Subject: Re: [PATCH] net: change capability used by socket options IP{,V6}_TRANSPARENT

From: Casey Schaufler <[email-masked]>
Date: Fri, 23 Sep 2011 09:36:57 -0700

On 9/22/2011 4:29 PM, Maciej Żenczykowski wrote:
> From: Maciej Żenczykowski <[email-masked]>
>
> Up till now the IP{,V6}_TRANSPARENT socket options (which actually set
> the same bit in the socket struct) have required CAP_NET_ADMIN
> privileges to set or clear the option.
>
> - we make clearing the bit not require any privileges.
> - we deprecate using CAP_NET_ADMIN for this purpose.
> - we introduce a new capability CAP_NET_TRANSPARENT,
>   which is tailored to allow setting just this bit.

Under what circumstances would a process that requires the
new capability not require CAP_NET_ADMIN? Is there a real
case where a process would be expected to require only this
new capability? Adding new capability values is somewhat
perilous and the granularity you are proposing, that of
controlling a single bit, would explode the list of
capabilities into the hundreds if it were applied throughout
the kernel.

> - we allow either one of CAP_NET_TRANSPARENT or CAP_NET_RAW
>   to set this bit, because raw sockets already effectively
>   allow you to emulate socket transparency, and make the
>   transition easier for apps not desiring to use a brand
>   new capability (because of header file or glibc support)
> - we print a warning (but allow it) if you try to set
>   the socket option with CAP_NET_ADMIN privs, but without
>   either one of CAP_NET_TRANSPARENT or CAP_NET_RAW.
>
> The reason for introducing a new capability is that while
> transparent sockets are potentially dangerous (and can let you
> spoof your source IP on traffic), they don't normally give you
> the full 'freedom' of eavesdropping and/or spoofing that raw sockets
> give you.
>
> Signed-off-by: Maciej Żenczykowski <[email-masked]>
> Acked-by: Balazs Scheidler <[email-masked]>
> Acked-by: David Miller <[email-masked]>
> ---
>  include/linux/capability.h |   13 +++++++++----
>  net/ipv4/ip_sockglue.c     |   26 ++++++++++++++++++++++----
>  net/ipv6/ipv6_sockglue.c   |   29 ++++++++++++++++++++++++-----
>  3 files changed, 55 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index c421123..a115ed4 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -198,7 +198,7 @@ struct cpu_vfs_cap_data {
>  /* Allow modification of routing tables */
>  /* Allow setting arbitrary process / process group ownership on
>     sockets */
> -/* Allow binding to any address for transparent proxying */
> +/* Allow binding to any address for transparent proxying (deprecated) */
>  /* Allow setting TOS (type of service) */
>  /* Allow setting promiscuous mode */
>  /* Allow clearing driver statistics */
> @@ -210,6 +210,7 @@ struct cpu_vfs_cap_data {
>  
>  /* Allow use of RAW sockets */
>  /* Allow use of PACKET sockets */
> +/* Allow binding to any address for transparent proxying */
>  
>  #define CAP_NET_RAW          13
>  
> @@ -332,7 +333,7 @@ struct cpu_vfs_cap_data {
>  
>  #define CAP_AUDIT_CONTROL    30
>  
> -#define CAP_SETFCAP	     31
> +#define CAP_SETFCAP          31
>  
>  /* Override MAC access.
>     The base kernel enforces no MAC policy.
> @@ -357,10 +358,14 @@ struct cpu_vfs_cap_data {
>  
>  /* Allow triggering something that will wake the system */
>  
> -#define CAP_WAKE_ALARM            35
> +#define CAP_WAKE_ALARM       35
> +
> +/* Allow binding to any address for transparent proxying */
> +
> +#define CAP_NET_TRANSPARENT  36
>  
>  
> -#define CAP_LAST_CAP         CAP_WAKE_ALARM
> +#define CAP_LAST_CAP         CAP_NET_TRANSPARENT
>  
>  #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
>  
> diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
> index 8905e92..44efa39 100644
> --- a/net/ipv4/ip_sockglue.c
> +++ b/net/ipv4/ip_sockglue.c
> @@ -961,12 +961,30 @@ mc_msf_out:
>  		break;
>  
>  	case IP_TRANSPARENT:
> -		if (!capable(CAP_NET_ADMIN)) {
> -			err = -EPERM;
> -			break;
> -		}
>  		if (optlen < 1)
>  			goto e_inval;
> +		/* Always allow clearing the transparent proxy socket option.
> +		 * The pre-3.2 permission for setting this was CAP_NET_ADMIN,
> +		 * and this is still supported - but deprecated.  As of Linux
> +		 * 3.2 the proper permission is one of CAP_NET_TRANSPARENT
> +		 * (preferred, a new capability) or CAP_NET_RAW.  The latter
> +		 * is supported to make the transition easier (and because
> +		 * raw sockets already effectively allow one to emulate
> +		 * socket transparency).
> +		 */
> +		if (!!val && !capable(CAP_NET_TRANSPARENT)
> +		          && !capable(CAP_NET_RAW)) {
> +			if (!capable(CAP_NET_ADMIN)) {
> +				err = -EPERM;
> +				break;
> +			}
> +			printk_once(KERN_WARNING "%s (%d): "
> +				 "deprecated: attempt to set socket option "
> +				 "IP_TRANSPARENT with CAP_NET_ADMIN but "
> +				 "without either one of CAP_NET_TRANSPARENT "
> +				 "or CAP_NET_RAW.\n",
> +				 current->comm, task_pid_nr(current));
> +		}
>  		inet->transparent = !!val;
>  		break;
>  
> diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
> index 2fbda5f..b8315c8 100644
> --- a/net/ipv6/ipv6_sockglue.c
> +++ b/net/ipv6/ipv6_sockglue.c
> @@ -343,13 +343,32 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
>  		break;
>  
>  	case IPV6_TRANSPARENT:
> -		if (!capable(CAP_NET_ADMIN)) {
> -			retv = -EPERM;
> -			break;
> -		}
>  		if (optlen < sizeof(int))
>  			goto e_inval;
> -		/* we don't have a separate transparent bit for IPV6 we use the one in the IPv4 socket */
> +		/* Always allow clearing the transparent proxy socket option.
> +		 * The pre-3.2 permission for setting this was CAP_NET_ADMIN,
> +		 * and this is still supported - but deprecated.  As of Linux
> +		 * 3.2 the proper permission is one of CAP_NET_TRANSPARENT
> +		 * (preferred, a new capability) or CAP_NET_RAW.  The latter
> +		 * is supported to make the transition easier (and because
> +		 * raw sockets already effectively allow one to emulate
> +		 * socket transparency).
> +		 */
> +		if (valbool && !capable(CAP_NET_TRANSPARENT)
> +		            && !capable(CAP_NET_RAW)) {
> +			if (!capable(CAP_NET_ADMIN)) {
> +				retv = -EPERM;
> +				break;
> +			}
> +			printk_once(KERN_WARNING "%s (%d): "
> +				 "deprecated: attempt to set socket option "
> +				 "IPV6_TRANSPARENT with CAP_NET_ADMIN but "
> +				 "without either one of CAP_NET_TRANSPARENT "
> +				 "or CAP_NET_RAW.\n",
> +				 current->comm, task_pid_nr(current));
> +		}
> +		/* we don't have a separate transparent bit for IPV6 we use the
> +		 * one in the IPv4 socket */
>  		inet_sk(sk)->transparent = valbool;
>  		retv = 0;
>  		break;

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