linux-kernel - Re: ?????: VFS, NFS security bug? Should CAP_MKNOD and CAP_LINUX_IMMUTABLE be added to CAP_FS

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Date:	Wed, 18 Mar 2009 09:17:41 -0700
From:	Casey Schaufler <casey@...aufler-ca.com>
To:	"Serge E. Hallyn" <serue@...ibm.com>
CC:	Stephen Smalley <sds@...ho.nsa.gov>,
	Igor Zhbanov <izh1979@...il.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	neilb@...e.de, Trond.Myklebust@...app.com,
	David Howells <dhowells@...hat.com>,
	Andrew Morgan <morgan@...nel.org>,
	James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	SELinux <selinux@...ho.nsa.gov>
Subject: Re: ?????: VFS, NFS security bug? Should CAP_MKNOD and CAP_LINUX_IMMUTABLE be added to CAP_FS_MASK?

Serge E. Hallyn wrote:
> Quoting Stephen Smalley (sds@...ho.nsa.gov):
>   
>> On Tue, 2009-03-17 at 12:39 -0500, Serge E. Hallyn wrote:
>>     
>>> Quoting Stephen Smalley (sds@...ho.nsa.gov):
>>>       
>>>>> So do you think it makes sense to have CAP_MAC_ADMIN and CAP_FOWNER
>>>>> in CAP_FS_MASK?  In other words are you objecting to CAP_SYS_ADMIN
>>>>> because of all of its other implications, or because you disagree
>>>>> that labels for security modules should be treated as mere fs data
>>>>> here?
>>>>>           
>>>> For CAP_FOWNER, yes (and it is already there).  CAP_MAC_ADMIN is less
>>>>         
>>> Sorry, I meant CAP_SETFCAP.  Should it be added?
>>>       
>> Sure - it is only used for filesystem operations.
>>     
>
> Ok, so then:
>
>   
>>>> ideal as it isn't clearly tied to filesystem accesses, and likewise for
>>>> CAP_MAC_OVERRIDE (but that one is included in CAP_FS_MASK already).
>>>>         
>>> So it is.  I didn't realize that.
>>>
>>>       
>>>> Ideally the capability space would be partitioned into capabilities that
>>>> affect filesystem accesses and the rest so that setfsuid() would yield
>>>> the expected behavior of only affecting filesystem access.
>>>> CAP_SYS_ADMIN is even less suitable due to its pervasive use outside of
>>>> the filesystem.  So that's the first concern.
>>>>
>>>> The second one is that we don't want CAP_SYS_ADMIN (or CAP_MAC_ADMIN) to
>>>> be required when setting SELinux labels.  Only the SELinux permission
>>>> checks should govern setting those labels (aside from the usual DAC
>>>> ownership || CAP_FOWNER check).
>>>>         
>>> So if a non-selinux kernel is booted, then you think only the usual
>>> DAC checks should be required to set selinux labels?
>>>       
>> I'm talking about the dumb NFS server case (non-SELinux NFS server
>> providing label and data storage to SELinux clients, MAC enforcement
>> handled client-side).  But we aren't there yet, so I don't think we have
>> to worry about it right now.
>>     
>
> But in cap_inode_setxattr, any security.* xattrs are controlled by
> CAP_SYS_ADMIN.  So do you think that this should be changed to a
> CAP_XATTR_SECURITY capability which can be added to CAP_FS_MASK?
>   

Hum. The intention of CAP_MAC_ADMIN was that it control the explicit
setting of the access control attributes used by the Smack LSM. I
personally prefer a single capability for the action over multiple
capabilities based on the objects involved. If you introduce
CAP_XATTR_SECURITY I would think that CAP_PROC_XATTR,
CAP_SVIPC_XATTR, CAP_NETWORK_XATTR, ... would follow in short order
and I hate the idea of having hundreds of capabilities. If you
must decouple the capability from MAC, how about a new name?

> Or do you think it's ok that fsuid=0 does not allow you to set
> security.selinux (or security.SMACK64, etc) xattrs when selinux is
> not compiled in?
>
> (You may have already answered this with your EOPNOTSUPP comment, but
> I want to make sure I understand right)
>
>   
>>> Does anyone know what the trusted xattrs are used for?
>>>       
>> Not beyond what attr(5) says about them.
>>     
>
> Well, if attr(5) says CAP_SYS_ADMIN being needed is the very
> thing that defines these xattrs, then changing that seems a
> bigger deal.  That really does seem akin to changing kernel-user
> API.
>
> thanks,
> -serge
> --
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>
>   

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