Date:   Tue, 29 Nov 2022 10:55:00 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Lai Jiangshan <jiangshanlai@...il.com>, rcu@...r.kernel.org
Subject: Re: [PATCH 0/3] rcu-tasks: Fix race against exiting pid_ns

On Mon, Nov 28, 2022 at 04:22:40PM -0800, Paul E. McKenney wrote:
> On Fri, Nov 25, 2022 at 02:54:57PM +0100, Frederic Weisbecker wrote:
> > Pengfei Xu has reported a deadlock involving calls to unshare(),
> > perf_event_open() and clone3() calls. It requires CAP_SYS_ADMIN
> > to reproduce (at least I don't see a way for a non privilege process to
> > reproduce).
> > 
> > See this thread for details: https://lore.kernel.org/all/Y3sOgrOmMQqPMItu@xpf.sh.intel.com/
> > And this document for the collaborative analysis with Boqun, Paul and Neeraj:
> > https://docs.google.com/document/d/1hJxgiZ5TMZ4YJkdJPLAkRvq7sYQ-A7svgA8no6i-v8k
> > 
> > The two first patches are small improvements. The fix is in the last patch.
> > 
> > git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> > 	rcu/dev
> > 
> > HEAD: 45ef5a0a4be4e0db9eadcc86e8f346d34c62e744
> 
> Hearing no objections, queued for further review and testing.
> 
> And thank you very much!  That race between synchronize_rcu_tasks() and
> zap_pid_ns_processes() certainly was more than a bit on the non-trivial
> side.  Good show!!!

Thanks!

Also please replace the last patch with the following to fix
a !CONFIG_RCU_TASKS issue:

---
From: Frederic Weisbecker <frederic@...nel.org>
Date: Thu, 24 Nov 2022 18:15:46 +0100
Subject: [PATCH] rcu-tasks: Fix synchronize_rcu_tasks() VS
 zap_pid_ns_processes()

RCU Tasks and PID-namespace unshare can interact in do_exit() in a
complicated circular dependency:

1) TASK A calls unshare(CLONE_NEWPID), this creates a new PID namespace
   that every subsequent child of TASK A will belong to. But TASK A
   doesn't itself belong to that new PID namespace.

2) TASK A forks() and creates TASK B. TASK A stays attached to its PID
   namespace (let's say PID_NS1) and TASK B is the first task belonging
   to the new PID namespace created by unshare()  (let's call it PID_NS2).

3) Since TASK B is the first task attached to PID_NS2, it becomes the
   PID_NS2 child reaper.

4) TASK A forks() again and creates TASK C which get attached to PID_NS2.
   Note how TASK C has TASK A as a parent (belonging to PID_NS1) but has
   TASK B (belonging to PID_NS2) as a pid_namespace child_reaper.

5) TASK B exits and since it is the child reaper for PID_NS2, it has to
   kill all other tasks attached to PID_NS2, and wait for all of them to
   die before getting reaped itself (zap_pid_ns_process()).

6) TASK A calls synchronize_rcu_tasks() which leads to
   synchronize_srcu(&tasks_rcu_exit_srcu).

7) TASK B is waiting for TASK C to get reaped. But TASK B is under a
   tasks_rcu_exit_srcu SRCU critical section (exit_notify() is between
   exit_tasks_rcu_start() and exit_tasks_rcu_finish()), blocking TASK A.

8) TASK C exits and since TASK A is its parent, it waits for it to reap
   TASK C, but it can't because TASK A waits for TASK B that waits for
   TASK C.

Pid_namespace semantics can hardly be changed at this point. But the
coverage of tasks_rcu_exit_srcu can be reduced instead.

The current task is assumed not to be concurrently reapable at this
stage of exit_notify() and therefore tasks_rcu_exit_srcu can be
temporarily relaxed without breaking its constraints, providing a way
out of the deadlock scenario.

