Subject: Re: [PATCH] net: Fix ns_capable check in sock_diag_put_filterinfo

From: Nicolas Dichtel <[email-masked]>
Date: Thu, 17 Apr 2014 11:21:20 +0200

[send again the reply without the disclaimer, sorry for that]

Le 17/04/2014 10:37, Eric W. Biederman a écrit :
> Andy Lutomirski <[email-masked]> writes:
>
>> The caller needs capabilities on the namespace being queried, not on
>> their own namespace.  This is a security bug, although it likely has
>> only a minor impact.
>
> Hmm.  Thinking this through.
>
> It would likely help to rename sk_user_ns to sk_opener_user_ns to make
> things clearer.
>
> As I read net/core/sock_diag.c anyone is allowed to open a diag socket
> and send messages to the kernel.
>
> Which means the code as written is definitely wrong as checking if we
> have CAP_NET_ADMIN in the user namespace in which we opened the netlink
> socket is meaningless.  We could very easily have unshared the user
> namespace and have no permissions whatsoever over the network namespace
> we are querrying.
>
> I see three possibilities here.
> 1) We simply don't care who gets to read the bpf filter of a socket.
> 2) We consider reading the bpf filter of a socket an information leak
>     about the socket and the opener of the socket.
> 3) We consider reading the bpf filter of a socket something we should
>     only let the administrator of a network namespace do.
>
> I honestly don't know the intent of the check, and what we are trying to
> protect against so I don't know why having permissions to protect the
> bpf filter is important.
>
> If we simply don't care we should just delete this permission check.
>
> If we want to protect the opener of the socket we probably want
> something looks a lot like ptrace_may_access(..., PTRACE_MODE_READ)
> applied to the creds of the socket.  Call it
>
> 	ns_capable((sock_opener_user_ns(sk), CAP_NET_ADMIN)
>
> for short.
>
>
> And if this is something we just want to limit to administrators
> of network stacks the check should be
>
> 	ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN)
>
> As Andy has coded below.
>
> Nicolas what was the intent of having a capability check to protect the
> bpf filter?  What were you trying to protect against with a capability
> check on the bpf filter?
Option 3 was the initial intention. BFP filter contains sensible informations
and thus we only want to disclose them to the admin.

Nicolas
>
> Eric
>
>
>> Cc: [email-masked]
>> Signed-off-by: Andy Lutomirski <[email-masked]>
>> ---
>>
>> Someone should check that I'm right.  I had trouble getting 'ss -b' to
>> work, even with plain old sudo.
>>
>>   include/linux/sock_diag.h | 2 +-
>>   net/core/sock_diag.c      | 4 ++--
>>   net/packet/diag.c         | 2 +-
>>   3 files changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/sock_diag.h b/include/linux/sock_diag.h
>> index 54f91d3..302ab80 100644
>> --- a/include/linux/sock_diag.h
>> +++ b/include/linux/sock_diag.h
>> @@ -23,7 +23,7 @@ int sock_diag_check_cookie(void *sk, __u32 *cookie);
>>   void sock_diag_save_cookie(void *sk, __u32 *cookie);
>>
>>   int sock_diag_put_meminfo(struct sock *sk, struct sk_buff *skb, int attr);
>> -int sock_diag_put_filterinfo(struct user_namespace *user_ns, struct sock *sk,
>> +int sock_diag_put_filterinfo(struct sock *sk,
>>   			     struct sk_buff *skb, int attrtype);
>>
>>   #endif
>> diff --git a/net/core/sock_diag.c b/net/core/sock_diag.c
>> index a0e9cf6..6a7fae2 100644
>> --- a/net/core/sock_diag.c
>> +++ b/net/core/sock_diag.c
>> @@ -49,7 +49,7 @@ int sock_diag_put_meminfo(struct sock *sk, struct sk_buff *skb, int attrtype)
>>   }
>>   EXPORT_SYMBOL_GPL(sock_diag_put_meminfo);
>>
>> -int sock_diag_put_filterinfo(struct user_namespace *user_ns, struct sock *sk,
>> +int sock_diag_put_filterinfo(struct sock *sk,
>>   			     struct sk_buff *skb, int attrtype)
>>   {
>>   	struct nlattr *attr;
>> @@ -57,7 +57,7 @@ int sock_diag_put_filterinfo(struct user_namespace *user_ns, struct sock *sk,
>>   	unsigned int len;
>>   	int err = 0;
>>
>> -	if (!ns_capable(user_ns, CAP_NET_ADMIN)) {
>> +	if (!ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN)) {
>>   		nla_reserve(skb, attrtype, 0);
>>   		return 0;
>>   	}
>> diff --git a/net/packet/diag.c b/net/packet/diag.c
>> index 533ce4f..435ff99 100644
>> --- a/net/packet/diag.c
>> +++ b/net/packet/diag.c
>> @@ -172,7 +172,7 @@ static int sk_diag_fill(struct sock *sk, struct sk_buff *skb,
>>   		goto out_nlmsg_trim;
>>
>>   	if ((req->pdiag_show & PACKET_SHOW_FILTER) &&
>> -	    sock_diag_put_filterinfo(user_ns, sk, skb, PACKET_DIAG_FILTER))
>> +	    sock_diag_put_filterinfo(sk, skb, PACKET_DIAG_FILTER))
>>   		goto out_nlmsg_trim;
>>
>>   	return nlmsg_end(skb, nlh);