[PATCH v4 1/4] security, lsm: Introduce security_create_user_ns()

[KP Singh]

On Mon, Aug 1, 2022 at 8:02 PM Frederick Lawler <fred at cloudflare.com> wrote:
>
> Preventing user namespace (privileged or otherwise) creation comes in a
> few of forms in order of granularity:
>
>         1. /proc/sys/user/max_user_namespaces sysctl
>         2. OS specific patch(es)
>         3. CONFIG_USER_NS
>
> To block a task based on its attributes, the LSM hook cred_prepare is a
> good candidate for use because it provides more granular control, and
> it is called before create_user_ns():
>
>         cred = prepare_creds()
>                 security_prepare_creds()
>                         call_int_hook(cred_prepare, ...
>         if (cred)
>                 create_user_ns(cred)
>
> Since security_prepare_creds() is meant for LSMs to copy and prepare
> credentials, access control is an unintended use of the hook. Therefore
> introduce a new function security_create_user_ns() with an accompanying
> userns_create LSM hook.
>
> This hook takes the prepared creds for LSM authors to write policy
> against. On success, the new namespace is applied to credentials,
> otherwise an error is returned.
>
> Signed-off-by: Frederick Lawler <fred at cloudflare.com>
> Reviewed-by: Christian Brauner (Microsoft) <brauner at kernel.org>

Reviewed-by: KP Singh <kpsingh at kernel.org>

This looks useful, and I would also like folks to consider the
observability aspects of BPF LSM as
brought up here:

https://lore.kernel.org/all/CAEiveUdPhEPAk7Y0ZXjPsD=Vb5hn453CHzS9aG-tkyRa8bf_eg@mail.gmail.com/

Frederick, what about adding the observability aspects to the commit
description as well.

- KP

>
> ---
> Changes since v3:
> - No changes
> Changes since v2:
> - Rename create_user_ns hook to userns_create
> Changes since v1:
> - Changed commit wording
> - Moved execution to be after id mapping check
> - Changed signature to only accept a const struct cred *
> ---
>  include/linux/lsm_hook_defs.h | 1 +
>  include/linux/lsm_hooks.h     | 4 ++++
>  include/linux/security.h      | 6 ++++++
>  kernel/user_namespace.c       | 5 +++++
>  security/security.c           | 5 +++++
>  5 files changed, 21 insertions(+)
>
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index eafa1d2489fd..7ff93cb8ca8d 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -223,6 +223,7 @@ LSM_HOOK(int, -ENOSYS, task_prctl, int option, unsigned long arg2,
>          unsigned long arg3, unsigned long arg4, unsigned long arg5)
>  LSM_HOOK(void, LSM_RET_VOID, task_to_inode, struct task_struct *p,
>          struct inode *inode)
> +LSM_HOOK(int, 0, userns_create, const struct cred *cred)
>  LSM_HOOK(int, 0, ipc_permission, struct kern_ipc_perm *ipcp, short flag)
>  LSM_HOOK(void, LSM_RET_VOID, ipc_getsecid, struct kern_ipc_perm *ipcp,
>          u32 *secid)
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 91c8146649f5..54fe534d0e01 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -799,6 +799,10 @@
>   *     security attributes, e.g. for /proc/pid inodes.
>   *     @p contains the task_struct for the task.
>   *     @inode contains the inode structure for the inode.
> + * @userns_create:
> + *     Check permission prior to creating a new user namespace.
> + *     @cred points to prepared creds.
> + *     Return 0 if successful, otherwise < 0 error code.
>   *
>   * Security hooks for Netlink messaging.
>   *
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 7fc4e9f49f54..a195bf33246a 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -435,6 +435,7 @@ int security_task_kill(struct task_struct *p, struct kernel_siginfo *info,
>  int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>                         unsigned long arg4, unsigned long arg5);
>  void security_task_to_inode(struct task_struct *p, struct inode *inode);
> +int security_create_user_ns(const struct cred *cred);
>  int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag);
>  void security_ipc_getsecid(struct kern_ipc_perm *ipcp, u32 *secid);
>  int security_msg_msg_alloc(struct msg_msg *msg);
> @@ -1185,6 +1186,11 @@ static inline int security_task_prctl(int option, unsigned long arg2,
>  static inline void security_task_to_inode(struct task_struct *p, struct inode *inode)
>  { }
>
> +static inline int security_create_user_ns(const struct cred *cred)
> +{
> +       return 0;
> +}
> +
>  static inline int security_ipc_permission(struct kern_ipc_perm *ipcp,
>                                           short flag)
>  {
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 5481ba44a8d6..3f464bbda0e9 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -9,6 +9,7 @@
>  #include <linux/highuid.h>
>  #include <linux/cred.h>
>  #include <linux/securebits.h>
> +#include <linux/security.h>
>  #include <linux/keyctl.h>
>  #include <linux/key-type.h>
>  #include <keys/user-type.h>
> @@ -113,6 +114,10 @@ int create_user_ns(struct cred *new)
>             !kgid_has_mapping(parent_ns, group))
>                 goto fail_dec;
>
> +       ret = security_create_user_ns(new);
> +       if (ret < 0)
> +               goto fail_dec;
> +
>         ret = -ENOMEM;
>         ns = kmem_cache_zalloc(user_ns_cachep, GFP_KERNEL);
>         if (!ns)
> diff --git a/security/security.c b/security/security.c
> index 188b8f782220..ec9b4696e86c 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1903,6 +1903,11 @@ void security_task_to_inode(struct task_struct *p, struct inode *inode)
>         call_void_hook(task_to_inode, p, inode);
>  }
>
> +int security_create_user_ns(const struct cred *cred)
> +{
> +       return call_int_hook(userns_create, 0, cred);
> +}
> +
>  int security_ipc_permission(struct kern_ipc_perm *ipcp, short flag)
>  {
>         return call_int_hook(ipc_permission, 0, ipcp, flag);
> --
> 2.30.2
>