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

From: Casey Schaufler <[email-masked]>
Date: Wed, 19 Jan 2011 12:40:12 -0800

On 1/19/2011 8:18 AM, Eric W. Biederman wrote:
> Casey Schaufler <[email-masked]> writes:
>
>> On 1/18/2011 9:45 PM, Eric W. Biederman wrote:

Quite a bit of work to do here, it would appear.

>>> 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.

Problem is that with a group list, capability sets, and LSM "context"
I don't have a perfectly good binary interface.

>>> 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.

This way leaves all parsing to user space, and uses well understood
facilities to create something that isn't going to require internal
counters and the like.

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

Sure, there could be a cmsg for each value, but if you know that
you're going to always use them all, why make them separate?

>> 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.

I'm seriously more concerned with the maintainability of cap sets.
I saw the pain level when they went to 2x32bit and any structure
that includes a cap set, especially a user visible interface, is
going to be an issue. Not to mention that there's a group list in
there as well.

> 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.

snprintf would be a simple change. The computation is just arithmetic.

> 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.

The issues with uid and gid translation appear to be easy to fix,
assuming that the cred_to_ucred() function provides a reasonable
example.

> 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.

Oh dear. Does that include capability sets and LSM blobs?

> 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.

The intent is to pull in the access control information so
that the (trusted) application can make decisions based on
its own set of criteria. The kernel is making access checks
(in SELinux and Smack at any rate) based on the packet label,
but it might make sense to provide both.

> 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);
>>>>  
>