Fixes: 3f95aa81d265 ("rcu: Make TASKS_RCU handle tasks that are almost done exiting")
Reported-by: Pengfei Xu <pengfei.xu@...el.com>
Suggested-by: Boqun Feng <boqun.feng@...il.com>
Suggested-by: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Suggested-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Eric W . Biederman <ebiederm@...ssion.com>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 include/linux/rcupdate.h |  2 ++
 kernel/pid_namespace.c   | 17 +++++++++++++++++
 kernel/rcu/tasks.h       | 15 +++++++++++++--
 3 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 89b3036746d2..a19d91d5461c 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -238,6 +238,7 @@ void synchronize_rcu_tasks_rude(void);
 
 #define rcu_note_voluntary_context_switch(t) rcu_tasks_qs(t, false)
 void exit_tasks_rcu_start(void);
+void exit_tasks_rcu_stop(void);
 void exit_tasks_rcu_finish(void);
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
 #define rcu_tasks_classic_qs(t, preempt) do { } while (0)
@@ -246,6 +247,7 @@ void exit_tasks_rcu_finish(void);
 #define call_rcu_tasks call_rcu
 #define synchronize_rcu_tasks synchronize_rcu
 static inline void exit_tasks_rcu_start(void) { }
+static inline void exit_tasks_rcu_stop(void) { }
 static inline void exit_tasks_rcu_finish(void) { }
 #endif /* #else #ifdef CONFIG_TASKS_RCU_GENERIC */
 
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index f4f8cb0435b4..fc21c5d5fd5d 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -244,7 +244,24 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 		set_current_state(TASK_INTERRUPTIBLE);
 		if (pid_ns->pid_allocated == init_pids)
 			break;
+		/*
+		 * Release tasks_rcu_exit_srcu to avoid following deadlock:
+		 *
+		 * 1) TASK A unshare(CLONE_NEWPID)
+		 * 2) TASK A fork() twice -> TASK B (child reaper for new ns)
+		 *    and TASK C
+		 * 3) TASK B exits, kills TASK C, waits for TASK A to reap it
+		 * 4) TASK A calls synchronize_rcu_tasks()
+		 *                   -> synchronize_srcu(tasks_rcu_exit_srcu)
+		 * 5) *DEADLOCK*
+		 *
+		 * It is considered safe to release tasks_rcu_exit_srcu here
+		 * because we assume the current task can not be concurrently
+		 * reaped at this point.
+		 */
+		exit_tasks_rcu_stop();
 		schedule();
+		exit_tasks_rcu_start();
 	}
 	__set_current_state(TASK_RUNNING);
 
diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 9a8114114b48..4dda8e6e5707 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1016,16 +1016,27 @@ void exit_tasks_rcu_start(void) __acquires(&tasks_rcu_exit_srcu)
  * task is exiting and may be removed from the tasklist. See
  * corresponding synchronize_srcu() for further details.
  */
-void exit_tasks_rcu_finish(void) __releases(&tasks_rcu_exit_srcu)
+void exit_tasks_rcu_stop(void) __releases(&tasks_rcu_exit_srcu)
 {
 	struct task_struct *t = current;
 
 	__srcu_read_unlock(&tasks_rcu_exit_srcu, t->rcu_tasks_idx);
-	exit_tasks_rcu_finish_trace(t);
+}
+
+/*
+ * Contribute to protect against tasklist scan blind spot while the
+ * task is exiting and may be removed from the tasklist. See
+ * corresponding synchronize_srcu() for further details.
+ */
+void exit_tasks_rcu_finish(void)
+{
+	exit_tasks_rcu_stop();
+	exit_tasks_rcu_finish_trace(current);
 }
 
 #else /* #ifdef CONFIG_TASKS_RCU */
 void exit_tasks_rcu_start(void) { }
+void exit_tasks_rcu_stop(void) { }
 void exit_tasks_rcu_finish(void) { exit_tasks_rcu_finish_trace(current); }
 #endif /* #else #ifdef CONFIG_TASKS_RCU */
 
-- 
2.25.1

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