Re: [PATCH 08/10] isolation: Prevent any child processes gaining capabilities

[Stefano Brivio <sbrivio@redhat.com>]

On Thu, 13 Oct 2022 20:33:34 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Thu, Oct 13, 2022 at 04:17:30AM +0200, Stefano Brivio wrote:
> > On Tue, 11 Oct 2022 16:40:16 +1100
> > David Gibson <david@gibson.dropbear.id.au> wrote:
> >   
> > > We drop our own capabilities, but it's possible that processes we exec()
> > > could gain extra privilege via file capabilities.  It shouldn't be possible
> > > for us to exec() anyway due to seccomp() and our filesystem isolation.  But
> > > just in case, zero the bounding and inheritable capability sets to prevent
> > > any such child from gainin privilege.
> > > 
> > > Note that we do this *after* spawning the pasta shell/command (if any),
> > > because we do want the user to be able to give that privilege if they want.
> > > 
> > > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > > ---
> > >  isolation.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 56 insertions(+)
> > > 
> > > diff --git a/isolation.c b/isolation.c
> > > index 2468f84..e1a024d 100644
> > > --- a/isolation.c
> > > +++ b/isolation.c
> > > @@ -120,6 +120,61 @@ static void drop_caps_ep_except(uint64_t keep)
> > >  	}
> > >  }
> > >  
> > > +/**
> > > + * clamp_caps() - Prevent any children from gaining caps  
> > 
> > "clamp" doesn't sound very specific or clear. caps_drop_inherit_bound()
> > would actually tell me what the function does, but it's a bit of a
> > mouthful in comparison. I'm fine with both, really, but if you have a
> > better idea...  
> 
> Yeah, I couldn't think of something that was both brief and specific,
> so I went with brief.
> 
> > > + *
> > > + * This drops all capabilities from both the inheritable and the
> > > + * bounding set.  This means that any exec()ed processes can't gain
> > > + * capabilities, even if they have file capabilities which would grant
> > > + * them.  We shouldn't ever exec() in any case, but this provides an
> > > + * additional layer of protection.  Executing this requires
> > > + * CAP_SETPCAP, which we will have within our userns.
> > > + *
> > > + * Note that dropping capabilites from the bounding set limits
> > > + * exec()ed processes, but does not remove them from the effective or
> > > + * permitted sets, so it doesn't reduce our own capabilities.
> > > + */
> > > +static void clamp_caps(void)
> > > +{
> > > +	struct __user_cap_header_struct hdr = {
> > > +		.version = CAP_VERSION,
> > > +		.pid = 0,
> > > +	};
> > > +	struct __user_cap_data_struct data[CAP_WORDS];  
> > 
> > For consistency, I'd move this before hdr.  
> 
> Ok.
> 
> > > +	int i;
> > > +
> > > +	for (i = 0; i < 64; i++) {
> > > +		/* Some errors can be ignored:
> > > +		 * - EINVAL, we'll get this for all values in 0..63
> > > +		 *   that are not actually allocated caps
> > > +		 * - EPERM, we'll get this if we don't have
> > > +		 *   CAP_SETPCAP, which can happen if using
> > > +		 *   --netns-only.  We don't need CAP_SETPCAP for
> > > +		 *   normal operation, so carry on without it.
> > > +		 */
> > > +		if (prctl(PR_CAPBSET_DROP, i, 0, 0, 0) &&
> > > +		    errno != EINVAL && errno != EPERM) {
> > > +			err("Couldn't drop cap %i from bounding set: %s",
> > > +			    i, strerror(errno));
> > > +			exit(EXIT_FAILURE);
> > > +		}
> > > +	}
> > > +
> > > +	if (syscall(SYS_capget, &hdr, data)) {
> > > +		err("Couldn't get current capabilities: %s", strerror(errno));
> > > +		exit(EXIT_FAILURE);
> > > +	}
> > > +
> > > +	for (i = 0; i < CAP_WORDS; i++)
> > > +		data[i].inheritable = 0;  
> > 
> > Any specific reason why? Initialisers can have variable sizes to some
> > extent, but if there's a reason why it can't be done, perhaps that
> > would warrant a comment here.  
> 
> Why what?  We're not trying to alter the permitted or effective sets
> here, so we're doing a capget() first, zeroing the inheritable field,
> then setting it back again.

Oops, never mind, of course, I missed the capget() for a moment.

-- 
Stefano