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Subject[PATCH 1/2] cgroup: prevent spurious transition into non-frozen state
Date
If freezing of a cgroup races with waking of a task from
the frozen state (like waiting in vfork() or in do_signal_stop()),
a spurious transition of the cgroup state can happen.

The task enters cgroup_leave_frozen(true), the cgroup->nr_frozen_tasks
counter decrements, and the cgroup is switched to the unfrozen state.

To prevent it, let's reserve cgroup_leave_frozen(true) for
terminating processes and use cgroup_leave_frozen(false) otherwise.

To avoid busy-looping in the signal handling loop waiting
for JOBCTL_TRAP_FREEZE set from the cgroup freezing path,
let's do it explicitly in cgroup_leave_frozen(), if the task
is going to stay frozen.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Roman Gushchin <guro@fb.com>
---
kernel/cgroup/freezer.c | 16 +++++-----------
kernel/signal.c | 2 +-
2 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/kernel/cgroup/freezer.c b/kernel/cgroup/freezer.c
index 3bfbb3c8baf3..c321e768f8d3 100644
--- a/kernel/cgroup/freezer.c
+++ b/kernel/cgroup/freezer.c
@@ -139,19 +139,13 @@ void cgroup_leave_frozen(bool always_leave)
cgroup_update_frozen(cgrp);
WARN_ON_ONCE(!current->frozen);
current->frozen = false;
+ } else if (!(current->jobctl & JOBCTL_TRAP_FREEZE)) {
+ spin_lock(&current->sighand->siglock);
+ current->jobctl |= JOBCTL_TRAP_FREEZE;
+ set_thread_flag(TIF_SIGPENDING);
+ spin_unlock(&current->sighand->siglock);
}
spin_unlock_irq(&css_set_lock);
-
- if (unlikely(current->frozen)) {
- /*
- * If the task remained in the frozen state,
- * make sure it won't reach userspace without
- * entering the signal handling loop.
- */
- spin_lock_irq(&current->sighand->siglock);
- recalc_sigpending();
- spin_unlock_irq(&current->sighand->siglock);
- }
}

/*
diff --git a/kernel/signal.c b/kernel/signal.c
index 095e0fc57b25..16b72f4f14df 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2514,7 +2514,7 @@ bool get_signal(struct ksignal *ksig)
*/
if (unlikely(cgroup_task_frozen(current))) {
spin_unlock_irq(&sighand->siglock);
- cgroup_leave_frozen(true);
+ cgroup_leave_frozen(false);
goto relock;
}

--
2.20.1
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