Re: [REVIEW][PATCH 2/2] sysfs: Restrict mounting sysfs

[James Hogan]

On 27/08/13 22:46, Eric W. Biederman wrote:
> 
> Don't allow mounting sysfs unless the caller has CAP_SYS_ADMIN rights
> over the net namespace.  The principle here is if you create or have
> capabilities over it you can mount it, otherwise you get to live with
> what other people have mounted.
> 
> Instead of testing this with a straight forward ns_capable call,
> perform this check the long and torturous way with kobject helpers,
> this keeps direct knowledge of namespaces out of sysfs, and preserves
> the existing sysfs abstractions.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

Hi,

Unfortunately this patch causes problems for me in v3.12-rc1 if
CONFIG_NET=n. Sysfs fails to mount with EPERM. Having networking enabled
seems an odd prerequisite for mounting sysfs :-P.

Cheers
James

> ---
>  fs/sysfs/mount.c           |   12 +++++++++---
>  include/linux/kobject_ns.h |    2 ++
>  lib/kobject.c              |   12 ++++++++++++
>  net/core/net-sysfs.c       |    8 ++++++++
>  4 files changed, 31 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
> index 4a2da3a..8c69ef4 100644
> --- a/fs/sysfs/mount.c
> +++ b/fs/sysfs/mount.c
> @@ -112,9 +112,15 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
>  	struct super_block *sb;
>  	int error;
>  
> -	if (!(flags & MS_KERNMOUNT) && !capable(CAP_SYS_ADMIN) &&
> -	    !fs_fully_visible(fs_type))
> -		return ERR_PTR(-EPERM);
> +	if (!(flags & MS_KERNMOUNT)) {
> +		if (!capable(CAP_SYS_ADMIN) && !fs_fully_visible(fs_type))
> +			return ERR_PTR(-EPERM);
> +
> +		for (type = KOBJ_NS_TYPE_NONE; type < KOBJ_NS_TYPES; type++) {
> +			if (!kobj_ns_current_may_mount(type))
> +				return ERR_PTR(-EPERM);
> +		}
> +	}
>  
>  	info = kzalloc(sizeof(*info), GFP_KERNEL);
>  	if (!info)
> diff --git a/include/linux/kobject_ns.h b/include/linux/kobject_ns.h
> index f66b065..df32d25 100644
> --- a/include/linux/kobject_ns.h
> +++ b/include/linux/kobject_ns.h
> @@ -39,6 +39,7 @@ enum kobj_ns_type {
>   */
>  struct kobj_ns_type_operations {
>  	enum kobj_ns_type type;
> +	bool (*current_may_mount)(void);
>  	void *(*grab_current_ns)(void);
>  	const void *(*netlink_ns)(struct sock *sk);
>  	const void *(*initial_ns)(void);
> @@ -50,6 +51,7 @@ int kobj_ns_type_registered(enum kobj_ns_type type);
>  const struct kobj_ns_type_operations *kobj_child_ns_ops(struct kobject *parent);
>  const struct kobj_ns_type_operations *kobj_ns_ops(struct kobject *kobj);
>  
> +bool kobj_ns_current_may_mount(enum kobj_ns_type type);
>  void *kobj_ns_grab_current(enum kobj_ns_type type);
>  const void *kobj_ns_netlink(enum kobj_ns_type type, struct sock *sk);
>  const void *kobj_ns_initial(enum kobj_ns_type type);
> diff --git a/lib/kobject.c b/lib/kobject.c
> index 4a1f33d..1853bd9 100644
> --- a/lib/kobject.c
> +++ b/lib/kobject.c
> @@ -915,6 +915,18 @@ const struct kobj_ns_type_operations *kobj_ns_ops(struct kobject *kobj)
>  	return kobj_child_ns_ops(kobj->parent);
>  }
>  
> +bool kobj_ns_current_may_mount(enum kobj_ns_type type)
> +{
> +	bool may_mount = false;
> +
> +	spin_lock(&kobj_ns_type_lock);
> +	if ((type > KOBJ_NS_TYPE_NONE) && (type < KOBJ_NS_TYPES) &&
> +	    kobj_ns_ops_tbl[type])
> +		may_mount = kobj_ns_ops_tbl[type]->current_may_mount();
> +	spin_unlock(&kobj_ns_type_lock);
> +
> +	return may_mount;
> +}
>  
>  void *kobj_ns_grab_current(enum kobj_ns_type type)
>  {
> diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> index 981fed3..0b0ff95 100644
> --- a/net/core/net-sysfs.c
> +++ b/net/core/net-sysfs.c
> @@ -1157,6 +1157,13 @@ static void remove_queue_kobjects(struct net_device *net)
>  #endif
>  }
>  
> +static bool net_current_may_mount(void)
> +{
> +	struct net *net = current->nsproxy->net_ns;
> +
> +	return !ns_capable(net->user_ns, CAP_SYS_ADMIN);
> +}
> +
>  static void *net_grab_current_ns(void)
>  {
>  	struct net *ns = current->nsproxy->net_ns;
> @@ -1179,6 +1186,7 @@ static const void *net_netlink_ns(struct sock *sk)
>  
>  struct kobj_ns_type_operations net_ns_type_operations = {
>  	.type = KOBJ_NS_TYPE_NET,
> +	.current_may_mount = net_current_may_mount,
>  	.grab_current_ns = net_grab_current_ns,
>  	.netlink_ns = net_netlink_ns,
>  	.initial_ns = net_initial_ns,
>

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