[PATCH v2] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj.

[PATCH v2] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj.

[Tetsuo Handa]

This patch reverts both commit 44a70adec910d692 ("mm, oom_adj: make sure
processes sharing mm have same view of oom_score_adj") and commit
97fd49c2355ffded ("mm, oom: kill all tasks sharing the mm") in order to
close a race and reduce the latency at __set_oom_adj(), and reduces the
warning at __oom_kill_process() in order to minimize the latency.

Commit 36324a990cf578b5 ("oom: clear TIF_MEMDIE after oom_reaper managed
to unmap the address space") introduced the worst case mentioned in
44a70adec910d692. But since the OOM killer skips mm with MMF_OOM_SKIP set,
only administrators can trigger the worst case.

Since 44a70adec910d692 did not take latency into account, we can "hold RCU
for minutes and trigger RCU stall warnings" by calling printk() on many
thousands of thread groups. Also, current code becomes a DoS attack vector
which will allow "stalling for more than one month in unkillable state"
simply printk()ing same messages when many thousands of thread groups
tried to iterate __set_oom_adj() on each other.

I also noticed that 44a70adec910d692 is racy [1], and trying to fix the
race will require a global lock which is too costly for rare events. And
Michal Hocko is thinking to change the oom_score_adj implementation to per
mm_struct (with shadowed score stored in per task_struct in order to
support vfork() => __set_oom_adj() => execve() sequence) so that we don't
need the global lock.

If the worst case in 44a70adec910d692 happened, it is an administrator's
request. Therefore, before changing the oom_score_adj implementation,
let's eliminate the DoS attack vector first.

[1] https://lkml.kernel.org/r/20181008011931epcms1p82dd01b7e5c067ea99946418bc97de46a@epcms1p8

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: Yong-Taek Lee <ytk.lee@samsung.com>
Nacked-by: Michal Hocko <mhocko@suse.com>
---
 fs/proc/base.c     | 46 ----------------------------------------------
 include/linux/mm.h |  2 --
 mm/oom_kill.c      | 10 ++++++----
 3 files changed, 6 insertions(+), 52 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 633a634..41ece8f 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1020,7 +1020,6 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count,
 static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
 {
 	static DEFINE_MUTEX(oom_adj_mutex);
-	struct mm_struct *mm = NULL;
 	struct task_struct *task;
 	int err = 0;
 
@@ -1050,55 +1049,10 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
 		}
 	}
 
-	/*
-	 * Make sure we will check other processes sharing the mm if this is
-	 * not vfrok which wants its own oom_score_adj.
-	 * pin the mm so it doesn't go away and get reused after task_unlock
-	 */
-	if (!task->vfork_done) {
-		struct task_struct *p = find_lock_task_mm(task);
-
-		if (p) {
-			if (atomic_read(&p->mm->mm_users) > 1) {
-				mm = p->mm;
-				mmgrab(mm);
-			}
-			task_unlock(p);
-		}
-	}
-
 	task->signal->oom_score_adj = oom_adj;
 	if (!legacy && has_capability_noaudit(current, CAP_SYS_RESOURCE))
 		task->signal->oom_score_adj_min = (short)oom_adj;
 	trace_oom_score_adj_update(task);
-
-	if (mm) {
-		struct task_struct *p;
-
-		rcu_read_lock();
-		for_each_process(p) {
-			if (same_thread_group(task, p))
-				continue;
-
-			/* do not touch kernel threads or the global init */
-			if (p->flags & PF_KTHREAD || is_global_init(p))
-				continue;
-
-			task_lock(p);
-			if (!p->vfork_done && process_shares_mm(p, mm)) {
-				pr_info("updating oom_score_adj for %d (%s) from %d to %d because it shares mm with %d (%s). Report if this is unexpected.\n",
-						task_pid_nr(p), p->comm,
-						p->signal->oom_score_adj, oom_adj,
-						task_pid_nr(task), task->comm);
-				p->signal->oom_score_adj = oom_adj;
-				if (!legacy && has_capability_noaudit(current, CAP_SYS_RESOURCE))
-					p->signal->oom_score_adj_min = (short)oom_adj;
-			}
-			task_unlock(p);
-		}
-		rcu_read_unlock();
-		mmdrop(mm);
-	}
 err_unlock:
 	mutex_unlock(&oom_adj_mutex);
 	put_task_struct(task);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 80bb640..28879c1 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2690,8 +2690,6 @@ static inline int in_gate_area(struct mm_struct *mm, unsigned long addr)
 }
 #endif	/* __HAVE_ARCH_GATE_AREA */
 
