Re: [PATCH 4/6] mm, oom: skip vforked tasks from being selected

[Michal Hocko <mhocko@kernel.org>]

On Mon 30-05-16 21:28:57, Oleg Nesterov wrote:
> On 05/30, Michal Hocko wrote:
> >
> > Make sure to not select vforked task as an oom victim by checking
> > vfork_done in oom_badness.
> 
> I agree, this look like a good change to me... But.
> 
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -176,11 +176,13 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
> >  
> >  	/*
> >  	 * Do not even consider tasks which are explicitly marked oom
> > -	 * unkillable or have been already oom reaped.
> > +	 * unkillable or have been already oom reaped or the are in
> > +	 * the middle of vfork
> >  	 */
> >  	adj = (long)p->signal->oom_score_adj;
> >  	if (adj == OOM_SCORE_ADJ_MIN ||
> > -			test_bit(MMF_OOM_REAPED, &p->mm->flags)) {
> > +			test_bit(MMF_OOM_REAPED, &p->mm->flags) ||
> > +			p->vfork_done) {
> 
> I don't think we can trust vfork_done != NULL.
> 
> copy_process() doesn't disallow CLONE_VFORK without CLONE_VM, so with this patch
> it would be trivial to make the exploit which hides a memory hog from oom-killer.

OK, I wasn't aware of this possibility. It sounds really weird because I
thought that the whole point of vfork is to prevent from MM copy
overhead for quick exec.

> So perhaps we need something like
> 
> 		bool in_vfork(p)
> 		{
> 			return	p->vfork_done &&
> 				p->real_parent->mm == mm;
> 
> 			
> 		}
> 
> task_lock() is not enough if CLONE_VM was used along with CLONE_PARENT... so this
> also needs rcu_read_lock() to access ->real_parent.
> 
> Or I am totally confused?

I cannot judge I am afraid. You are definitely much more familiar with
all these subtle details than me.

So what do you think about this follow up:
---
>From 9300b4f59b0b108f0013ff44a5da93e8c6b12b46 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Tue, 31 May 2016 07:45:45 +0200
Subject: [PATCH 3/3] fold me "mm, oom: skip vforked tasks from being selected"

per Oleg:
- copy_process() doesn't disallow CLONE_VFORK without CLONE_VM, so with
  this patch it would be trivial to make the exploit which hides a
  memory hog from oom-killer.

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 include/linux/sched.h | 16 ++++++++++++++++
 mm/oom_kill.c         |  2 +-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ec636400669f..e1877f4da4cc 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1883,6 +1883,22 @@ extern int arch_task_struct_size __read_mostly;
 #define TNF_FAULT_LOCAL	0x08
 #define TNF_MIGRATE_FAIL 0x10
 
+/* expects to be called with task_lock held */
+static inline bool in_vfork(struct task_struct *tsk)
+{
+	bool ret;
+
+	/*
+	 * need RCU to access ->real_parent if CLONE_VM was used along with
+	 * CLONE_PARENT
+	 */
+	rcu_read_lock();
+	ret = tsk->vfork_done && tsk->real_parent->mm == tsk->mm;
+	rcu_read_unlock();
+
+	return ret;
+}
+
 #ifdef CONFIG_NUMA_BALANCING
 extern void task_numa_fault(int last_node, int node, int pages, int flags);
 extern pid_t task_numa_group_id(struct task_struct *p);
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index aa28315ac310..d1e24deb79b9 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -182,7 +182,7 @@ unsigned long oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
 	adj = (long)p->signal->oom_score_adj;
 	if (adj == OOM_SCORE_ADJ_MIN ||
 			test_bit(MMF_OOM_REAPED, &p->mm->flags) ||
-			p->vfork_done) {
+			in_vfork(p)) {
 		task_unlock(p);
 		return 0;
 	}
-- 
2.8.1


-- 
Michal Hocko
SUSE Labs