Re: [PATCH RFC] audit: move the tree pruning to a dedicated thread

Imre Palik
2015-01-15T09:33:32Z

On 01/13/15 02:47, Paul Moore wrote:
> On Monday, January 12, 2015 09:11:21 AM Imre Palik wrote:
>> On 01/08/15 22:53, Paul Moore wrote:
>>> On Tuesday, January 06, 2015 03:51:20 PM Imre Palik wrote:
>>>> From: "Palik, Imre" <imrep@amazon.de>
>>>>
>>>> When file auditing is enabled, during a low memory situation, a memory
>>>> allocation with __GFP_FS can lead to pruning the inode cache. Which can,
>>>> in turn lead to audit_tree_freeing_mark() being called. This can call
>>>> audit_schedule_prune(), that tries to fork a pruning thread, and
>>>> waits until the thread is created. But forking needs memory, and the
>>>> memory allocations there are done with __GFP_FS.
>>>>
>>>> So we are waiting merrily for some __GFP_FS memory allocations to
>>>> complete,
>>>> while holding some filesystem locks. This can take a while ...
>>>>
>>>> This patch creates a single thread for pruning the tree from
>>>> audit_add_tree_rule(), and thus avoids the deadlock that the on-demand
>>>> thread creation can cause.
>>>>
>>>> Reported-by: Matt Wilson <msw@amazon.com>
>>>> Cc: Matt Wilson <msw@amazon.com>
>>>> Signed-off-by: Imre Palik <imrep@amazon.de>
>>>
>>> Thanks for sticking with this and posting a revised patch, my comments are
>>> inline with the patch below ... also as a FYI, when sending a revised
>>> patch it is often helpful to put a revision indicator in the subject
>>> line, as an>
>>> example:
>>> "[RFC PATCH v2] audit: make audit less awful"
>>>
>>> It's not strictly necessary, but it makes my life just a little bit
>>> easier.
>>
>> Sorry for that. I realised that I botched the subject immediately after
>> sending the mail :-(
>
> No worries, you'll take care of it next time.
>
>>>> diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
>>>> index 0caf1f8..0ada577 100644
>>>> --- a/kernel/audit_tree.c
>>>> +++ b/kernel/audit_tree.c
>>>
>>> ...
>>>
>>>> +static int launch_prune_thread(void)
>>>> +{
>>>> + prune_thread = kthread_create(prune_tree_thread, NULL,
>>>> + "audit_prune_tree");
>>>> + if (IS_ERR(prune_thread)) {
>>>> + audit_panic("cannot start thread audit_prune_tree");
>>>
>>> I'm not certain audit_panic() is warranted here, pr_err() might be a
>>> better choice. What is the harm if the thread doesn't start and we return
>>> an error code?
>>
>> I thought disabling the bigger part of the file auditing would warrant a
>> bigger bang than pr_err(). If you think, it is an overkill, then I can
>> change it.
>>
>> But see my comment below in audit_schedule_prune()
>
> My concern with audit_panic() is that it only generates a panic() in the
> *very* rare circumstance that someone has configured it that way. While there
> are some users who do configure their systems with AUDIT_FAIL_PANIC, I think
> it is safe to say that most do not. Further, audit_panic() can be rate
> limited or even silenced in the case of AUDIT_FAIL_SILENT.
>
> The choice of pr_err() is not perfect for all scenarios, but I think it is the
> better choice for most of the common scenarios.

Ok. so pr_err() is the one with reliably the bigger bang.

>>>> + prune_thread = NULL;
>>>> + return -ENOSYS;
>>>
>>> Out of curiosity, why ENOSYS?
>>
>> I thought it is a bit less confusing than ESRCH.
>
> Originally I was thinking about -ENOMEM, thoughts?

That might be better in the sense that the application programmer's response
to that case might be the one we want here.

>
>>>> + } else {
>>>> + wake_up_process(prune_thread);
>>>> + return 0;
>>>> + }
>>>> +}
>>>
>>> See my comments below in audit_schedule_prune().
>>>
>>>> /* called with audit_filter_mutex */
>>>> int audit_add_tree_rule(struct audit_krule *rule)
>>>> {
>>>>
>>>> @@ -663,6 +713,12 @@ int audit_add_tree_rule(struct audit_krule *rule)
>>>>
>>>> /* do not set rule->tree yet */
>>>> mutex_unlock(&audit_filter_mutex);
>>>>
>>>> + if (unlikely(!prune_thread)) {
>>>> + err = launch_prune_thread();
>>>> + if (err)
>>>> + goto Err;
>>>> + }
>>>> +
>>>
>>> Why not put this at the top of audit_add_tree_rule()?
>>
>> I would like to do this without holding audit_filter_mutex.
>
> Sorry, I forgot that audit_add_tree_rule() is called with the
> audit_filter_mutex locked.
>
>>>> err = kern_path(tree->pathname, 0, &path);
>>>> if (err)
>>>>
>>>> goto Err;
>>>>
>>>> @@ -713,6 +769,9 @@ int audit_tag_tree(char *old, char *new)
>>>>
>>>> struct vfsmount *tagged;
>>>> int err;
>>>>
>>>> + if (!prune_thread)
>>>> + return -ENOSYS;
>>>
>>> Help me out - why?
>
> Still, why?

You are right, it is not needed.
The codepath I was worried about is impossible.

