Re: [PATCH 1/4] ns: add bpf hooks

[Matt Bobrowski <mattbobrowski@google.com>]

On Fri, Feb 20, 2026 at 01:38:29AM +0100, Christian Brauner wrote:
> Add the three namespace lifecycle hooks and make them available to bpf
> lsm program types. This allows bpf to supervise namespace creation. I'm
> in the process of adding various "universal truth" bpf programs to
> systemd that will make use of this. This e.g., allows to lock in a
> program into a given set of namespaces.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>  include/linux/bpf_lsm.h | 21 +++++++++++++++++++++
>  kernel/bpf/bpf_lsm.c    | 25 +++++++++++++++++++++++++
>  kernel/nscommon.c       |  9 ++++++++-
>  kernel/nsproxy.c        |  7 +++++++
>  4 files changed, 61 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
> index 643809cc78c3..5ae438fdf567 100644
> --- a/include/linux/bpf_lsm.h
> +++ b/include/linux/bpf_lsm.h
> @@ -12,6 +12,9 @@
>  #include <linux/bpf_verifier.h>
>  #include <linux/lsm_hooks.h>
>  
> +struct ns_common;
> +struct nsset;
> +
>  #ifdef CONFIG_BPF_LSM
>  
>  #define LSM_HOOK(RET, DEFAULT, NAME, ...) \
> @@ -48,6 +51,11 @@ void bpf_lsm_find_cgroup_shim(const struct bpf_prog *prog, bpf_func_t *bpf_func)
>  
>  int bpf_lsm_get_retval_range(const struct bpf_prog *prog,
>  			     struct bpf_retval_range *range);
> +
> +int bpf_lsm_namespace_alloc(struct ns_common *ns);
> +void bpf_lsm_namespace_free(struct ns_common *ns);
> +int bpf_lsm_namespace_install(struct nsset *nsset, struct ns_common *ns);
> +
>  int bpf_set_dentry_xattr_locked(struct dentry *dentry, const char *name__str,
>  				const struct bpf_dynptr *value_p, int flags);
>  int bpf_remove_dentry_xattr_locked(struct dentry *dentry, const char *name__str);
> @@ -104,6 +112,19 @@ static inline bool bpf_lsm_has_d_inode_locked(const struct bpf_prog *prog)
>  {
>  	return false;
>  }
> +
> +static inline int bpf_lsm_namespace_alloc(struct ns_common *ns)
> +{
> +	return 0;
> +}
> +static inline void bpf_lsm_namespace_free(struct ns_common *ns)
> +{
> +}
> +static inline int bpf_lsm_namespace_install(struct nsset *nsset,
> +					    struct ns_common *ns)
> +{
> +	return 0;
> +}
>  #endif /* CONFIG_BPF_LSM */
>  
>  #endif /* _LINUX_BPF_LSM_H */
> diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> index 0c4a0c8e6f70..f6378db46220 100644
> --- a/kernel/bpf/bpf_lsm.c
> +++ b/kernel/bpf/bpf_lsm.c
> @@ -30,10 +30,32 @@ __weak noinline RET bpf_lsm_##NAME(__VA_ARGS__)	\
>  #include <linux/lsm_hook_defs.h>
>  #undef LSM_HOOK
>  
> +__bpf_hook_start();
> +
> +__weak noinline int bpf_lsm_namespace_alloc(struct ns_common *ns)
> +{
> +	return 0;
> +}
> +
> +__weak noinline void bpf_lsm_namespace_free(struct ns_common *ns)
> +{
> +}

I'm wondering how you foresee this hook functioning in a scenario
where the BPF LSM program is attached to this new hook point, although
with its attachment type being set to BPF_LSM_CGROUP instead of
BPF_LSM_MAC? You probably wouldn't want to utilize something like
BPF_LSM_CGROUP for your specific use case, but as things stand
currently I don't believe there's anyhthing preventing you from using
BPF_LSM_CGROUP with a hook like bpf_lsm_namespace_free().

Notably, the BPF_LSM_CGROUP infrastructure is designed to execute BPF
programs based on the cgroup of the currently executing task. There
could be some surprises if the bpf_lsm_namespace_free() hook were to
ever be called from a context (e.g, kworker) other than the one
specified whilst attaching the BPF LSM program with type
BPF_LSM_CGROUP.

