Subject: Re: [PATCH] scm: provide full privilege set via SCM_PRIVILEGE

From: Casey Schaufler <[email-masked]>
Date: Wed, 19 Jan 2011 06:57:31 -0800

On 1/18/2011 9:45 PM, Eric W. Biederman wrote:
> Casey Schaufler <[email-masked]> writes:
>
>> Subject: [PATCH] scm: provide full privilege set via SCM_PRIVILEGE
>>
>> The SCM mechanism currently provides interfaces for delivering
>> the uid/gid and the "security context" (LSM information) of the
>> peer on a UDS socket. All of the security credential information
>> is available, but there is no interface available to obtain it.
>> Further, the existing interfaces require that the user chose
>> between the uid/gid and the context as the existing interfaces
>> are exclusive.
>>
>> This patch introduces an additional interface that provides
>> a complete set of security information from the peer credential.
>> No additional work is required to provide the information
>> internally, it is all being passed, just not exposed.
> In ascii text?

As is commonly done in /proc interfaces.

> A bitmap in hex?

As is done in /proc/<pid>/status. I seriously doubt
that anyone would want the kernel doing the capability
set to text conversion.

> Maybe it is just me, but this seems harder to deal with than
> if the data had been transferred in binary.

There are a couple of issues with passing a binary structure
in the modern cred case. First is the capability set, which
has been proven to grow over time. Sure, it took a while to
get past 32 bits, and hopefully will never go beyond 64, but
given the long term problems caused by 16 bit uids (some of
us still remember) I would hate to get bitten by this in my
old age. Second is the LSM specific security context, which
may not be there at all and if it is the size will depend on
the LSM in use.

There are classic C language techniques for dealing with
both of these issues, and I've used them enough times to
want to avoid them where possible. This is the same logic
that the aforementioned /proc interface implementers have
been using for some time. And while there are problems
with formatting, passing and parsing a string they pale
in comparison to maintaining multiple versions of kernel
interface structures that are themselves variable depending
on the kernel configuration.

