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

[Serge E. Hallyn]

Quoting Eric W. Biederman (ebiederm at xmission.com):
> 
> 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 <ebiederm at xmission.com>
> ---
>  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.

> +			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;
> -- 
> 1.6.5.2.143.g8cc62
> 
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