Re: [RFC] capabilities: add capability cgroup controller

[Topi Miettinen]

On 06/19/16 20:01, serge@xxxxxxxxxx wrote:
> apologies for top posting, this phone doesn't support inline)
> 
> Where are you preventing less privileged tasks from limiting the caps of a more privileged task?  It looks like you are relying on the cgroupfs for that?

I didn't think that aspect. Some of that could be dealt with by
preventing tasks which don't have CAP_SETPCAP to make other tasks join
or set the bounding set. One problem is that the privileges would not be
checked at cgroup.procs open(2) time but only when writing. In general,
less privileged tasks should not be able to gain new capabilities even
if they were somehow able to join the cgroup and also your case must be
addressed in full.

> 
> Overall I'm not a fan of this for several reasons.  Can you tell us precisely what your use case is?

There are two.

1. Capability use tracking at cgroup level. There is no way to know
which capabilities have been used and which could be trimmed. With
cgroup approach, we can also keep track of how subprocesses use
capabilities. Thus the administrator can quickly get a reasonable
estimate of a bounding set just by reading the capability.used file.

2. cgroup approach to capability management. Currently the capabilities
are inherited with bounding set and ambient capabilities taking their
part. With cgroups, additional limits can be set which apply to the
whole group. I admit that the difference to the current model is small.

Could you list the several reasons you mentioned?

