Re: [RFC PATCH v4] audit: move the tree pruning to a dedicated thread

[Richard Guy Briggs <rgb@redhat.com>]

On 15/01/30, 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>

Reviewed-by: Richard Guy Briggs <rgb@redhat.com>

> ---
>  kernel/audit_tree.c |   88 +++++++++++++++++++++++++++++++++++----------------
>  1 file changed, 60 insertions(+), 28 deletions(-)
> 
> diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
> index 2e0c974..71fd1f2 100644
> --- a/kernel/audit_tree.c
> +++ b/kernel/audit_tree.c
> @@ -37,6 +37,7 @@ struct audit_chunk {
>  
>  static LIST_HEAD(tree_list);
>  static LIST_HEAD(prune_list);
> +static struct task_struct *prune_thread;
>  
>  /*
>   * One struct chunk is attached to each inode of interest.
> @@ -651,6 +652,57 @@ static int tag_mount(struct vfsmount *mnt, void *arg)
>  	return tag_chunk(mnt->mnt_root->d_inode, arg);
>  }
>  
> +/*
> + * 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)
> +{
> +	for (;;) {
> +		set_current_state(TASK_INTERRUPTIBLE);
> +		if (list_empty(&prune_list))
> +			schedule();
> +		__set_current_state(TASK_RUNNING);
> +
> +		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 int audit_launch_prune(void)
> +{
> +	if (prune_thread)
> +		return 0;
> +	prune_thread = kthread_create(prune_tree_thread, NULL,
> +				"audit_prune_tree");
> +	if (IS_ERR(prune_thread)) {
> +		pr_err("cannot start thread audit_prune_tree");
> +		prune_thread = NULL;
> +		return -ENOMEM;
> +	} else {
> +		wake_up_process(prune_thread);
> +		return 0;
> +	}
> +}
> +
>  /* called with audit_filter_mutex */
>  int audit_add_tree_rule(struct audit_krule *rule)
>  {
> @@ -674,6 +726,12 @@ 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 = audit_launch_prune();
> +		if (err)
> +			goto Err;
> +	}
> +
>  	err = kern_path(tree->pathname, 0, &path);
>  	if (err)
>  		goto Err;
> @@ -811,36 +869,10 @@ 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");
> +	wake_up_process(prune_thread);
>  }
>  
>  /*
> @@ -907,9 +939,9 @@ 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);
>  }
>  
>  static int audit_tree_handle_event(struct fsnotify_group *group,
> -- 
> 1.7.9.5
> 
> --
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- RGB

--
Richard Guy Briggs <rbriggs@redhat.com>
Senior Software Engineer, Kernel Security, AMER ENG Base Operating Systems, Red Hat
Remote, Ottawa, Canada
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