Re: NULL pointer dereference in selinux_ip_postroute_compat

[Casey Schaufler <casey@schaufler-ca.com>]

On 8/8/2012 2:54 PM, Eric Dumazet wrote:

By the way, once this proved to be an issue that involved
more than just SELinux it needed to go onto the LSM list as
well.

> On Wed, 2012-08-08 at 16:46 -0400, Paul Moore wrote:
>> On Wednesday, August 08, 2012 10:32:52 PM Eric Dumazet wrote:
>>> On Wed, 2012-08-08 at 22:09 +0200, Eric Dumazet wrote:
>>>> On Wed, 2012-08-08 at 15:59 -0400, Eric Paris wrote:
>>>>> Seems wrong.  We shouldn't ever need ifdef CONFIG_SECURITY in core
>>>>> code.
>>>> Sure but it seems include file misses an accessor for this.
>>>>
>>>> We could add it on a future cleanup patch, as Paul mentioned.
>>> I cooked following patch.
>>> But smack/smack_lsm.c makes a reference to
>>> smk_of_current()... so it seems we are in a hole...
>>>
>>> It makes little sense to me to have any kind of security on this
>>> internal sockets.
>>>
>>> Maybe selinux should not crash if sk->sk_security is NULL ?
>> I realize our last emails probably passed each other mid-flight, but hopefully 
>> it explains why we can't just pass packets when sk->sk_security is NULL.
>>
>> Regardless, some quick comments below ...
>>
>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>>> index 6c77f63..459eca6 100644
>>> --- a/security/selinux/hooks.c
>>> +++ b/security/selinux/hooks.c
>>> @@ -4289,10 +4289,13 @@ out:
>>>  	return 0;
>>>  }
>>>
>>> -static int selinux_sk_alloc_security(struct sock *sk, int family, ...
>>> +static int selinux_sk_alloc_security(struct sock *sk, int family, ...
>>>  {
>>>  	struct sk_security_struct *sksec;
>>>
>>> +	if (check && sk->sk_security)
>>> +		return 0;
>>> +
>>>  	sksec = kzalloc(sizeof(*sksec), priority);
>>>  	if (!sksec)
>>>  		return -ENOMEM;
>> I think I might replace the "check" boolean with a "kern/kernel" boolean so 
>> that in addition to the allocation we can also initialize the socket to 
>> SECINITSID_KERNEL/kernel_t here in the case when the boolean is set.  The only 
>> place that would set the boolean to true would be ip_send_unicast_reply(), all 
>> other callers would set it to false.
>>
>>> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
>>> index 8221514..8965cf1 100644
>>> --- a/security/smack/smack_lsm.c
>>> +++ b/security/smack/smack_lsm.c
>>> @@ -1754,11 +1754,14 @@ static void smack_task_to_inode(struct task_struct
>>> *p, struct inode *inode) *
>>>   * Returns 0 on success, -ENOMEM is there's no memory
>>>   */
>>> -static int smack_sk_alloc_security(struct sock *sk, int family, gfp_t
>>> gfp_flags) +static int smack_sk_alloc_security(struct sock *sk, int family,
>>> gfp_t gfp_flags, bool check) {
>>>  	char *csp = smk_of_current();
>>>  	struct socket_smack *ssp;
>>>
>>> +	if (check && sk->sk_security)
>>> +		return 0;
>>> +
>>>  	ssp = kzalloc(sizeof(struct socket_smack), gfp_flags);
>>>  	if (ssp == NULL)
>>>  		return -ENOMEM;
>> In the case of Smack, when the kernel boolean is true I think the right 
>> solution is to use smack_net_ambient.

I confess that my understanding of unicast is limited.
If the intention is to send an unlabeled packet then
indeed smack_net_ambient is the way to go.

>>
> cool, here the last version :
>
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 4e5a73c..4d8e454 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -1601,7 +1601,7 @@ struct security_operations {
>  	int (*socket_sock_rcv_skb) (struct sock *sk, struct sk_buff *skb);
>  	int (*socket_getpeersec_stream) (struct socket *sock, char __user *optval, int __user *optlen, unsigned len);
>  	int (*socket_getpeersec_dgram) (struct socket *sock, struct sk_buff *skb, u32 *secid);
> -	int (*sk_alloc_security) (struct sock *sk, int family, gfp_t priority);
> +	int (*sk_alloc_security) (struct sock *sk, int family, gfp_t priority, bool kernel);

Is there no information already available in the sock
that will tell us this is a unicast operation?