-Topi

> On 6/18/16 14:31 Topi Miettinen wrote:
> Add a new cgroup controller for enforcement of and monitoring of
> capabilities in the cgroup.
> 
> Test case (boot to rdshell);
> BusyBox v1.22.1 (Debian 1:1.22.0-19) built-in shell (ash)
> Enter 'help' for a list of built-in commands.
> 
> (initramfs) cd /sys/fs
> (initramfs) mount -t cgroup2 cgroup cgroup
> (initramfs) cd cgroup
> (initramfs) echo +capability > cgroup.subtree_control
> (initramfs) mkdir test; cd test
> (initramfs) ls
> capability.bounding_set  cgroup.controllers       cgroup.procs
> capability.used          cgroup.events            cgroup.subtree_control
> (initramfs) sh
> 
> BusyBox v1.22.1 (Debian 1:1.22.0-19) built-in shell (ash)
> Enter 'help' for a list of built-in commands.
> 
> (initramfs) echo $$ >cgroup.procs
> (initramfs) cat capability.used
> 0000000000000000
> (initramfs) mknod /dev/z1 c 1 2
> (initramfs) cat capability.used
> 0000000008000000
> (initramfs) exit
> (initramfs) echo 0000000000000000 > capability.bounding_set
> (initramfs) sh
> 
> BusyBox v1.22.1 (Debian 1:1.22.0-19) built-in shell (ash)
> Enter 'help' for a list of built-in commands.
> 
> (initramfs) echo $$ >cgroup.procs
> (initramfs) mknod /dev/z2 c 1 2
> mknod: /dev/z2: Operation not permitted
> (initramfs) exit
> 
> Signed-off-by: Topi Miettinen <toiwoton@xxxxxxxxx>
> ---
>  include/linux/capability_cgroup.h |   7 ++
>  include/linux/cgroup_subsys.h     |   4 +
>  init/Kconfig                      |   6 ++
>  kernel/capability.c               |   2 +
>  security/Makefile                 |   1 +
>  security/capability_cgroup.c      | 216 ++++++++++++++++++++++++++++++++++++++
>  6 files changed, 236 insertions(+)
>  create mode 100644 include/linux/capability_cgroup.h
>  create mode 100644 security/capability_cgroup.c
> 
> diff --git a/include/linux/capability_cgroup.h b/include/linux/capability_cgroup.h
> new file mode 100644
> index 0000000..c03b58d
> --- /dev/null
> +++ b/include/linux/capability_cgroup.h
> @@ -0,0 +1,7 @@
> +#ifdef CONFIG_CGROUP_CAPABILITY
> +void capability_cgroup_update_used(int cap);
> +#else
> +static inline void capability_cgroup_update_used(int cap)
> +{
> +}
> +#endif
> diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
> index 0df0336a..a5161d0 100644
> --- a/include/linux/cgroup_subsys.h
> +++ b/include/linux/cgroup_subsys.h
> @@ -56,6 +56,10 @@ SUBSYS(hugetlb)
>  SUBSYS(pids)
>  #endif
>  
> +#if IS_ENABLED(CONFIG_CGROUP_CAPABILITY)
> +SUBSYS(capability)
> +#endif
> +
>  /*
>   * The following subsystems are not supported on the default hierarchy.
>   */
> diff --git a/init/Kconfig b/init/Kconfig
> index f755a60..098ce66 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1141,6 +1141,12 @@ config CGROUP_PERF
>  
>  	  Say N if unsure.
>  
> +config CGROUP_CAPABILITY
> +	bool "Capability controller"
> +	help
> +	  Provides a simple controller for enforcement of and monitoring of
> +	  capabilities in the cgroup.
> +
>  config CGROUP_DEBUG
>  	bool "Example controller"
>  	default n
> diff --git a/kernel/capability.c b/kernel/capability.c
> index 45432b5..b57d7f9 100644
> --- a/kernel/capability.c
> +++ b/kernel/capability.c
> @@ -17,6 +17,7 @@
>  #include <linux/syscalls.h>
>  #include <linux/pid_namespace.h>
>  #include <linux/user_namespace.h>
> +#include <linux/capability_cgroup.h>
>  #include <asm/uaccess.h>
>  
>  /*
> @@ -380,6 +381,7 @@ bool ns_capable(struct user_namespace *ns, int cap)
>  	}
>  
>  	if (security_capable(current_cred(), ns, cap) == 0) {
> +		capability_cgroup_update_used(cap);
>  		current->flags |= PF_SUPERPRIV;
>  		return true;
>  	}
> diff --git a/security/Makefile b/security/Makefile
> index f2d71cd..2bb04f1 100644
> --- a/security/Makefile
> +++ b/security/Makefile
> @@ -25,6 +25,7 @@ obj-$(CONFIG_SECURITY_APPARMOR)		+= apparmor/
>  obj-$(CONFIG_SECURITY_YAMA)		+= yama/
>  obj-$(CONFIG_SECURITY_LOADPIN)		+= loadpin/
>  obj-$(CONFIG_CGROUP_DEVICE)		+= device_cgroup.o
> +obj-$(CONFIG_CGROUP_CAPABILITY)		+= capability_cgroup.o
>  
>  # Object integrity file lists
>  subdir-$(CONFIG_INTEGRITY)		+= integrity
> diff --git a/security/capability_cgroup.c b/security/capability_cgroup.c
> new file mode 100644
> index 0000000..6e03fce
> --- /dev/null
> +++ b/security/capability_cgroup.c
> @@ -0,0 +1,216 @@
> +/*
> + * Capability cgroup
> + *
> + * Copyright 2016 Topi Miettinen
> + *
> + * This file is subject to the terms and conditions of the GNU General
> + * Public License.  See the file COPYING in the main directory of the
> + * Linux distribution for more details.
> + */
> +
> +#include <linux/capability.h>
> +#include <linux/capability_cgroup.h>
> +#include <linux/cgroup.h>
> +#include <linux/cred.h>
> +#include <linux/security.h>
> +#include <linux/seq_file.h>
> +#include <linux/slab.h>
> +
> +static DEFINE_MUTEX(capcg_mutex);
> +
> +struct capcg_cgroup {
> +	struct cgroup_subsys_state css;
