[PATCH 15/19] capabilities: Allow privileged user in s_user_ns to set file caps

[Seth Forshee]

On Fri, Dec 04, 2015 at 01:42:06PM -0600, Serge E. Hallyn wrote:
> Quoting Seth Forshee (seth.forshee at canonical.com):
> > A privileged user in a super block's s_user_ns is privileged
> > towards that file system and thus should be allowed to set file
> > capabilities. The file capabilities will not be trusted outside
> > of s_user_ns, so an unprivileged user cannot use this to gain
> > privileges in a user namespace where they are not already
> > privileged.
> > 
> > Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
> > ---
> >  security/commoncap.c | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/security/commoncap.c b/security/commoncap.c
> > index 2119421613f6..d6c80c19c449 100644
> > --- a/security/commoncap.c
> > +++ b/security/commoncap.c
> > @@ -653,15 +653,17 @@ int cap_bprm_secureexec(struct linux_binprm *bprm)
> >  int cap_inode_setxattr(struct dentry *dentry, const char *name,
> >  		       const void *value, size_t size, int flags)
> >  {
> > +	struct user_namespace *user_ns = dentry->d_sb->s_user_ns;
> > +
> >  	if (!strcmp(name, XATTR_NAME_CAPS)) {
> > -		if (!capable(CAP_SETFCAP))
> > +		if (!ns_capable(user_ns, CAP_SETFCAP))
> >  			return -EPERM;
> 
> This, for file capabilities, is fine,
> 
> >  		return 0;
> >  	}
> >  
> >  	if (!strncmp(name, XATTR_SECURITY_PREFIX,
> >  		     sizeof(XATTR_SECURITY_PREFIX) - 1) &&
> > -	    !capable(CAP_SYS_ADMIN))
> > +	    !ns_capable(user_ns, CAP_SYS_ADMIN))
> 
> but this is for all other security.*.
> 
> It's probably still ok, but let's think about it a sec.  MAC like
> selinux or smack should be orthogonal to DAC.  Capabilities are the
> same in essence, but the reason they can be treated differently here
> is because capabilties are in fact targated at a user namespace.
> Apparmor namespaces, for instance, are completely orthogonal to user
> namespaces, as are contexts in selinux.
> 
> Now, if smack or selinux xattrs are being set then those modules
> should be gating these writes.  Booting a kernel without those
> modules should be a challenge for an untrusted user.  But such a
> situation could be exploited opportunistically if it were to happen.
> 
> The problem with simply not changing this here is that if selinux
> or smack authorizes the xattr write, then commoncap shouldn't be
> denying it.

This is partly the logic behind the change, the other part being that
the user could already insert the xattrs directly into the backing store
so the LSMs must be prepared not to trust them in any case. But the
commit message doesn't explain that, my mistake. And it's a question
worth revisiting.

> I get the feeling we need cooperation among the modules (i.e. "if
> the write is to 'security.$lsm' and $lsm is not loaded, then require
> capable(CAP_SYS_ADMIN), else just allow)  But that's not how things are
> structured right now.
> 
> Maybe security.ko could grow central logic to 'assign' security.*
> capabilities to specific lsms and gate writes to those if $lsm is not
> loaded.

I don't see any meaningful difference between this case and the case of
inserting them into the backing store before mounting. We can't do
anything to prevent the latter, so LSMs just have to be aware of
unprivileged mounts and handle them with care. Previous patches do this
for SELinux and Smack by adopting a policy that doesn't respect security
labels on disk for these mounts. So I don't think that refusing to set
security.* xattrs for an LSM that isn't loaded really accomplishes
anything.

Then there's the case of setting xattrs for an LSM that is currently
loaded. I think that SELinux and Smack are both going to refuse these
writes, Smack rather directly by seeing that the user lacks global
CAP_MAC_ADMIN and SELinux by virtue of the fact that the previous patch
in this series applies mountpoint labeling to these mounts. As far as I
can tell the other LSMs don't take security policy from xattrs.

So, as far as I can tell, removing the check doesn't create any
vulnerabilities.

But that's not to say it's the right thing to do. After reconsidering
it, I'm inclined to be more conservative and to keep requiring
capable(CAP_SYS_ADMIN) until such time as there's a use case for
allowing a user privileged only in s_user_ns to write these xattrs.

> Does anything break if the second hunk in each fn in this patch is
> not applied?

Not that I'm aware of, no.

Seth