-extern bool process_shares_mm(struct task_struct *p, struct mm_struct *mm);
-
 #ifdef CONFIG_SYSCTL
 extern int sysctl_drop_caches;
 int drop_caches_sysctl_handler(struct ctl_table *, int,
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index f0e8cd9..c7005b1 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -478,7 +478,7 @@ static void dump_header(struct oom_control *oc, struct task_struct *p)
  * task's threads: if one of those is using this mm then this task was also
  * using it.
  */
-bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
+static bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
 {
 	struct task_struct *t;
 
@@ -896,12 +896,14 @@ static void __oom_kill_process(struct task_struct *victim)
 			continue;
 		if (same_thread_group(p, victim))
 			continue;
-		if (is_global_init(p)) {
+		if (is_global_init(p) ||
+		    p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) {
 			can_oom_reap = false;
-			set_bit(MMF_OOM_SKIP, &mm->flags);
-			pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
+			if (!test_bit(MMF_OOM_SKIP, &mm->flags))
+				pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
 					task_pid_nr(victim), victim->comm,
 					task_pid_nr(p), p->comm);
+			set_bit(MMF_OOM_SKIP, &mm->flags);
 			continue;
 		}
 		/*
-- 
1.8.3.1

Re: [PATCH v2] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj.

[Michal Hocko]

On Thu 31-01-19 07:49:35, Tetsuo Handa wrote:
> This patch reverts both commit 44a70adec910d692 ("mm, oom_adj: make sure
> processes sharing mm have same view of oom_score_adj") and commit
> 97fd49c2355ffded ("mm, oom: kill all tasks sharing the mm") in order to
> close a race and reduce the latency at __set_oom_adj(), and reduces the
> warning at __oom_kill_process() in order to minimize the latency.
> 
> Commit 36324a990cf578b5 ("oom: clear TIF_MEMDIE after oom_reaper managed
> to unmap the address space") introduced the worst case mentioned in
> 44a70adec910d692. But since the OOM killer skips mm with MMF_OOM_SKIP set,
> only administrators can trigger the worst case.
> 
> Since 44a70adec910d692 did not take latency into account, we can "hold RCU
> for minutes and trigger RCU stall warnings" by calling printk() on many
> thousands of thread groups. Also, current code becomes a DoS attack vector
> which will allow "stalling for more than one month in unkillable state"
> simply printk()ing same messages when many thousands of thread groups
> tried to iterate __set_oom_adj() on each other.
> 
> I also noticed that 44a70adec910d692 is racy [1], and trying to fix the
> race will require a global lock which is too costly for rare events. And
> Michal Hocko is thinking to change the oom_score_adj implementation to per
> mm_struct (with shadowed score stored in per task_struct in order to
> support vfork() => __set_oom_adj() => execve() sequence) so that we don't
> need the global lock.
> 
> If the worst case in 44a70adec910d692 happened, it is an administrator's
> request. Therefore, before changing the oom_score_adj implementation,
> let's eliminate the DoS attack vector first.

This is really ridiculous. I have already nacked the previous version
and provided two ways around. The simplest one is to drop the printk.
The second one is to move oom_score_adj to the mm struct. Could you
explain why do you still push for this?

