From 7d24e2df52f596a1ea922e4f84a61f2fb24fbb70 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Wed, 10 Dec 2014 15:55:02 -0800 Subject: exit: reparent: fix the cross-namespace PR_SET_CHILD_SUBREAPER reparenting find_new_reaper() assumes that "has_child_subreaper" logic is safe as long as we are not the exiting ->child_reaper and this is doubly wrong: 1. In fact it is safe if "pid_ns->child_reaper == father"; there must be no children after zap_pid_ns_processes() returns, so it doesn't matter what we return in this case and even pid_ns->child_reaper is wrong otherwise: we can't reparent to ->child_reaper == current. This is not a bug, but this is confusing. 2. It is not safe if we are not pid_ns->child_reaper but from the same thread group. We drop tasklist_lock before zap_pid_ns_processes(), so another thread can lock it and choose the new reaper from the upper namespace if has_child_subreaper == T, and this is obviously wrong. This is not that bad, zap_pid_ns_processes() won't return until the the new reaper reaps all zombies, but this should be fixed anyway. We could change for_each_thread() loop to use ->exit_state instead of PF_EXITING which we had to use until 8aac62706ada, or we could change copy_signal() to check CLONE_NEWPID before setting has_child_subreaper, but lets change this code so that it is clear we can't look outside of our namespace, otherwise same_thread_group(reaper, child_reaper) check will look wrong and confusing anyway. We can simply start from "father" and fix the problem. We can't wrongly return a thread from the same thread group if ->is_child_subreaper == T, we know that all threads have PF_EXITING set. Signed-off-by: Oleg Nesterov Cc: Aaron Tomlin Cc: "Eric W. Biederman" Cc: Kay Sievers Cc: Lennart Poettering Cc: Sterling Alexander Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/exit.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) (limited to 'kernel') diff --git a/kernel/exit.c b/kernel/exit.c index fd38a8f04367..9babd47a36e2 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -492,7 +492,9 @@ static struct task_struct *find_new_reaper(struct task_struct *father) zap_pid_ns_processes(pid_ns); write_lock_irq(&tasklist_lock); - } else if (father->signal->has_child_subreaper) { + } + + if (father->signal->has_child_subreaper) { struct task_struct *reaper; /* @@ -502,7 +504,7 @@ static struct task_struct *find_new_reaper(struct task_struct *father) * PID namespace. However we still need the check above, see * http://marc.info/?l=linux-kernel&m=131385460420380 */ - for (reaper = father->real_parent; + for (reaper = father; reaper != &init_task; reaper = reaper->real_parent) { if (same_thread_group(reaper, pid_ns->child_reaper)) -- cgit v1.2.3