[PATCH v2 2/4] capabilities: Add securebit to restrict userns caps

[Serge E. Hallyn]

On Mon, Jun 10, 2024 at 02:46:06AM -0700, Jonathan Calmels wrote:
> On Sun, Jun 09, 2024 at 09:33:01PM GMT, Serge E. Hallyn wrote:
> > On Sun, Jun 09, 2024 at 03:43:35AM -0700, Jonathan Calmels wrote:
> > > This patch adds a new capability security bit designed to constrain a
> > > task’s userns capability set to its bounding set. The reason for this is
> > > twofold:
> > > 
> > > - This serves as a quick and easy way to lock down a set of capabilities
> > >   for a task, thus ensuring that any namespace it creates will never be
> > >   more privileged than itself is.
> > > - This helps userspace transition to more secure defaults by not requiring
> > >   specific logic for the userns capability set, or libcap support.
> > > 
> > > Example:
> > > 
> > >     # capsh --secbits=$((1 << 8)) --drop=cap_sys_rawio -- \
> > >             -c 'unshare -r grep Cap /proc/self/status'
> > >     CapInh: 0000000000000000
> > >     CapPrm: 000001fffffdffff
> > >     CapEff: 000001fffffdffff
> > >     CapBnd: 000001fffffdffff
> > >     CapAmb: 0000000000000000
> > >     CapUNs: 000001fffffdffff
> > 
> > But you are not (that I can see, in this or the previous patch)
> > keeping SECURE_USERNS_STRICT_CAPS in securebits on the next
> > level unshare.  Though I think it's ok, because by then both
> > cap_userns and cap_bset are reduced and cap_userns can't be
> > expanded.  (Sorry, just thinking aloud here)
> 
> Right this is safe to reset, but maybe we do keep it if the secbit is
> locked? This is kind of a special case compared to the other bits.

I don't think it would be worth the extra complication in the
secbits code, and it's semantically very different from the
cap_userns.

> > > +	/* Limit userns capabilities to our parent's bounding set. */
> > 
> > In the case of userns_install(), it will be the target user namespace
> > creator's bounding set, right?  Not "our parent's"?
> 
> Good point, I should reword this comment.