbc349.tar.gz lwn-3fd37226216620c1a468afa999739d5016fbc349.zip |
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>
Diffstat (limited to 'kernel/fork.c')
-rw-r--r-- | kernel/fork.c | 8 |
1 files 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); |