Re: [RFC PATCH 5 1/7] kmod - add workqueue service thread store

[Jeff Layton]

On Tue, 31 Mar 2015 11:14:42 +0800
Ian Kent <raven@xxxxxxxxxx> wrote:

> From: Ian Kent <ikent@xxxxxxxxxx>
> 
> Persistent use of process information is needed for contained
> execution in a namespace other than the root init namespace.
> 
> Use a simple random token as a key to create and store thread
> information in a hashed list for use by the usermode helper
> thread runner.
> 
> Signed-off-by: Ian Kent <ikent@xxxxxxxxxx>
> Cc: Benjamin Coddington <bcodding@xxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: J. Bruce Fields <bfields@xxxxxxxxxxxx>
> Cc: David Howells <dhowells@xxxxxxxxxx>
> Cc: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
> Cc: Oleg Nesterov <onestero@xxxxxxxxxx>
> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx>
> ---
>  include/linux/kmod.h |    3 +
>  kernel/kmod.c        |  179 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 181 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/kmod.h b/include/linux/kmod.h
> index 0555cc6..fa46722 100644
> --- a/include/linux/kmod.h
> +++ b/include/linux/kmod.h
> @@ -66,6 +66,9 @@ struct subprocess_info {
>  	void *data;
>  };
>  
> +extern int umh_wq_get_token(int token, const char *service);
> +extern void umh_wq_put_token(int token);
> +
>  extern int
>  call_usermodehelper(char *path, char **argv, char **envp, int wait);
>  
> diff --git a/kernel/kmod.c b/kernel/kmod.c
> index 2777f40..55d20ce 100644
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -40,13 +40,30 @@
>  #include <linux/ptrace.h>
>  #include <linux/async.h>
>  #include <asm/uaccess.h>
> +#include <linux/hash.h>
> +#include <linux/list.h>
> +#include <linux/random.h>
>  
>  #include <trace/events/module.h>
>  
>  extern int max_threads;
>  
> +#define KHELPER		"khelper"
>  static struct workqueue_struct *khelper_wq;
>  
> +#define UMH_WQ_HASH_SHIFT  6
> +#define UMH_WQ_HASH_SIZE   1 << UMH_WQ_HASH_SHIFT
> +
> +struct umh_wq_entry {
> +	int token;
> +	unsigned int count;
> +	struct workqueue_struct *wq;
> +	struct hlist_node umh_wq_hlist;
> +};
> +
> +static DEFINE_SPINLOCK(umh_wq_hash_lock);
> +static struct hlist_head umh_wq_hash[UMH_WQ_HASH_SIZE];
> +
>  #define CAP_BSET	(void *)1
>  #define CAP_PI		(void *)2
>  
> @@ -475,6 +492,165 @@ static void helper_unlock(void)
>  		wake_up(&running_helpers_waitq);
>  }
>  
> +static void umh_wq_hash_init(void)
> +{
> +	int i;
> +
> +	for (i = 0; i < UMH_WQ_HASH_SIZE; i++)
> +		INIT_HLIST_HEAD(&umh_wq_hash[i]);
> +}
> +
> +static struct umh_wq_entry *umh_wq_find_entry(int token)
> +{
> +	struct umh_wq_entry *this, *entry;
> +	struct hlist_head *bucket;
> +	unsigned int hash;
> +
> +	hash = hash_32((unsigned long) token, UMH_WQ_HASH_SHIFT);
> +	bucket = &umh_wq_hash[hash];
> +
> +	entry = ERR_PTR(-ENOENT);
> +	if (hlist_empty(bucket))
> +		goto out;
> +
> +	hlist_for_each_entry(this, bucket, umh_wq_hlist) {
> +		if (this->token == token) {
> +			entry = this;
> +			break;
> +		}
> +	}
> +out:
> +	return entry;
> +}
> +
> +static struct workqueue_struct *umh_find_wq(int token, unsigned int nowait)

nit: there's no caller of this in this patch, but one is added in patch #2.

