Re: [COMMERCIAL] Re: [PATCH 2/3] lib/kobject_uevent.c: add uevent forwarding function

[Michael J Coss]

On 9/10/2015 8:54 PM, Eric W. Biederman wrote:
> "Michael J. Coss" <michael.coss@xxxxxxxxxxxxxxxxxx> writes:
>
>> Adds capability to allow userspace programs to forward a given event to
>> a specific network namespace as determined by the provided pid.  In
>> addition, support for a per-namespace kobject_sequence counter was
>> added.  Sysfs was modified to return the correct event counter based on
>> the current network namespace.
> So this patch is buggy.
>
>> Signed-off-by: Michael J. Coss <michael.coss@xxxxxxxxxxxxxxxxxx>
>> ---
>>  include/linux/kobject.h     |  3 ++
>>  include/net/net_namespace.h |  3 ++
>>  kernel/ksysfs.c             | 12 ++++++
>>  lib/kobject_uevent.c        | 90 +++++++++++++++++++++++++++++++++++++++++++++
>>  4 files changed, 108 insertions(+)
>>
>> diff --git a/include/linux/kobject.h b/include/linux/kobject.h
>> index 637f670..d1bb509 100644
>> --- a/include/linux/kobject.h
>> +++ b/include/linux/kobject.h
>> @@ -215,6 +215,9 @@ extern struct kobject *firmware_kobj;
>>  int kobject_uevent(struct kobject *kobj, enum kobject_action action);
>>  int kobject_uevent_env(struct kobject *kobj, enum kobject_action action,
>>  			char *envp[]);
>> +#if defined(CONFIG_UDEVNS) || defined(CONFIG_UDEVNS_MODULE)
>> +int kobject_uevent_forward(char *buf, size_t len, pid_t pid);
>> +#endif
>>  
>>  __printf(2, 3)
>>  int add_uevent_var(struct kobj_uevent_env *env, const char *format, ...);
>> diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
>> index e951453..a4013e5 100644
>> --- a/include/net/net_namespace.h
>> +++ b/include/net/net_namespace.h
>> @@ -134,6 +134,9 @@ struct net {
>>  #if IS_ENABLED(CONFIG_MPLS)
>>  	struct netns_mpls	mpls;
>>  #endif
>> +#if defined(CONFIG_UDEVNS) || defined(CONFIG_UDEVNS_MODULE)
>> +	u64				kevent_seqnum;
>> +#endif
>>  	struct sock		*diag_nlsk;
>>  	atomic_t		fnhe_genid;
>>  };
>> diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
>> index 6683cce..4bc15fd 100644
>> --- a/kernel/ksysfs.c
>> +++ b/kernel/ksysfs.c
>> @@ -21,6 +21,9 @@
>>  #include <linux/compiler.h>
>>  
>>  #include <linux/rcupdate.h>	/* rcu_expedited */
>> +#if defined(CONFIG_UDEVNS) || defined(CONFIG_UDEVNS_MODULE)
>> +#include <net/net_namespace.h>
>> +#endif
>>  
>>  #define KERNEL_ATTR_RO(_name) \
>>  static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
>> @@ -33,6 +36,15 @@ static struct kobj_attribute _name##_attr = \
>>  static ssize_t uevent_seqnum_show(struct kobject *kobj,
>>  				  struct kobj_attribute *attr, char *buf)
>>  {
>> +#if defined(CONFIG_UDEVNS) || defined(CONFIG_UDEVNS_MODULE)
>> +	pid_t p = task_pid_vnr(current);
>> +	struct net *n = get_net_ns_by_pid(p);
>> +
>> +	if (n != ERR_PTR(-ESRCH)) {
>> +		if (!net_eq(n, &init_net))
>> +			return sprintf(buf, "%llu\n", n->kevent_seqnum);
>> +	}
>> +#endif
> The test here is completely wrong.
> a) Each sysfs instance already has a network namespace attached to it's
>    superblock so using the pid of the caller is wrong.
>
> b) You return kevent_seqnum even in network namespaces where you are not
>    sending uevents from userspace which is concerning.
>
> c) Is this all we need to do to sysfs?  Otherwise it may be best to
>    simply fake this file, and remove the need to play games with the
>    sequence number in kobject_uevent_forward.
I will take a look at the sysfs network namespace, I was unaware that
there was an existing namespace attached.  If the broadcasting is
disabled,  then no events are ever sent to a non-host namespace except
as a function of a user space daemon.  And as such the kevent_seqnum
will be 0 and rightfully so.  If broadcasts are selectively disabled
then we clearly need to look this in that light.  The sequence number is
a single value generated as uevents occur, it's not clear to me how to
get a user space application to fake this out, as the generated number
is inserted into the uevent message.
>>  	return sprintf(buf, "%llu\n", (unsigned long long)uevent_seqnum);
>>  }
