[lxc-devel] [PATCH RFC] Introduce new security.nscapability xattr

[Jann Horn]

On Fri, Dec 04, 2015 at 02:21:16PM -0600, Serge E. Hallyn wrote:
> Quoting Eric W. Biederman (ebiederm at xmission.com):
> > "Serge E. Hallyn" <serge.hallyn at ubuntu.com> writes:
> > 
> > > A common way for daemons to run with minimal privilege is to start as root,
> > > perhaps setuid-root, choose a desired capability set, set PR_SET_KEEPCAPS,
> > > then change uid to non-root.  A simpler way to achieve this is to set file
> > > capabilities on a not-setuid-root binary.  However, when installing a package
> > > inside a (user-namespaced) container, packages cannot be installed with file
> > > capabilities.  For this reason, containers must install ping setuid-root.
> > 
> > Don't ping sockets avoid that specific problem?
> > 
> > I expect the general case still holds.
> > 
> > > To achieve this, we would need for containers to be able to request file
> > > capabilities be added to a file without causing these to be honored in the
> > > initial user namespace.
> > >
> > > To this end, the patch below introduces a new capability xattr format.  The
> > > main enhancement over the existing security.capability xattr is that we
> > > tag capability sets with a uid - the uid of the root user in the namespace
> > > where the capabilities are set.  The capabilities will be ignored in any
> > > other namespace.  The special case of uid == -1 (which must only ever be
> > > able to be set by kuid 0) means use the capabilities in all
> > > namespaces.
> 
> really since security.capability xattrs are currently honored in all
> namespaces this isn't really necessary.  Until and unless Seth's set
> changes that.
> 
> > 
> > A quick comment on this.
> > 
> > We currently allow capabilities that have been gained to be valid in all
> > descendent user namespaces.
> > 
> > Applying this principle to the on-disk capabilities would make it so
> > that uid 0 would mean capabilities in all namespaces.
> > 
> > It might be worth it to introduce a fixed sized array with a length
> > parameter of perhaps 32 entries which is a path of root uids as seen by
> > the initial user namespace.  That way the entire construction of the
> > user namespace could be verified.  AKA verify the current user namespace
> > and the parent and the parents parent.  Up to the user namespace the
> > current filesystem is mounted in. We would look at how much space
> > allows an xattr to be stored without causing filesystems a challenge
> > to properly size such an array.
> > 
> > Given that uids are fundamentally flat that might not be particularly
> > useful.  If we add an alternative way of identifying user namespaces
> > say a privileged operation that set a uuid, then the complete path would
> > be more interesting.
> > 
> > > An alternative format would use a pair of uids to indicate a range of rootids.
> > > This would allow root in a user namespace with uids 100000-165536 mapped to
> > > set the xattr once on a file, then launch nested containers wherein the file
> > > could be used with privilege.  That's not what this patch does, but would be
> > > a trivial change if people think it would be worthwhile.
> > >
> > > This patch does not actually address the real problem, which is setting the
> > > xattrs from inside containers.  For that, I think the best solution is to
> > > add a pair of new system calls, setfcap and getfcap. Userspace would for
> > > instance call fsetfcap(fd, cap_user_header_t, cap_user_data_t), to which
> > > the kernel would, if not in init_user_ns, react by writing an appropriate
> > > security.nscapability xattr.
> > 
> > That feels hard to maintain, but you may be correct that we have a small
> > enough userspace that it would not be a problem.
> > 
> > Eric
> > 
> > 
> > > The libcap2 library's cap_set_file/cap_get_file could be switched over
> > > transparently to use this to hide its use from all callers.
> > >
> > > Comments appreciated.
> > >
> > > Note - In this patch, file capabilities only work for containers which have
> > > a root uid defined.  We may want to allow -1 uids to work in all
> > > namespaces.  There certainly would be uses for this, but I'm a bit unsettled
> > > about the implications of allowing a program privilege in a container where
> > > there is no uid with privilege.  This needs more thought.
> 
> So for actually enabling (user-namespaced) containers to use these, there
> are a few possibilities that come to mine.
> 
> 1. A new setfcap (/getfcap) syscall.  Uses mapped uid 0 from
> current_user_ns() to write a value in the security.nscapability xattr.
> Userspace doesn't need to worry at all about namespace issues.

Your patch has an "ncaps" field and supports giving (possibly different)
privileges to different namespace root users.
I think that with a setfcap() syscall as you describe, it would be hard
to add more than one security.nscapability entry to a file without
poking holes into stuff:

If setfcap() requires inode write access, every namespace root for whom
a security.nscapability entry can be added can overwrite the file, so
namespace roots might be able to execute code as each other.

If setfcap() doesn't require inode write access, that isn't a big
security issue, but it allows unprivileged users to waste disk space
in a writable-mounted filesystem on which they have no write access
anywhere. I'm not sure whether that is a sufficiently big problem for
anyone to care.


Another issue with this would be that it makes restoring sub-namespace
capabilities from a backup somewhat ugly, at least if you're not in the
init ns: You'd have to parse the capabilities attribute, then, for every
attribute you want to restore whose rootid doesn't refer to your
namespace, clone(CLONE_NEWUSER), map the target uid to 0 from the parent
process, seteuid(0), do the setfcap() and exit.

It would require less synchronization with my ptrace hardening patch
that checks whether uids are mapped (if that lands, I don't think it has
yet?) because that would allow you to first setuid, then clone(), map
the uid to 0 inside the namespace and setfcap(), but it'd still require
special-case code in backup software.


> 2. Just expect userspace to write a xattr;  kernel checks that no values
> are changed for any other namespaces.  This could be a lot of parsing and
> verifying in the kernel.

This option would allow the first problem I described with option 1 (if
it is a problem?) to be worked around by letting a helper in the outer
namespace add capabilities for lower namespaces - but it wouldn't be
pretty.

I think it would also allow tar, which already supports xattrs, to
deal with namespace capabilities without needing any additional code.
I think that would be nice to have.


> 3. Switch the xattr scheme - instead of one security.nscapability xattr
> with multiple entries, use security.nscapability.$(rootid).  Now the
> kernel only needs to verify that the $rootid is valid for the writing
> task, and we don't need a new syscall.  OTOH userspace needs to know
> what it's doing.  Of course we can still hide that behind libcap2's helpers.

This doesn't sound so good - how would the namespace know its rootid?
If it is just one level below init_ns, it can grab it from
/proc/self/uid_map, but for deeper levels that won't work.


> Any opinions on which way seems best?  1 does seem cleanest (and supports
> use of seccomp if we want to forbit its use by some containers), but
> involves a new pair of syscalls.  2 seems to me to be right out, but
> others might disagree...
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