> [1] https://lkml.kernel.org/r/20181008011931epcms1p82dd01b7e5c067ea99946418bc97de46a@epcms1p8
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Reported-by: Yong-Taek Lee <ytk.lee@samsung.com>
> Nacked-by: Michal Hocko <mhocko@suse.com>
> ---
>  fs/proc/base.c     | 46 ----------------------------------------------
>  include/linux/mm.h |  2 --
>  mm/oom_kill.c      | 10 ++++++----
>  3 files changed, 6 insertions(+), 52 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 633a634..41ece8f 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1020,7 +1020,6 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count,
>  static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
>  {
>  	static DEFINE_MUTEX(oom_adj_mutex);
> -	struct mm_struct *mm = NULL;
>  	struct task_struct *task;
>  	int err = 0;
>  
> @@ -1050,55 +1049,10 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
>  		}
>  	}
>  
> -	/*
> -	 * Make sure we will check other processes sharing the mm if this is
> -	 * not vfrok which wants its own oom_score_adj.
> -	 * pin the mm so it doesn't go away and get reused after task_unlock
> -	 */
> -	if (!task->vfork_done) {
> -		struct task_struct *p = find_lock_task_mm(task);
> -
> -		if (p) {
> -			if (atomic_read(&p->mm->mm_users) > 1) {
> -				mm = p->mm;
> -				mmgrab(mm);
> -			}
> -			task_unlock(p);
> -		}
> -	}
> -
>  	task->signal->oom_score_adj = oom_adj;
>  	if (!legacy && has_capability_noaudit(current, CAP_SYS_RESOURCE))
>  		task->signal->oom_score_adj_min = (short)oom_adj;
>  	trace_oom_score_adj_update(task);
> -
> -	if (mm) {
> -		struct task_struct *p;
> -
> -		rcu_read_lock();
> -		for_each_process(p) {
> -			if (same_thread_group(task, p))
> -				continue;
> -
> -			/* do not touch kernel threads or the global init */
> -			if (p->flags & PF_KTHREAD || is_global_init(p))
> -				continue;
> -
> -			task_lock(p);
> -			if (!p->vfork_done && process_shares_mm(p, mm)) {
> -				pr_info("updating oom_score_adj for %d (%s) from %d to %d because it shares mm with %d (%s). Report if this is unexpected.\n",
> -						task_pid_nr(p), p->comm,
> -						p->signal->oom_score_adj, oom_adj,
> -						task_pid_nr(task), task->comm);
> -				p->signal->oom_score_adj = oom_adj;
> -				if (!legacy && has_capability_noaudit(current, CAP_SYS_RESOURCE))
> -					p->signal->oom_score_adj_min = (short)oom_adj;
> -			}
> -			task_unlock(p);
> -		}
> -		rcu_read_unlock();
> -		mmdrop(mm);
> -	}
>  err_unlock:
>  	mutex_unlock(&oom_adj_mutex);
>  	put_task_struct(task);
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 80bb640..28879c1 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2690,8 +2690,6 @@ static inline int in_gate_area(struct mm_struct *mm, unsigned long addr)
>  }
>  #endif	/* __HAVE_ARCH_GATE_AREA */
>  
> -extern bool process_shares_mm(struct task_struct *p, struct mm_struct *mm);
> -
>  #ifdef CONFIG_SYSCTL
>  extern int sysctl_drop_caches;
>  int drop_caches_sysctl_handler(struct ctl_table *, int,
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index f0e8cd9..c7005b1 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -478,7 +478,7 @@ static void dump_header(struct oom_control *oc, struct task_struct *p)
>   * task's threads: if one of those is using this mm then this task was also
>   * using it.
>   */
> -bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
> +static bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
>  {
>  	struct task_struct *t;
>  
> @@ -896,12 +896,14 @@ static void __oom_kill_process(struct task_struct *victim)
>  			continue;
>  		if (same_thread_group(p, victim))
>  			continue;
> -		if (is_global_init(p)) {
> +		if (is_global_init(p) ||
> +		    p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) {
>  			can_oom_reap = false;
> -			set_bit(MMF_OOM_SKIP, &mm->flags);
> -			pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
> +			if (!test_bit(MMF_OOM_SKIP, &mm->flags))
> +				pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
>  					task_pid_nr(victim), victim->comm,
>  					task_pid_nr(p), p->comm);
> +			set_bit(MMF_OOM_SKIP, &mm->flags);
>  			continue;
>  		}
>  		/*
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj.

[Tetsuo Handa]