> +{
> +	struct umh_wq_entry *entry;
> +	unsigned long flags;
> +
> +	if (!token)
> +		return khelper_wq;
> +
> +	if (nowait)
> +		spin_lock_irqsave(&umh_wq_hash_lock, flags);
> +	else
> +		spin_lock(&umh_wq_hash_lock);
> +	entry = umh_wq_find_entry(token);
> +	if (nowait)
> +		spin_unlock_irqrestore(&umh_wq_hash_lock, flags);
> +	else
> +		spin_unlock(&umh_wq_hash_lock);
> +
> +	return entry->wq;
> +}
> +
> +/**
> + * umh_wq_get_token - create service thread and return an identifying token
> + * @token: token of an existing service thread or 0 to create a new
> + * 	   service thread.
> + * @name: service name to be appended to "khelper" for identification.
> + *
> + * Returns a token that used with calls to call_usermode_helper_service().
> + * If token corresponds to an existing service thread its reference count
> + * is increased and the token returned. On failure returns a negative errno.
> + */
> +int umh_wq_get_token(int token, const char *service)
> +{
> +	struct workqueue_struct *wq;
> +	char *wq_name;
> +	int wq_name_len;
> +	struct umh_wq_entry *entry;
> +	struct hlist_head *bucket;
> +	unsigned int hash;
> +	unsigned int new_token;
> +
> +	if (token) {
> +		spin_lock(&umh_wq_hash_lock);
> +		entry = umh_wq_find_entry(token);
> +		if (entry) {
> +			entry->count++;
> +			spin_unlock(&umh_wq_hash_lock);
> +			return token;
> +		}
> +		spin_unlock(&umh_wq_hash_lock);
> +	}
> +
> +	if (!service)
> +		return -EINVAL;
> +
> +	wq_name_len = sizeof(KHELPER) + strlen(service) + 1;
> +	wq_name = kmalloc(wq_name_len, GFP_KERNEL);
> +	if (!wq_name)
> +		return -ENOMEM;
> +	strcpy(wq_name, "KHELPER-");
> +	strcat(wq_name, service);
> +
> +	entry = kzalloc(sizeof(struct umh_wq_entry), GFP_KERNEL);
> +	if (!entry) {
> +		kfree(wq_name);
> +		return -ENOMEM;
> +	}
> +
> +	wq = create_singlethread_workqueue(wq_name);
> +	if (IS_ERR(wq)) {
> +		kfree(wq_name);
> +		kfree(entry);
> +		return PTR_ERR(wq);
> +	}
> +	kfree(wq_name);
> +	entry->wq = wq;
> +
> +	do {
> +		new_token = get_random_int();
> +		if (!new_token)
> +			continue;

Why a random value here? Is there some attack that can be done by
guessing the token?

ISTM that that could end up with you reusing a token soon after it has
been removed from the hash if you were really unlucky. If so, then that
could potentially be dangerous.

If there isn't a potential attack vector that involves guessing this
value, then a simple counter might mean less chance that the token
would be reused (particularly if the token were 64 bits).

> +		spin_lock(&umh_wq_hash_lock);
> +		entry = umh_wq_find_entry(new_token);
> +		if (likely(IS_ERR(entry))) {
> +			hash = hash_32(new_token, UMH_WQ_HASH_SHIFT);
> +			bucket = &umh_wq_hash[hash];
> +			hlist_add_head(&entry->umh_wq_hlist, bucket);
> +			entry->token = (long) new_token;

Why cast to long here? Shouldn't that be (int)?

> +			spin_unlock(&umh_wq_hash_lock);
> +			break;
> +		}
> +		spin_unlock(&umh_wq_hash_lock);
> +	} while (1);
> +
> +	return (int) new_token;
> +}
> +EXPORT_SYMBOL(umh_wq_get_token);
> +
> +/**
> + * umh_ns_put_token - stop a service thread if it's not longer in use
> + * 		      and remove it from the serive thread list
> + * @token: token of a service thread.
> + */
> +void umh_wq_put_token(int token)
> +{
> +	struct umh_wq_entry *entry;
> +
> +	if (!token)
> +		return;
> +
> +	spin_lock(&umh_wq_hash_lock);
> +	entry = umh_wq_find_entry(token);
> +	if (unlikely(IS_ERR(entry)))
> +		spin_unlock(&umh_wq_hash_lock);
> +	else {
> +		if (--entry->count)
> +			spin_unlock(&umh_wq_hash_lock);
> +		else {
> +			hlist_del(&entry->umh_wq_hlist);
> +			spin_unlock(&umh_wq_hash_lock);
> +			destroy_workqueue(entry->wq);
> +			kfree(entry);
> +		}
> +	}
> +	return;
> +}
> +EXPORT_SYMBOL(umh_wq_put_token);
> +
>  /**
>   * call_usermodehelper_setup - prepare to call a usermode helper
>   * @path: path to usermode executable
> @@ -689,6 +865,7 @@ struct ctl_table usermodehelper_table[] = {
>  
>  void __init usermodehelper_init(void)
>  {
> -	khelper_wq = create_singlethread_workqueue("khelper");
> +	umh_wq_hash_init();
> +	khelper_wq = create_singlethread_workqueue(KHELPER);
>  	BUG_ON(!khelper_wq);
>  }
> 


-- 
Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx>
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