On Tue, Nov 29, 2022 at 10:55:00AM +0100, Frederic Weisbecker wrote: > 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@xxxxxxxxxxxxxxxx/ > > > 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: Like this? ;-) a0c355bbdfee ("squash! rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes()") Thanx, Paul > --- > From: Frederic Weisbecker <frederic@xxxxxxxxxx> > 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@xxxxxxxxx> > Suggested-by: Boqun Feng <boqun.feng@xxxxxxxxx> > Suggested-by: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx> > Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > Cc: Oleg Nesterov <oleg@xxxxxxxxxx> > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx> > Cc: Eric W . Biederman <ebiederm@xxxxxxxxxxxx> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx> > --- > 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 >