On 2019/01/31 16:11, Michal Hocko wrote:
> On Thu 31-01-19 07:49:35, Tetsuo Handa wrote:
>> This patch reverts both commit 44a70adec910d692 ("mm, oom_adj: make sure
>> processes sharing mm have same view of oom_score_adj") and commit
>> 97fd49c2355ffded ("mm, oom: kill all tasks sharing the mm") in order to
>> close a race and reduce the latency at __set_oom_adj(), and reduces the
>> warning at __oom_kill_process() in order to minimize the latency.
>>
>> Commit 36324a990cf578b5 ("oom: clear TIF_MEMDIE after oom_reaper managed
>> to unmap the address space") introduced the worst case mentioned in
>> 44a70adec910d692. But since the OOM killer skips mm with MMF_OOM_SKIP set,
>> only administrators can trigger the worst case.
>>
>> Since 44a70adec910d692 did not take latency into account, we can "hold RCU
>> for minutes and trigger RCU stall warnings" by calling printk() on many
>> thousands of thread groups. Also, current code becomes a DoS attack vector
>> which will allow "stalling for more than one month in unkillable state"
>> simply printk()ing same messages when many thousands of thread groups
>> tried to iterate __set_oom_adj() on each other.
>>
>> I also noticed that 44a70adec910d692 is racy [1], and trying to fix the
>> race will require a global lock which is too costly for rare events. And
>> Michal Hocko is thinking to change the oom_score_adj implementation to per
>> mm_struct (with shadowed score stored in per task_struct in order to
>> support vfork() => __set_oom_adj() => execve() sequence) so that we don't
>> need the global lock.
>>
>> If the worst case in 44a70adec910d692 happened, it is an administrator's
>> request. Therefore, before changing the oom_score_adj implementation,
>> let's eliminate the DoS attack vector first.
> 
> This is really ridiculous. I have already nacked the previous version
> and provided two ways around. The simplest one is to drop the printk.
> The second one is to move oom_score_adj to the mm struct. Could you
> explain why do you still push for this?

Dropping printk() does not close the race.
You must propose an alternative patch if you dislike this patch.

> 
>> [1] https://lkml.kernel.org/r/20181008011931epcms1p82dd01b7e5c067ea99946418bc97de46a@epcms1p8
>>
>> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
>> Reported-by: Yong-Taek Lee <ytk.lee@samsung.com>
>> Nacked-by: Michal Hocko <mhocko@suse.com>
>> ---
>>  fs/proc/base.c     | 46 ----------------------------------------------
>>  include/linux/mm.h |  2 --
>>  mm/oom_kill.c      | 10 ++++++----
>>  3 files changed, 6 insertions(+), 52 deletions(-)
>>

Re: [PATCH v2] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj.

[Michal Hocko]

On Fri 01-02-19 05:59:55, Tetsuo Handa wrote:
> On 2019/01/31 16:11, Michal Hocko wrote:
> > On Thu 31-01-19 07:49:35, Tetsuo Handa wrote:
> >> This patch reverts both commit 44a70adec910d692 ("mm, oom_adj: make sure
> >> processes sharing mm have same view of oom_score_adj") and commit
> >> 97fd49c2355ffded ("mm, oom: kill all tasks sharing the mm") in order to
> >> close a race and reduce the latency at __set_oom_adj(), and reduces the
> >> warning at __oom_kill_process() in order to minimize the latency.
> >>
> >> Commit 36324a990cf578b5 ("oom: clear TIF_MEMDIE after oom_reaper managed
> >> to unmap the address space") introduced the worst case mentioned in
> >> 44a70adec910d692. But since the OOM killer skips mm with MMF_OOM_SKIP set,
> >> only administrators can trigger the worst case.
> >>
> >> Since 44a70adec910d692 did not take latency into account, we can "hold RCU
> >> for minutes and trigger RCU stall warnings" by calling printk() on many
> >> thousands of thread groups. Also, current code becomes a DoS attack vector
> >> which will allow "stalling for more than one month in unkillable state"
> >> simply printk()ing same messages when many thousands of thread groups
> >> tried to iterate __set_oom_adj() on each other.
> >>
> >> I also noticed that 44a70adec910d692 is racy [1], and trying to fix the
> >> race will require a global lock which is too costly for rare events. And
> >> Michal Hocko is thinking to change the oom_score_adj implementation to per
> >> mm_struct (with shadowed score stored in per task_struct in order to
> >> support vfork() => __set_oom_adj() => execve() sequence) so that we don't
> >> need the global lock.
> >>
> >> If the worst case in 44a70adec910d692 happened, it is an administrator's
> >> request. Therefore, before changing the oom_score_adj implementation,
> >> let's eliminate the DoS attack vector first.
> > 
> > This is really ridiculous. I have already nacked the previous version
> > and provided two ways around. The simplest one is to drop the printk.
> > The second one is to move oom_score_adj to the mm struct. Could you
> > explain why do you still push for this?
> 
> Dropping printk() does not close the race.

But it does remove the source of a long operation from the RCU context.
If you are not willing to post such a trivial patch I will do so.

> You must propose an alternative patch if you dislike this patch.

I will eventually get there.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH v2] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj.

