Re: [RFC PATCH] selinux: add a fallback to defcontext for native labeling

[Stephen Smalley]

On 10/02/2018 02:48 PM, Taras Kondratiuk wrote:
> Quoting Stephen Smalley (2018-09-21 07:40:58)
> > If we set the inode sid to the superblock def_sid on an invalid
> > context, then we lose the association to the original context value.
> > The support for deferred mapping of contexts requires allocating a new
> > SID for the invalid context and storing that SID in the inode, so that
> > if the context later becomes valid upon a policy update/reload, the
> > inode SID will refer to the now valid context.
> > To combine the two, we would need security_context_to_sid_core() to
> > save the def_sid in the context structure for invalid contexts, and
> > change sidtab_search_core() to use that value instead of
> > SECINITSID_UNLABELED for invalid SIDs.  Then the inode would be
> > treated as having the defcontext for access control and getxattr() w/o
> > CAP_MAC_ADMIN purposes, but a subsequent policy update/reload that
> > makes the context valid would automatically cause subsequent accesses
> > to the inode to start using the original context value for access
> > control and getxattr() purposes.  I think that's the behavior you
> > want.
>
> While implementing the change I've realized that storing default context
> for sidtab_search_core() in the context structure is not enough to
> achieve the desired behavior. The same invalid context may exist in two
> mounts with different 'defcontext', so default context can't be a
> property of a context structure.
>
> One way to address it is to propagate default context to sidtab_search() all
> the way from inode hooks. But that will be a bit intrusive. Something like
> avc_has_perm_default() will need to be added.
>
> Other way is to check for context validity in inode hooks and provide a default
> context to avc_has_perm() if the inode's sid is invalid. But this may have
> performance implication since validity check will be done each time in fast
> path.
>
> Do you see any other options?

I think the original approach is still best.  The def_sid can be part of 
the context structure; you just need to update context_cmp() to compare 
it (as part of the first return statement) so that the same invalid 
context in two mounts with different defcontext= values will allocate 
different SIDs.  Likewise context_cpy() needs to copy the def_sid.

As a separate matter, you should also modify 
security_context_to_sid_force() to pass down the sbsec->def_sid so that 
when userspace with CAP_MAC_ADMIN sets an invalid context on an inode, 
the default context will be used in the same manner.