Subject: Re: [PATCH 6/8] scm: Capture the full credentials of the scm sender.

From: [email-masked] (Eric W. Biederman)
Date: Tue, 15 Jun 2010 15:08:01 -0700

"Serge E. Hallyn" <[email-masked]> writes:

> Quoting Eric W. Biederman ([email-masked]):
>> 
>> Start capturing not only the userspace pid, uid and gid values of the
>> sending process but also the struct pid and struct cred of the sending
>> process as well.
>> 
>> This is in preparation for properly supporting SCM_CREDENTIALS for
>> sockets that have different uid and/or pid namespaces at the different
>> ends.
>> 
>> Signed-off-by: Eric W. Biederman <[email-masked]>
>> ---
>>  include/net/scm.h |   28 ++++++++++++++++++++++++----
>>  net/core/scm.c    |   24 ++++++++++++++++++++++++
>>  2 files changed, 48 insertions(+), 4 deletions(-)
>> 
>> diff --git a/include/net/scm.h b/include/net/scm.h
>> index 17d9d2e..3165650 100644
>> --- a/include/net/scm.h
>> +++ b/include/net/scm.h
>> @@ -19,6 +19,8 @@ struct scm_fp_list {
>>  };
>>  
>>  struct scm_cookie {
>> +	struct pid		*pid;		/* Skb credentials */
>> +	const struct cred	*cred;
>>  	struct scm_fp_list	*fp;		/* Passed files		*/
>>  	struct ucred		creds;		/* Skb credentials	*/
>>  #ifdef CONFIG_SECURITY_NETWORK
>> @@ -42,8 +44,27 @@ static __inline__ void unix_get_peersec_dgram(struct socket *sock, struct scm_co
>>  { }
>>  #endif /* CONFIG_SECURITY_NETWORK */
>>  
>> +static __inline__ void scm_set_cred(struct scm_cookie *scm,
>> +				    struct pid *pid, const struct cred *cred)
>> +{
>> +	scm->pid  = get_pid(pid);
>> +	scm->cred = get_cred(cred);
>> +	cred_to_ucred(pid, cred, &scm->creds);
>> +}
>> +
>> +static __inline__ void scm_destroy_cred(struct scm_cookie *scm)
>> +{
>> +	put_pid(scm->pid);
>> +	scm->pid  = NULL;
>> +
>> +	if (scm->cred)
>> +		put_cred(scm->cred);
>> +	scm->cred = NULL;
>> +}
>> +
>>  static __inline__ void scm_destroy(struct scm_cookie *scm)
>>  {
>> +	scm_destroy_cred(scm);
>>  	if (scm && scm->fp)
>>  		__scm_destroy(scm);
>>  }
>> @@ -51,10 +72,7 @@ static __inline__ void scm_destroy(struct scm_cookie *scm)
>>  static __inline__ int scm_send(struct socket *sock, struct msghdr *msg,
>>  			       struct scm_cookie *scm)
>>  {
>> -	struct task_struct *p = current;
>> -	scm->creds.uid = current_uid();
>> -	scm->creds.gid = current_gid();
>> -	scm->creds.pid = task_tgid_vnr(p);
>> +	scm_set_cred(scm, task_tgid(current), current_cred());
>>  	scm->fp = NULL;
>>  	unix_get_peersec_dgram(sock, scm);
>>  	if (msg->msg_controllen <= 0)
>> @@ -96,6 +114,8 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
>>  	if (test_bit(SOCK_PASSCRED, &sock->flags))
>>  		put_cmsg(msg, SOL_SOCKET, SCM_CREDENTIALS, sizeof(scm->creds), &scm->creds);
>>  
>> +	scm_destroy_cred(scm);
>> +
>>  	scm_passec(sock, msg, scm);
>>  
>>  	if (!scm->fp)
>> diff --git a/net/core/scm.c b/net/core/scm.c
>> index b88f6f9..681c976 100644
>> --- a/net/core/scm.c
>> +++ b/net/core/scm.c
>> @@ -170,6 +170,30 @@ int __scm_send(struct socket *sock, struct msghdr *msg, struct scm_cookie *p)
>>  			err = scm_check_creds(&p->creds);
>>  			if (err)
>>  				goto error;
>> +
>
> I think this hunk needs to be documented.  I.e. given that scm_send()
> will call scm_set_cred() before calling __scm_send, I don't see how
> these conditions could happen?  If the condition can legitimately
> happen, then given all of the pid_t vs struct pid and 'cred' vs. 'creds'
> in these two hunks, I think a comment over each would be nice.

I think if you have the full context of __scm_send it becomes pretty obvious.

		case SCM_CREDENTIALS:
			if (cmsg->cmsg_len != CMSG_LEN(sizeof(struct ucred)))
				goto error;
			memcpy(&p->creds, CMSG_DATA(cmsg), sizeof(struct ucred));
			err = scm_check_creds(&p->creds);
			if (err)
				goto error;

At this point we have just copied ucred from userspace.  We have done
scm_check_creds to ensure we allow the user to send the pid, uid, and
gid they have passed in.

These tests catch the case where the user is legitimately sending
something other than their own credentials.

>> +			if (pid_vnr(p->pid) != p->creds.pid) {
>> +				struct pid *pid;
>> +				err = -ESRCH;
>> +				pid = find_get_pid(p->creds.pid);
>> +				if (!pid)
>> +					goto error;
>> +				put_pid(p->pid);
>> +				p->pid = pid;
>> +			}
>> +
>> +			if ((p->cred->euid != p->creds.uid) ||
>> +				(p->cred->egid != p->creds.gid)) {
>> +				struct cred *cred;
>> +				err = -ENOMEM;
>> +				cred = prepare_creds();
>> +				if (!cred)
>> +					goto error;
>> +
>> +				cred->uid = cred->euid = p->creds.uid;
>> +				cred->gid = cred->egid = p->creds.uid;
>> +				put_cred(p->cred);
>> +				p->cred = cred;
>> +			}
>>  			break;
>>  		default:
>>  			goto error;

Eric