>>  KERNEL_ATTR_RO(uevent_seqnum);
>> diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
>> index d791e33..7589745 100644
>> --- a/lib/kobject_uevent.c
>> +++ b/lib/kobject_uevent.c
>> @@ -379,6 +379,96 @@ int kobject_uevent(struct kobject *kobj, enum kobject_action action)
>>  }
>>  EXPORT_SYMBOL_GPL(kobject_uevent);
>>  
>> +#if defined(CONFIG_UDEVNS) || defined(CONFIG_UDEVNS_MODULE)
>> +/**
>> + * kobject_uevent_forward - forward event to specified network namespace
>> + *
>> + * @buf: event buffer
>> + * @len: event length
>> + * @pid: pid of network namespace
> This function should just take a network namespace all of the weird bits
> should be left to the user/kernel interface.
>
> The pid should be translated into a network namespace at the edge of
> user space.  Not down here in a helper function.
Agreed.  It could just pass the pointer to the namespace, as part of the
message processing in the module.
>> + * Returns 0 if kobject_uevent_forward() is completed with success or the
>> + * corresponding error when it fails.
>> + */
>> +int kobject_uevent_forward(char *buf, size_t len, pid_t pid)
>> +{
>> +	int retval = 0;
>> +#if defined(CONFIG_NET)
>> +	struct uevent_sock *ue_sk;
>> +	struct net *pns;
>> +	char *p;
>> +	u64 num;
>> +
>> +	/* grab the network namespace of the provided pid */
>> +	pns = get_net_ns_by_pid(pid);
>> +	if (pns == ERR_PTR(-ESRCH))
>> +		return -ESRCH;
>> +
>> +	/* find sequence number in buffer */
>> +	p = buf;
>> +	num = 0;
>> +	while (p < (buf + len)) {
>> +		if (strncmp(p, "SEQNUM=", 7) == 0) {
>> +			int r;
>> +
>> +			p += 7;
>> +			r = kstrtoull(p, 10, &num);
>> +			if (r) {
>> +				put_net(pns);
>> +				return r;
>> +			}
>> +			break;
>> +		}
>> +		p += (strlen(p) + 1);
>> +	}
> Do we really need to parse the sequence number out of the packet?  I
> suspect it would be easier to simply have a sequence number that
> increments every time a message passes through.
The original uevent message don't have a sequence number,  one is
generated and  inserted into the uevent message before it is sent to
userspace.  As the messages are being pushed back out kernel->user
space->kernel, I just wanted to track the highest sequence number seen
in this particular namespace.
>> +
>> +	/* if we didn't see a valid seqnum, or none was present, return error */
>> +	if (num == 0) {
>> +		put_net(pns);
>> +		return -EINVAL;
>> +	}
>> +	/* update per namespace sequence number as needed */
>> +	if (pns->kevent_seqnum < num)
>> +		pns->kevent_seqnum = num;
>> +
>> +	list_for_each_entry(ue_sk, &uevent_sock_list, list) {
>> +		struct sock *uevent_sock = ue_sk->sk;
>> +		struct sk_buff *skb;
>> +
>> +		if (!netlink_has_listeners(uevent_sock, 1))
>> +			continue;
>> +		/*
>> +		 * only send to sockets share the same network namespace
>> +		 * as the passed pid
>> +		 */
>> +		if (!net_eq(sock_net(uevent_sock), pns))
>> +			continue;
> This look is terribly inefficient.  You could just go directly to the
> network namespace uevent_sock that you want from your network namespace.
>
> I wonder if we could arrange things so that the same skb you pass in is
> the skb that gets broadcast out.  That would simplify a lot of things.
>
>> +		/* allocate message with the maximum possible size */
>> +		skb = alloc_skb(len, GFP_KERNEL);
>> +		if (skb) {
>> +			char *p;
>> +
>> +			p = skb_put(skb, len);
>> +			memcpy(p, buf, len);
>> +			NETLINK_CB(skb).dst_group = 1;
>> +			retval = netlink_broadcast(uevent_sock, skb, 0, 1,
>> +						   GFP_KERNEL);
>> +
>> +			/* ENOBUFS should be handled in userspace */
>> +			if (retval == -ENOBUFS || retval == -ESRCH)
>> +				retval = 0;
>> +		} else {
>> +			retval = -ENOMEM;
>> +		}
>> +	}
>> +	put_net(pns);
>> +#endif
>> +	return retval;
>> +}
>> +EXPORT_SYMBOL_GPL(kobject_uevent_forward);
>> +#endif
>> +
>>  /**
>>   * add_uevent_var - add key value string to the environment buffer
>>   * @env: environment buffer structure
> --
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Thanks.

-- 
---Michael J Coss

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