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

[Stefano Brivio <sbrivio@redhat.com>]

Well, this drop_caps() is pretty much the same as patch 8/10, so it
actually did something. :)

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,

>  }
>  
>  /**
> @@ -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;
>  }

-- 
Stefano