> Eric
>
>> Signed-off-by: Casey Schaufler <[email-masked]>
>> ---
>>
>>  include/asm-generic/socket.h |    1 +
>>  include/linux/net.h          |    1 +
>>  include/linux/socket.h       |    1 +
>>  include/net/scm.h            |   80 +++++++++++++++++++++++++++++++++++++++++-
>>  net/core/sock.c              |   11 ++++++
>>  5 files changed, 93 insertions(+), 1 deletions(-)
>> diff --git a/include/asm-generic/socket.h b/include/asm-generic/socket.h
>> index 9a6115e..7aa8e84 100644
>> --- a/include/asm-generic/socket.h
>> +++ b/include/asm-generic/socket.h
>> @@ -64,4 +64,5 @@
>>  #define SO_DOMAIN		39
>>  
>>  #define SO_RXQ_OVFL             40
>> +#define SO_PASSPRIV		41
>>  #endif /* __ASM_GENERIC_SOCKET_H */
>> diff --git a/include/linux/net.h b/include/linux/net.h
>> index 16faa13..159a929 100644
>> --- a/include/linux/net.h
>> +++ b/include/linux/net.h
>> @@ -71,6 +71,7 @@ struct net;
>>  #define SOCK_NOSPACE		2
>>  #define SOCK_PASSCRED		3
>>  #define SOCK_PASSSEC		4
>> +#define SOCK_PASSPRIV		5
>>  
>>  #ifndef ARCH_HAS_SOCKET_TYPES
>>  /**
>> diff --git a/include/linux/socket.h b/include/linux/socket.h
>> index 86b652f..e9cfd68 100644
>> --- a/include/linux/socket.h
>> +++ b/include/linux/socket.h
>> @@ -147,6 +147,7 @@ static inline struct cmsghdr * cmsg_nxthdr (struct msghdr *__msg, struct cmsghdr
>>  #define	SCM_RIGHTS	0x01		/* rw: access rights (array of int) */
>>  #define SCM_CREDENTIALS 0x02		/* rw: struct ucred		*/
>>  #define SCM_SECURITY	0x03		/* rw: security label		*/
>> +#define SCM_PRIVILEGES  0x04		/* rw: privilege set		*/
>>  
>>  struct ucred {
>>  	__u32	pid;
>> diff --git a/include/net/scm.h b/include/net/scm.h
>> index 3165650..4b8db21 100644
>> --- a/include/net/scm.h
>> +++ b/include/net/scm.h
>> @@ -101,6 +101,83 @@ static inline void scm_passec(struct socket *sock, struct msghdr *msg, struct sc
>>  { }
>>  #endif /* CONFIG_SECURITY_NETWORK */
>>  
>> +static __inline__ void scm_passpriv(struct socket *sock, struct msghdr *msg,
>> +				struct scm_cookie *scm)
>> +{
>> +	const struct cred *credp = scm->cred;
>> +	const struct group_info *gip;
>> +	char *result;
>> +	char *cp;
>> +	int i;
>> +#ifdef CONFIG_SECURITY_NETWORK
>> +	char *secdata;
>> +	u32 seclen;
>> +	int err;
>> +#endif /* CONFIG_SECURITY_NETWORK */
>> +
>> +	if (!test_bit(SOCK_PASSPRIV, &sock->flags))
>> +		return;
>> +
>> +	gip = credp->group_info;
>> +
>> +	/*
>> +	 * uid + euid + gid + egid + group-list + capabilities
>> +	 *     + "uid=" + "euid=" + "gid=" + "egid=" + "grps="
>> +	 *     + "cap-e=" + "cap-p=" + "cap-i="
>> +	 * 10  + 10   + 10  + 10   + (ngrps * 10) + ecap + pcap + icap
>> +	 *     + 4 + 5 + 4 + 5 + 5 + 6 + 6 + 6
>> +	 */
>> +	i = ((4 + gip->ngroups) * 11) + (3 * (_KERNEL_CAPABILITY_U32S * 8 + 1))
>> +		+ 41;
>> +
>> +#ifdef CONFIG_SECURITY_NETWORK
>> +	err = security_secid_to_secctx(scm->secid, &secdata, &seclen);
>> +	if (!err)
>> +		/*
>> +		 * " context="
>> +		 */
>> +		i += seclen + 10;
>> +#endif /* CONFIG_SECURITY_NETWORK */
>> +
>> +	result = kzalloc(i, GFP_KERNEL);
>> +	if (result == NULL)
>> +		return;
>> +
>> +	cp = result + sprintf(result, "euid=%d uid=%d egid=%d gid=%d",
>> +				credp->euid, credp->uid,
>> +				credp->egid, credp->gid);
>> +
>> +	if (gip != NULL && gip->ngroups > 0) {
>> +		cp += sprintf(cp, " grps=%d", GROUP_AT(gip, 0));
>> +		for (i = 1 ; i < gip->ngroups; i++)
>> +			cp += sprintf(cp, ",%d", GROUP_AT(gip, i));
>> +	}
>> +
>> +	cp += sprintf(cp, " cap-e=");
>> +	CAP_FOR_EACH_U32(i)
>> +		cp += sprintf(cp, "%08x", credp->cap_effective.cap[i]);
>> +	cp += sprintf(cp, " cap-p=");
>> +	CAP_FOR_EACH_U32(i)
>> +		cp += sprintf(cp, "%08x", credp->cap_permitted.cap[i]);
>> +	cp += sprintf(cp, " cap-i=");
>> +	CAP_FOR_EACH_U32(i)
>> +		cp += sprintf(cp, "%08x", credp->cap_inheritable.cap[i]);
>> +
>> +#ifdef CONFIG_SECURITY_NETWORK
>> +	cp += sprintf(cp, " context=");
>> +	strncpy(cp, secdata, seclen);
>> +	cp += seclen;
>> +	*cp = '\0';
>> +
>> +	security_release_secctx(secdata, seclen);
>> +#endif /* CONFIG_SECURITY_NETWORK */
>> +
>> +	put_cmsg(msg, SOL_SOCKET, SCM_PRIVILEGES, strlen(result)+1, result);
>> +
>> +	kfree(result);
>> +}
>> +
>> +
>>  static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
>>  				struct scm_cookie *scm, int flags)
>>  {
>> @@ -114,6 +191,8 @@ 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_passpriv(sock, msg, scm);
>> +
>>  	scm_destroy_cred(scm);
>>  
>>  	scm_passec(sock, msg, scm);
>> @@ -124,6 +203,5 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
>>  	scm_detach_fds(msg, scm);
>>  }
>>  
>> -
>>  #endif /* __LINUX_NET_SCM_H */
>>  
>> diff --git a/net/core/sock.c b/net/core/sock.c
>> index fb60801..f134126 100644
>> --- a/net/core/sock.c
>> +++ b/net/core/sock.c
>> @@ -725,6 +725,13 @@ set_rcvbuf:
>>  		else
>>  			clear_bit(SOCK_PASSSEC, &sock->flags);
>>  		break;
>> +
>> +	case SO_PASSPRIV:
>> +		if (valbool)
>> +			set_bit(SOCK_PASSPRIV, &sock->flags);
>> +		else
>> +			clear_bit(SOCK_PASSPRIV, &sock->flags);
>> +		break;
>>  	case SO_MARK:
>>  		if (!capable(CAP_NET_ADMIN))
>>  			ret = -EPERM;
>> @@ -950,6 +957,10 @@ int sock_getsockopt(struct socket *sock, int level, int optname,
>>  		v.val = test_bit(SOCK_PASSSEC, &sock->flags) ? 1 : 0;
>>  		break;
>>  
>> +	case SO_PASSPRIV:
>> +		v.val = test_bit(SOCK_PASSPRIV, &sock->flags) ? 1 : 0;
>> +		break;
>> +
>>  	case SO_PEERSEC:
>>  		return security_socket_getpeersec_stream(sock, optval, optlen, len);
>>  
>