[SRU][Utopic][PATCH 1/2] sched/numa: Fix unsafe get_task_struct() in task_numa_assign()
Gavin Guo
gavin.guo at canonical.com
Sat Mar 12 14:09:10 UTC 2016
From: Kirill Tkhai <ktkhai at parallels.com>
BugLink: https://bugs.launchpad.net/bugs/1527643
Unlocked access to dst_rq->curr in task_numa_compare() is racy.
If curr task is exiting this may be a reason of use-after-free:
task_numa_compare() do_exit()
... current->flags |= PF_EXITING;
... release_task()
... ~~delayed_put_task_struct()~~
... schedule()
rcu_read_lock() ...
cur = ACCESS_ONCE(dst_rq->curr) ...
... rq->curr = next;
... context_switch()
... finish_task_switch()
... put_task_struct()
... __put_task_struct()
... free_task_struct()
task_numa_assign() ...
get_task_struct() ...
As noted by Oleg:
<<The lockless get_task_struct(tsk) is only safe if tsk == current
and didn't pass exit_notify(), or if this tsk was found on a rcu
protected list (say, for_each_process() or find_task_by_vpid()).
IOW, it is only safe if release_task() was not called before we
take rcu_read_lock(), in this case we can rely on the fact that
delayed_put_pid() can not drop the (potentially) last reference
until rcu_read_unlock().
And as Kirill pointed out task_numa_compare()->task_numa_assign()
path does get_task_struct(dst_rq->curr) and this is not safe. The
task_struct itself can't go away, but rcu_read_lock() can't save
us from the final put_task_struct() in finish_task_switch(); this
reference goes away without rcu gp>>
The patch provides simple check of PF_EXITING flag. If it's not set,
this guarantees that call_rcu() of delayed_put_task_struct() callback
hasn't happened yet, so we can safely do get_task_struct() in
task_numa_assign().
Locked dst_rq->lock protects from concurrency with the last schedule().
Reusing or unmapping of cur's memory may happen without it.
Suggested-by: Oleg Nesterov <oleg at redhat.com>
Signed-off-by: Kirill Tkhai <ktkhai at parallels.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Link: http://lkml.kernel.org/r/1413962231.19914.130.camel@tkhai
Signed-off-by: Ingo Molnar <mingo at kernel.org>
(cherry picked from commit 1effd9f19324efb05fccc7421530e11a52db0278)
Signed-off-by: Gavin Guo <gavin.guo at canonical.com>
---
kernel/sched/fair.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 123078569a96..ca3829734199 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1142,9 +1142,19 @@ static void task_numa_compare(struct task_numa_env *env,
long imp = (groupimp > 0) ? groupimp : taskimp;
rcu_read_lock();
- cur = ACCESS_ONCE(dst_rq->curr);
- if (cur->pid == 0) /* idle */
+
+ raw_spin_lock_irq(&dst_rq->lock);
+ cur = dst_rq->curr;
+ /*
+ * No need to move the exiting task, and this ensures that ->curr
+ * wasn't reaped and thus get_task_struct() in task_numa_assign()
+ * is safe under RCU read lock.
+ * Note that rcu_read_lock() itself can't protect from the final
+ * put_task_struct() after the last schedule().
+ */
+ if ((cur->flags & PF_EXITING) || is_idle_task(cur))
cur = NULL;
+ raw_spin_unlock_irq(&dst_rq->lock);
/*
* "imp" is the fault differential for the source task between the
--
2.0.0
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