Re: [PATCH 07/10] isolation: Replace drop_caps() with a version that actually does something

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Oct 13, 2022 at 04:18:24AM +0200, Stefano Brivio wrote:
> Well, this drop_caps() is pretty much the same as patch 8/10, so it
> actually did something. :)

Yes, but not what we wanted :).

> On Tue, 11 Oct 2022 16:40:15 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > The current implementation of drop_caps() doesn't really work because it
> > attempts to drop capabilities from the bounding set.  hat's not the set
> > that really matters: the bounding set is about limiting the abilities of
> > otherwise things we might later exec() rather than our own capabilities.
> > In addition altering the bounding set requires CAP_SETPCAP which we won't
> > usually have.
> > 
> > Replace it with a new version which uses setcap(2) to drop capabilities
> > from the effective and permitted sets, which is what actually matters for
> > most purposes.  For now we leave the inheritable set alone, since we don't
> > want to preclude the user from passing inheritable capabilities to the
> > command spawed by pasta.
> > 
> > Correctly dropping caps reveals that we actually need CAP_SYS_ADMIN within
> > the userns we create/join in pasta mode, so that we can later setns() to
> > the netns within it.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  isolation.c | 52 ++++++++++++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 44 insertions(+), 8 deletions(-)
> > 
> > diff --git a/isolation.c b/isolation.c
> > index 4aa75e6..2468f84 100644
> > --- a/isolation.c
> > +++ b/isolation.c
> > @@ -86,18 +86,37 @@
> >  #include "passt.h"
> >  #include "isolation.h"
> >  
> > +#define CAP_VERSION	_LINUX_CAPABILITY_VERSION_3
> > +#define CAP_WORDS	_LINUX_CAPABILITY_U32S_3
> > +
> >  /**
> > - * drop_caps() - Drop capabilities we might have except for CAP_NET_BIND_SERVICE
> > + * drop_caps_ep_except() - Drop capabilities from effective & permitted sets
> > + * @keep:	Capabilities to keep
> >   */
> > -static void drop_caps(void)
> > +static void drop_caps_ep_except(uint64_t keep)
> >  {
> > +	struct __user_cap_header_struct hdr = {
> > +		.version = CAP_VERSION,
> > +		.pid = 0,
> > +	};
> > +	struct __user_cap_data_struct data[CAP_WORDS];
> >  	int i;
> >  
> > -	for (i = 0; i < 64; i++) {
> > -		if (i == CAP_NET_BIND_SERVICE)
> > -			continue;
> > +	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++) {
> > +		uint32_t mask = keep >> (32 * i);
> > +
> > +		data[i].effective &= mask;
> > +		data[i].permitted &= mask;
> > +	}
> >  
> > -		prctl(PR_CAPBSET_DROP, i, 0, 0, 0);
> > +	if (syscall(SYS_capset, &hdr, data)) {
> > +		err("Couldn't drop capabilities: %s", strerror(errno));
> > +		exit(EXIT_FAILURE);
> >  	}
> >  }
> >  
> > @@ -111,7 +130,11 @@ static void drop_caps(void)
> >   */
> >  void isolate_initial(void)
> >  {
> > -	drop_caps();
> > +	/* We want to keep CAP_NET_BIND_SERVICE in the initial
> > +	 * namespace if we have it, so that we can forward low ports
> > +	 * into the guest/namespace
> > +	 */
> > +	drop_caps_ep_except((1UL << CAP_NET_BIND_SERVICE));
> 
> You could use BIT() (util.h) here,

Ah, yes, done.

> >  }
> >  
> >  /**
> > @@ -211,6 +234,7 @@ void isolate_user(uid_t uid, gid_t gid, bool use_userns, const char *userns)
> >  int isolate_prefork(struct ctx *c)
> >  {
> >  	int flags = CLONE_NEWIPC | CLONE_NEWNS | CLONE_NEWUTS;
> > +	uint64_t ns_caps = 0;
> >  
> >  	/* If we run in foreground, we have no chance to actually move to a new
> >  	 * PID namespace. For passt, use CLONE_NEWPID anyway, in case somebody
> > @@ -251,7 +275,19 @@ int isolate_prefork(struct ctx *c)
> >  		return -errno;
> >  	}
> >  
> > -	drop_caps();	/* Relative to the new user namespace this time. */
> > +	/* Drop capabilites in our new userns */
> > +	if (c->mode == MODE_PASTA) {
> > +		/* Keep CAP_SYS_ADMIN, so that we can setns() to the
> > +		 * netns when we need to act upon it
> > +		 */
> > +		ns_caps |= 1UL << CAP_SYS_ADMIN;
> 
> here,
> 
> > +		/* Keep CAP_NET_BIND_SERVICE, so we can splice
> > +		 * outbound connections to low port numbers
> > +		 */
> > +		ns_caps |= 1UL << CAP_NET_BIND_SERVICE;
> 
> and here.
> 
> > +	}
> > +
> > +	drop_caps_ep_except(ns_caps);
> >  
> >  	return 0;
> >  }
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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