Re: [PATCH 1/3] Security: Add disablenetwork interface. (v4)

[Serge E. Hallyn]

Quoting Michael Stone (michael@xxxxxxxxxx):
> Daniel Bernstein has observed [1] that security-conscious userland processes
> may benefit from the ability to irrevocably remove their ability to create,
> bind, connect to, or send messages except in the case of previously connected
> sockets or AF_UNIX filesystem sockets.
> 
> This patch provides
> 
>    * a new configuration option named CONFIG_SECURITY_DISABLENETWORK,
>    * a new prctl option-pair (PR_SET_NETWORK, PR_GET_NETWORK),
>    * a new prctl(PR_SET_NETWORK) flag named PR_NETWORK_OFF, and
>    * a new task_struct flags field named "network"
> 
> Signed-off-by: Michael Stone <michael@xxxxxxxxxx>
> ---
>   include/linux/prctl.h         |    7 +++++
>   include/linux/prctl_network.h |    7 +++++
>   include/linux/sched.h         |    4 +++
>   kernel/sys.c                  |   53 +++++++++++++++++++++++++++++++++++++++++
>   security/Kconfig              |   11 ++++++++
>   5 files changed, 82 insertions(+), 0 deletions(-)
>   create mode 100644 include/linux/prctl_network.h
> 
> diff --git a/include/linux/prctl.h b/include/linux/prctl.h
> index a3baeb2..4eb4110 100644
> --- a/include/linux/prctl.h
> +++ b/include/linux/prctl.h
> @@ -102,4 +102,11 @@
>   
>   #define PR_MCE_KILL_GET 34
>   
> +/* Get/set process disable-network flags */
> +#define PR_SET_NETWORK	35
> +#define PR_GET_NETWORK	36
> +# define PR_NETWORK_ON        0
> +# define PR_NETWORK_OFF       1
> +# define PR_NETWORK_ALL_FLAGS 1
> +
>   #endif /* _LINUX_PRCTL_H */
> diff --git a/include/linux/prctl_network.h b/include/linux/prctl_network.h
> new file mode 100644
> index 0000000..d18f8cb
> --- /dev/null
> +++ b/include/linux/prctl_network.h
> @@ -0,0 +1,7 @@
> +#ifndef _LINUX_PRCTL_NETWORK_H
> +#define _LINUX_PRCTL_NETWORK_H
> +
> +extern long prctl_get_network(unsigned long*);
> +extern long prctl_set_network(unsigned long*);
> +
> +#endif /* _LINUX_PRCTL_NETWORK_H */
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index f2f842d..6fcaef8 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1403,6 +1403,10 @@ struct task_struct {
>   #endif
>   	seccomp_t seccomp;
>   
> +#ifdef CONFIG_SECURITY_DISABLENETWORK
> +  unsigned long network;
> +#endif
> +
>   /* Thread group tracking */
>      	u32 parent_exec_id;
>      	u32 self_exec_id;
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 26a6b73..b48f021 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -35,6 +35,7 @@
>   #include <linux/cpu.h>
>   #include <linux/ptrace.h>
>   #include <linux/fs_struct.h>
> +#include <linux/prctl_network.h>
>   
>   #include <linux/compat.h>
>   #include <linux/syscalls.h>
> @@ -1578,6 +1579,12 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>   			else
>   				error = PR_MCE_KILL_DEFAULT;
>   			break;
> +		case PR_SET_NETWORK:
> +			error = prctl_set_network((unsigned long*)arg2);
> +			break;
> +		case PR_GET_NETWORK:
> +			error = prctl_get_network((unsigned long*)arg2);
> +			break;
>   		default:
>   			error = -EINVAL;
>   			break;
> @@ -1585,6 +1592,52 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>   	return error;
>   }
>   
> +#ifdef CONFIG_SECURITY_DISABLENETWORK
> +
> +long prctl_get_network(unsigned long* user)
> +{
> +	return put_user(current->network, user);
> +}
> +
> +long prctl_set_network(unsigned long* user)
> +{
> +	unsigned long network_flags;
> +	long ret;
> +
> +	ret = -EFAULT;
> +	if (copy_from_user(&network_flags, user, sizeof(network_flags)))
> +		goto out;

Do you expect to pass more than 32 bits through this interface at
some point?  If not, how about avoiding the copy, and just passing
a long into prctl_set_network(), and having prctl_get_network
return 0 or a positive value indicating the active bits?

So

long prctl_get_network(void)
{
	return current->network;
}

long prctl_set_network(unsigned long network_flags)
{
	if (network_flags & ~PR_NETWORK_ALL_FLAGS)
		return -EINVAL;
	if (current->network & ~network_flags)
		return -EPERM;
	current->network = network_flags;
	return 0;
}

> +	ret = -EINVAL;
> +	if (network_flags & ~PR_NETWORK_ALL_FLAGS)
> +		goto out;
> +
> +	/* only dropping access is permitted */
> +	ret = -EPERM;
> +        if (current->network & ~network_flags)

whitespace.

> +		goto out;
> +
> +	current->network = network_flags;
> +	ret = 0;
> +
> +out:
> +	return ret;
> +}
> +
> +#else
> +
> +long prctl_get_network(unsigned long* user)
> +{
> +	return -ENOSYS;
> +}
> +
> +long prctl_set_network(unsigned long* user)
> +{
> +	return -ENOSYS;
> +}
> +
> +#endif /* ! CONFIG_SECURITY_DISABLENETWORK */
> +
>   SYSCALL_DEFINE3(getcpu, unsigned __user *, cpup, unsigned __user *, nodep,
>   		struct getcpu_cache __user *, unused)
>   {
> diff --git a/security/Kconfig b/security/Kconfig
> index 226b955..afd7f76 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -137,6 +137,17 @@ config LSM_MMAP_MIN_ADDR
>   	  this low address space will need the permission specific to the
>   	  systems running LSM.
>   
> +config SECURITY_DISABLENETWORK
> +	bool "Socket and networking discretionary access control"
> +	depends on SECURITY_NETWORK
> +	help
> +    This enables processes to drop networking privileges via
> +    prctl(PR_SET_NETWORK, PR_NETWORK_OFF).
> +
> +    See Documentation/disablenetwork.txt for more information.
> +
> +	  If you are unsure how to answer this question, answer N.
> +
>   source security/selinux/Kconfig
>   source security/smack/Kconfig
>   source security/tomoyo/Kconfig
> -- 
> 1.6.6.rc2
> --
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