> +	kernel_cap_t cap_bset; /* Capability bounding set */
> +	kernel_cap_t cap_used; /* Capabilities actually used */
> +};
> +
> +static inline struct capcg_cgroup *css_to_capcg(struct cgroup_subsys_state *s)
> +{
> +	return s ? container_of(s, struct capcg_cgroup, css) : NULL;
> +}
> +
> +static inline struct capcg_cgroup *task_to_capcg(struct task_struct *task)
> +{
> +	return css_to_capcg(task_css(task, capability_cgrp_id));
> +}
> +
> +static struct cgroup_subsys_state *capcg_css_alloc(struct cgroup_subsys_state
> +						   *parent)
> +{
> +	struct capcg_cgroup *caps;
> +
> +	caps = kzalloc(sizeof(*caps), GFP_KERNEL);
> +	if (!caps)
> +		return ERR_PTR(-ENOMEM);
> +
> +	caps->cap_bset = CAP_FULL_SET;
> +	cap_clear(caps->cap_used);
> +	return &caps->css;
> +}
> +
> +static void capcg_css_free(struct cgroup_subsys_state *css)
> +{
> +	kfree(css_to_capcg(css));
> +}
> +
> +/**
> + * capcg_apply_bset - apply cgroup bounding set to all task's capabilities
> + */
> +static int capcg_task_apply_bset(struct task_struct *task, kernel_cap_t bset)
> +{
> +	struct cred *new;
> +	const struct cred *old;
> +	kernel_cap_t bounding, effective, inheritable, permitted;
> +	int ret;
> +
> +	new = prepare_creds();
> +	if (!new)
> +		return -ENOMEM;
> +
> +	ret = security_capget(task, 
> +			      &effective, &inheritable, &permitted);
> +	if (ret < 0)
> +		goto abort_cred;
> +
> +	old = get_task_cred(task);
> +	bounding = cap_intersect(bset, old->cap_bset);
> +	effective = cap_intersect(bset, effective);
> +	inheritable = cap_intersect(bset, inheritable);
> +	permitted = cap_intersect(bset, permitted);
> +
> +	/* security_capset() also updates ambient capabilities */
> +	ret = security_capset(new, old,
> +			      &effective, &inheritable, &permitted);
> +	new->cap_bset = bounding;
> +		
> +	put_cred(old);
> +	if (ret < 0)
> +		goto abort_cred;
> +
> +	ret = commit_creds(new);
> +	return ret;
> +
> + abort_cred:
> +	abort_creds(new);
> +	return ret;
> +}
> +
> +static void capcg_attach(struct cgroup_taskset *tset)
> +{
> +	struct task_struct *task;
> +	struct cgroup_subsys_state *css;
> +
> +	rcu_read_lock();
> +	cgroup_taskset_for_each(task, css, tset) {
> +		struct capcg_cgroup *caps = css_to_capcg(css);
> +		
> +		capcg_task_apply_bset(task, caps->cap_bset);
> +	}
> +	rcu_read_unlock();
> +}
> +
> +/** capcg_write_bset - update css tree and their tasks with new
> + *  bounding capability
> + */
> +static ssize_t capcg_write_bset(struct kernfs_open_file *of, char *buf,
> +				size_t nbytes, loff_t off)
> +{
> +	struct cgroup_subsys_state *css = of_css(of), *pos;
> +	struct capcg_cgroup *caps = css_to_capcg(css);
> +	u32 capi;
> +	int err;
> +	kernel_cap_t new_bset;
> +
> +	buf = strstrip(buf);
> +
> +	CAP_FOR_EACH_U32(capi) {
> +		char buf2[9]; /* for each 32 bit block */
> +		u32 capv;
> +
> +		memcpy(buf2, &buf[capi * 8], 8);
> +		buf2[8] = '\0';
> +		err = kstrtou32(buf2, 16, &capv);
> +		if (err)
> +			return err;
> +		new_bset.cap[CAP_LAST_U32 - capi] = capv;
> +	}
> +
> +	mutex_lock(&capcg_mutex);
> +	caps->cap_bset = cap_intersect(caps->cap_bset, new_bset);
> +	mutex_unlock(&capcg_mutex);
> +
> +	rcu_read_lock();
> +	css_for_each_child(pos, css) {
> +		struct css_task_iter it;
> +		struct task_struct *task;
> +
> +		css_task_iter_start(pos, &it);
> +		while ((task = css_task_iter_next(&it)))
> +			capcg_task_apply_bset(task, new_bset);
> +	}
> +	rcu_read_unlock();
> +
> +	return nbytes;
> +}
> +
> +static int capcg_seq_show_cap(struct seq_file *m, kernel_cap_t *cap)
> +{
> +	u32 capi;
> +
> +	rcu_read_lock();
> +
> +	CAP_FOR_EACH_U32(capi) {
> +		seq_printf(m, "%08x",
> +			   cap->cap[CAP_LAST_U32 - capi]);
> +	}
> +	seq_putc(m, '\n');
> +
> +	rcu_read_unlock();
> +
> +	return 0;
> +}
> +
> +static int capcg_seq_show_bset(struct seq_file *m, void *v)
> +{
> +	struct capcg_cgroup *capcg = css_to_capcg(seq_css(m));
> +
> +	return capcg_seq_show_cap(m, &capcg->cap_bset);
> +}
> +
> +static int capcg_seq_show_used(struct seq_file *m, void *v)
> +{
> +	struct capcg_cgroup *capcg = css_to_capcg(seq_css(m));
> +
> +	return capcg_seq_show_cap(m, &capcg->cap_used);
> +}
> +
> +static struct cftype capcg_files[] = {
> +	{
> +		.name = "bounding_set",
> +		.seq_show = capcg_seq_show_bset,
> +		.write = capcg_write_bset,
> +		.flags = CFTYPE_NOT_ON_ROOT,
> +	},
> +	{
> +		.name = "used",
> +		.seq_show = capcg_seq_show_used,
> +		.flags = CFTYPE_NOT_ON_ROOT,
> +	},
> +	{ }	/* terminate */
> +};
> +
> +struct cgroup_subsys capability_cgrp_subsys = {
> +	.css_alloc = capcg_css_alloc,
> +	.css_free = capcg_css_free,
> +	.attach = capcg_attach,
> +	.dfl_cftypes = capcg_files,
> +};
> +
> +void capability_cgroup_update_used(int cap)
> +{
> +	struct capcg_cgroup *caps = task_to_capcg(current);
> +
> +	mutex_lock(&capcg_mutex);
> +	cap_raise(caps->cap_used, cap);
> +	mutex_unlock(&capcg_mutex);
> +}
>