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linux-kernel: Re: ?????: VFS, NFS security bug? Should CAP_MKNOD

Re: ?????: VFS, NFS security bug? Should CAP_MKNOD and CAP_LINUX_IMMUTABLE be added to CAP_FS_MASK?

From: Casey Schaufler <casey_at_nospam>
Date: Wed Mar 18 2009 - 16:17:41 GMT
To: "Serge E. Hallyn" <>

Serge E. Hallyn wrote:
> Quoting Stephen Smalley (
>> On Tue, 2009-03-17 at 12:39 -0500, Serge E. Hallyn wrote: >> >>> Quoting Stephen Smalley ( >>> >>>>> 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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