>  	void (*sk_free_security) (struct sock *sk);
>  	void (*sk_clone_security) (const struct sock *sk, struct sock *newsk);
>  	void (*sk_getsecid) (struct sock *sk, u32 *secid);
> @@ -2539,7 +2539,7 @@ int security_sock_rcv_skb(struct sock *sk, struct sk_buff *skb);
>  int security_socket_getpeersec_stream(struct socket *sock, char __user *optval,
>  				      int __user *optlen, unsigned len);
>  int security_socket_getpeersec_dgram(struct socket *sock, struct sk_buff *skb, u32 *secid);
> -int security_sk_alloc(struct sock *sk, int family, gfp_t priority);
> +int security_sk_alloc(struct sock *sk, int family, gfp_t priority, bool kernel);
>  void security_sk_free(struct sock *sk);
>  void security_sk_clone(const struct sock *sk, struct sock *newsk);
>  void security_sk_classify_flow(struct sock *sk, struct flowi *fl);
> @@ -2667,7 +2667,7 @@ static inline int security_socket_getpeersec_dgram(struct socket *sock, struct s
>  	return -ENOPROTOOPT;
>  }
>  
> -static inline int security_sk_alloc(struct sock *sk, int family, gfp_t priority)
> +static inline int security_sk_alloc(struct sock *sk, int family, gfp_t priority, bool kernel)
>  {
>  	return 0;
>  }
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 8f67ced..e00cadf 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1186,7 +1186,7 @@ static struct sock *sk_prot_alloc(struct proto *prot, gfp_t priority,
>  	if (sk != NULL) {
>  		kmemcheck_annotate_bitfield(sk, flags);
>  
> -		if (security_sk_alloc(sk, family, priority))
> +		if (security_sk_alloc(sk, family, priority, false))
>  			goto out_free;
>  
>  		if (!try_module_get(prot->owner))
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 76dde25..b233d6e 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -1524,6 +1524,8 @@ void ip_send_unicast_reply(struct net *net, struct sk_buff *skb, __be32 daddr,
>  	sk->sk_priority = skb->priority;
>  	sk->sk_protocol = ip_hdr(skb)->protocol;
>  	sk->sk_bound_dev_if = arg->bound_dev_if;
> +	if (security_sk_alloc(sk, PF_INET, GFP_ATOMIC, true))
> +		goto out;
>  	sock_net_set(sk, net);
>  	__skb_queue_head_init(&sk->sk_write_queue);
>  	sk->sk_sndbuf = sysctl_wmem_default;
> @@ -1539,7 +1541,7 @@ void ip_send_unicast_reply(struct net *net, struct sk_buff *skb, __be32 daddr,
>  		skb_set_queue_mapping(nskb, skb_get_queue_mapping(skb));
>  		ip_push_pending_frames(sk, &fl4);
>  	}
> -
> +out:
>  	put_cpu_var(unicast_sock);
>  
>  	ip_rt_put(rt);
> diff --git a/security/security.c b/security/security.c
> index 860aeb3..23cf297 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1146,9 +1146,9 @@ int security_socket_getpeersec_dgram(struct socket *sock, struct sk_buff *skb, u
>  }
>  EXPORT_SYMBOL(security_socket_getpeersec_dgram);
>  
> -int security_sk_alloc(struct sock *sk, int family, gfp_t priority)
> +int security_sk_alloc(struct sock *sk, int family, gfp_t priority, bool kernel)
>  {
> -	return security_ops->sk_alloc_security(sk, family, priority);
> +	return security_ops->sk_alloc_security(sk, family, priority, kernel);
>  }
>  
>  void security_sk_free(struct sock *sk)
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 6c77f63..ccd4374 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -4289,10 +4289,13 @@ out:
>  	return 0;
>  }
>  
> -static int selinux_sk_alloc_security(struct sock *sk, int family, gfp_t priority)
> +static int selinux_sk_alloc_security(struct sock *sk, int family, gfp_t priority, bool kernel)
>  {
>  	struct sk_security_struct *sksec;
>  
> +	if (kernel && sk->sk_security)
> +		return 0;
> +
>  	sksec = kzalloc(sizeof(*sksec), priority);
>  	if (!sksec)
>  		return -ENOMEM;
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 8221514..207d9cc 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -1749,20 +1749,25 @@ static void smack_task_to_inode(struct task_struct *p, struct inode *inode)
>   * @sk: the socket
>   * @family: unused
>   * @gfp_flags: memory allocation flags
> + * @kernel: true if we should check sk_security being already set
>   *
>   * Assign Smack pointers to current
>   *
>   * Returns 0 on success, -ENOMEM is there's no memory
>   */
> -static int smack_sk_alloc_security(struct sock *sk, int family, gfp_t gfp_flags)
> +static int smack_sk_alloc_security(struct sock *sk, int family, gfp_t gfp_flags, bool kernel)
>  {
>  	char *csp = smk_of_current();
>  	struct socket_smack *ssp;
>  
> +	if (kernel && sk->sk_security)
> +		return 0;
> +
>  	ssp = kzalloc(sizeof(struct socket_smack), gfp_flags);
>  	if (ssp == NULL)
>  		return -ENOMEM;
> -
> +	/* kernel is true if called from ip_send_unicast_reply() */
> +	csp = kernel ? smack_net_ambient : smk_of_current();

How about ...

        if (kernel)
            csp = smack_net_ambient;

... as csp is set to smk_of_current() in the declaration.
That, or change the declaration.

>  	ssp->smk_in = csp;
>  	ssp->smk_out = csp;
>  	ssp->smk_packet = NULL;
>
>
> --
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