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

From: [email-masked] (Eric W. Biederman)
Date: Wed, 19 Jan 2011 08:18:34 -0800

Casey Schaufler <[email-masked]> writes:

> 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.

But when you have a perfectly good binary interface when reducing the
encoding efficiency for effectively no gain.

>> 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.

Sure but you have to use an interface that properly handles variable
length binary data, to get to the string.  It feels like you are
violating the even more classic one value per file rule.

Maybe I am missing something but is there any reason you can't have
multiple cmsg types?  

> 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.

If you are worrying about variable size structures that vary
depending on kernel configuration I pretty certain you are doing
it wrong.

The use of sprintf (not snprintf) and the crazy size computation needed
for your string also worries me.  That part of the implementation
appears to be just asking for trouble.

Having a giant function like your scm_passpriv inline in
include/net/scm.h also seems very questionable.

I think there is a real impedance mismatch here between the interface
you are using and the way you are returning the data.

There is a show stopper bug.  You don't translate uid/gid in the
receivers user namespace so passing a message between two processes in
different user namespaces can pass deceptive credentials. Given that the
reason we have the struct cred on unix domain sockets in the first place
is that we handled the user namespace conversion issues so we could
cross namespaces without security issues not handling that case in a new
scm cred is just inexcusable.

On that note Serge and I are slowly working to get credentials to be
namespace local as well, so shortly the credentials will also need to be
translated as well as just uid's and gid's.

The scm->secid is if I understand correctly a per packet label.  Is that
what you want here?  I thought you were interested in the information
off of struct cred.

In principle returning all of the credential should be fine.  In
practice I think this patch has a lot of poorly thought through details
that will be a nightmare to maintain in practice.

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);
>>>  
>>