>>>> err = kern_path(new, 0, &path2);
>>>> if (err)
>>>>
>>>> return err;
>>>>
>>>> @@ -800,36 +859,11 @@ int audit_tag_tree(char *old, char *new)
>>>>
>>>> return failed;
>>>>
>>>> }
>>>>
>>>> -/*
>>>> - * That gets run when evict_chunk() ends up needing to kill audit_tree.
>>>> - * Runs from a separate thread.
>>>> - */
>>>> -static int prune_tree_thread(void *unused)
>>>> -{
>>>> - mutex_lock(&audit_cmd_mutex);
>>>> - mutex_lock(&audit_filter_mutex);
>>>> -
>>>> - while (!list_empty(&prune_list)) {
>>>> - struct audit_tree *victim;
>>>> -
>>>> - victim = list_entry(prune_list.next, struct audit_tree, list);
>>>> - list_del_init(&victim->list);
>>>> -
>>>> - mutex_unlock(&audit_filter_mutex);
>>>> -
>>>> - prune_one(victim);
>>>> -
>>>> - mutex_lock(&audit_filter_mutex);
>>>> - }
>>>> -
>>>> - mutex_unlock(&audit_filter_mutex);
>>>> - mutex_unlock(&audit_cmd_mutex);
>>>> - return 0;
>>>> -}
>>>>
>>>> static void audit_schedule_prune(void)
>>>> {
>>>>
>>>> - kthread_run(prune_tree_thread, NULL, "audit_prune_tree");
>>>> + BUG_ON(!prune_thread);
>>>> + wake_up_process(prune_thread);
>>>>
>>>> }
>>>
>>> First, I probably wasn't clear last time so I'll be more clear now: no
>>> BUG_ON() here, handle the error.
>>
>> As far as I see, I disabled all the codepaths that can reach this point with
>> !prune_thread. So I thought leaving the BUG_ON() here doesn't really
>> matter.
>
> If it doesn't do anything, then you can remove it ;)

Will do.

>
> BUG_ON()/BUG() does have its uses, but I'm not sure this in one of those
> cases.
>
>>> Second, and closely related to the last sentence, perhaps the right
>>> approach is to merge the launch_prune_thread() code with
>>> audit_schedule_prune() such that we only have one function which starts
>>> the thread if it isn't present, and wakes it up if it is, something like
>>> the following:
>>>
>>> static int audit_schedule_prune(void)
>>> {
>>>
>>> if (!prune_thread) {
>>>
>>> prune_thread = kthread_create(...);
>>> if (IS_ERR(prune_thread)) {
>>>
>>> pr_err("cannot start thread audit_prune_tree");
>>> prune_thread = NULL;
>>> return -ENOSYS;
>>>
>>> }
>>>
>>> }
>>>
>>> wake_up_process(prune_thread);
>>> return 0;
>>>
>>> }
>>
>> if we do the thread creation in audit_schedule_prune, we won't necessarily
>> need the dedicated thread. This would be the alternative approach I
>> mentioned in the comment part of my original mail. Sorry if I was not
>> clear enough.
>
> The code in the snippet I provided starts the thread if it doesn't exist, and
> wakes the thread if it exists. I don't understand how that is different from
> what you were doing, I just happen to think it is a little more robust.

The thing that worries me there, is that thread creation still getting called
from filesystem code. As far as I see no filesystem locks are held on the
codepath right now, but if things change, we will be hit with the same error
I am trying to fix here. I.e., it might be more robust in one sense, but it is
brittle too.

Moreover it needs an intricate dance to serialise the thread creation (so we
won't leak threads), without holding a mutex during the thread creation itself.

>
>> Basically I see the following approaches:
>>
>> 1) dedicated thread created on syscall entry, with disabling file auditing
>> as long as the thread cannot be created.
>>
>> 2) on-demand thread-creation/destruction with some serialisation to ensure
>> that we won't create too many threads.
>>
>> 3) dedicated thread created on syscall entry, with fallback to thread
>> creation at cleanup time, if original thread creation fails.
>>
>> Am I right that you would like to see the third one?
>
> I don't want #2, and I think in general we should do whatever we can to
> recover from errors such that we don't disable auditing. That is just good
> practice.
>
>>>> /*
>>>>
>>>> @@ -896,9 +930,10 @@ static void evict_chunk(struct audit_chunk *chunk)
>>>>
>>>> for (n = 0; n < chunk->count; n++)
>>>>
>>>> list_del_init(&chunk->owners[n].list);
>>>>
>>>> spin_unlock(&hash_lock);
>>>>
>>>> + mutex_unlock(&audit_filter_mutex);
>>>>
>>>> if (need_prune)
>>>>
>>>> audit_schedule_prune();
>>>>
>>>> - mutex_unlock(&audit_filter_mutex);
>>>> +
>>>>
>>>> }
>>>
>>> Remove that trailing empty vertical whitespace please. If you aren't
>>> using it already, you should look into scripts/checkpatch.pl to sanity
>>> check your patches before sending.
>>
>> Could you point me to that whitespace? I cannot see it in the patch, and
>> scripts/checkpatch.pl was not complaining either.
>
> In your patch it looks like there is a blank empty line at the end of
> evict_chunk(); it appears like you are replacing the last mutex_unlock() with
> blank line.
>