[Tetsuo Handa]

On 2019/02/01 18:14, Michal Hocko wrote:
> On Fri 01-02-19 05:59:55, Tetsuo Handa wrote:
>> On 2019/01/31 16:11, Michal Hocko wrote:
>>> This is really ridiculous. I have already nacked the previous version
>>> and provided two ways around. The simplest one is to drop the printk.
>>> The second one is to move oom_score_adj to the mm struct. Could you
>>> explain why do you still push for this?
>>
>> Dropping printk() does not close the race.
> 
> But it does remove the source of a long operation from the RCU context.
> If you are not willing to post such a trivial patch I will do so.
> 
>> You must propose an alternative patch if you dislike this patch.
> 
> I will eventually get there.
> 

This is really ridiculous. "eventually" cannot be justified as a reason for
rejecting this patch. I want a patch which can be easily backported _now_ .

If vfork() => __set_oom_adj() => execve() sequence is permitted, someone can
try vfork() => clone() => __set_oom_adj() => execve() sequence. And below
program demonstrates that task->vfork_done based exemption in __set_oom_adj()
is broken. It is not always the task_struct who called vfork() that will call
execve().

----------------------------------------
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sched.h>

static int thread1(void *unused)
{
	char *args[3] = { "/bin/true", "true", NULL };
	int fd = open("/proc/self/oom_score_adj", O_WRONLY);
	write(fd, "1000", 4);
	close(fd);
	execve(args[0], args, NULL);
	return 0;
}
int main(int argc, char *argv[])
{
	printf("PID=%d\n", getpid());
	if (vfork() == 0) {
		clone(thread1, malloc(8192) + 8192,
		      CLONE_VM | CLONE_FS | CLONE_FILES, NULL);
		sleep(1);
		_exit(0);
	}
	return 0;
}
----------------------------------------

  PID=8802
  [ 1138.425255] updating oom_score_adj for 8802 (a.out) from 0 to 1000 because it shares mm with 8804 (a.out). Report if this is unexpected.

Current loop to enforce same oom_score_adj is 99%+ ending in vain.
And even your "eventually" will remove this loop.

Re: [PATCH v2] mm, oom: Tolerate processes sharing mm with different view of oom_score_adj.

[Michal Hocko]

On Sat 02-02-19 20:06:07, Tetsuo Handa wrote:
> int main(int argc, char *argv[])
> {
> 	printf("PID=%d\n", getpid());
> 	if (vfork() == 0) {
> 		clone(thread1, malloc(8192) + 8192,
> 		      CLONE_VM | CLONE_FS | CLONE_FILES, NULL);
> 		sleep(1);
> 		_exit(0);
> 	}
> 	return 0;
> }

This program is not correct AFAIU:
   Standard description
       (From POSIX.1) The vfork() function has the same effect as
       fork(2), except that the behavior is undefined if the process
       created by vfork() either modifies any data other than a variable
       of type pid_t used to store the return value from vfork(), or
       returns from the function in which vfork() was called, or calls
       any other function before successfully calling _exit(2) or one of
       the exec(3) family of functions.
> 
>   PID=8802
>   [ 1138.425255] updating oom_score_adj for 8802 (a.out) from 0 to 1000 because it shares mm with 8804 (a.out). Report if this is unexpected.
> 
> Current loop to enforce same oom_score_adj is 99%+ ending in vain.
> And even your "eventually" will remove this loop.

But it keeps the semantic of the mm shared processes share the same
oomd_score_adj so that we do not have to add kludges to the OOM code to
handle with potential corner cases.

Really, this nagging is both unproductive and annoying. You are right
that the printk is overzealous and it can be dropped. The printk is
more than two years old and we haven't heard anybody to care. So the
first and the most obvious thing to do is to remove it. The patch is
trivial and if I was not buried in the backlog I would have posted it
already.  Regarding a potentially expensive for_each_process. This is
unfortunate but the thing we have to pay for in other paths as well
(e.g. exit path) so closing this only here just doesn't really help
much if you are concerned about security and potential stalls will
explode the machine scenarios.. So even though this sucks it is not
earth shattering. CLONE_VM withtout CLONE_SIGHAND simply sucks and
nobody should be using this threading model.

So, please calm down, try to be more productive and try to understand
what people try to tell you rather than shout around "i want my pony".
-- 
Michal Hocko
SUSE Labs