> +__weak noinline int bpf_lsm_namespace_install(struct nsset *nsset,
> +					  struct ns_common *ns)
> +{
> +	return 0;
> +}
> +
> +__bpf_hook_end();
> +
>  #define LSM_HOOK(RET, DEFAULT, NAME, ...) BTF_ID(func, bpf_lsm_##NAME)
>  BTF_SET_START(bpf_lsm_hooks)
>  #include <linux/lsm_hook_defs.h>
>  #undef LSM_HOOK
> +BTF_ID(func, bpf_lsm_namespace_alloc)
> +BTF_ID(func, bpf_lsm_namespace_free)
> +BTF_ID(func, bpf_lsm_namespace_install)
>  BTF_SET_END(bpf_lsm_hooks)
>  
>  BTF_SET_START(bpf_lsm_disabled_hooks)
> @@ -383,6 +405,8 @@ BTF_ID(func, bpf_lsm_task_prctl)
>  BTF_ID(func, bpf_lsm_task_setscheduler)
>  BTF_ID(func, bpf_lsm_task_to_inode)
>  BTF_ID(func, bpf_lsm_userns_create)
> +BTF_ID(func, bpf_lsm_namespace_alloc)
> +BTF_ID(func, bpf_lsm_namespace_install)
>  BTF_SET_END(sleepable_lsm_hooks)
>  
>  BTF_SET_START(untrusted_lsm_hooks)
> @@ -395,6 +419,7 @@ BTF_ID(func, bpf_lsm_sk_alloc_security)
>  BTF_ID(func, bpf_lsm_sk_free_security)
>  #endif /* CONFIG_SECURITY_NETWORK */
>  BTF_ID(func, bpf_lsm_task_free)
> +BTF_ID(func, bpf_lsm_namespace_free)
>  BTF_SET_END(untrusted_lsm_hooks)
>  
>  bool bpf_lsm_is_sleepable_hook(u32 btf_id)
> diff --git a/kernel/nscommon.c b/kernel/nscommon.c
> index bdc3c86231d3..c3613cab3d41 100644
> --- a/kernel/nscommon.c
> +++ b/kernel/nscommon.c
> @@ -1,6 +1,7 @@
>  // SPDX-License-Identifier: GPL-2.0-only
>  /* Copyright (c) 2025 Christian Brauner <brauner@kernel.org> */
>  
> +#include <linux/bpf_lsm.h>
>  #include <linux/ns_common.h>
>  #include <linux/nstree.h>
>  #include <linux/proc_ns.h>
> @@ -77,6 +78,7 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
>  		ret = proc_alloc_inum(&ns->inum);
>  	if (ret)
>  		return ret;
> +
>  	/*
>  	 * Tree ref starts at 0. It's incremented when namespace enters
>  	 * active use (installed in nsproxy) and decremented when all
> @@ -86,11 +88,16 @@ int __ns_common_init(struct ns_common *ns, u32 ns_type, const struct proc_ns_ope
>  		atomic_set(&ns->__ns_ref_active, 1);
>  	else
>  		atomic_set(&ns->__ns_ref_active, 0);
> -	return 0;
> +
> +	ret = bpf_lsm_namespace_alloc(ns);
> +	if (ret && !inum)
> +		proc_free_inum(ns->inum);
> +	return ret;
>  }
>  
>  void __ns_common_free(struct ns_common *ns)
>  {
> +	bpf_lsm_namespace_free(ns);
>  	proc_free_inum(ns->inum);
>  }
>  
> diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
> index 259c4b4f1eeb..5742f9664dbb 100644
> --- a/kernel/nsproxy.c
> +++ b/kernel/nsproxy.c
> @@ -9,6 +9,7 @@
>   *             Pavel Emelianov <xemul@openvz.org>
>   */
>  
> +#include <linux/bpf_lsm.h>
>  #include <linux/slab.h>
>  #include <linux/export.h>
>  #include <linux/nsproxy.h>
> @@ -379,6 +380,12 @@ static int prepare_nsset(unsigned flags, struct nsset *nsset)
>  
>  static inline int validate_ns(struct nsset *nsset, struct ns_common *ns)
>  {
> +	int ret;
> +
> +	ret = bpf_lsm_namespace_install(nsset, ns);
> +	if (ret)
> +		return ret;
> +
>  	return ns->ops->install(nsset, ns);
>  }
>  
> 
> -- 
> 2.47.3
>