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

[Matt Bobrowski <mattbobrowski@google.com>]

On Mon, Feb 23, 2026 at 12:12:28PM +0100, Christian Brauner wrote:
> On Mon, Feb 23, 2026 at 10:36:19AM +0000, Matt Bobrowski wrote:
> > 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)
> > > +{
> > > +}
> > > +
> > > +__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);
> > >  }
> > 
> > What's the reason for not adding these new hook points to the generic
> > set of hooks that are currently being exposed directly by the LSM
> > framework? Honestly, it seems a little odd to be providing
> > declarations/definitions for a set of new hook points which are to be
> > exclusively siloed to BPF LSM implementations only. I'd argue that
> > some other LSM implementations could very well find namespace
> > lifecycle events possibly interesting.
> 
> The LSM layer is of the opinion that adding new security hooks is only
> acceptable if an implementation for an in-tree LSM is provided alongside
> it (cf. [1]). IOW, your bpf lsm needs are not sufficient justification
> for adding new security hooks. So if you want to add security hooks that
> a bpf lsm makes use of then you need to come up with an implementation
> for another in-tree LSM.

I apologize. I didn't realize that adding these as new generic LSM
hooks points had already been proposed and discussed with the LSM
maintainers. I just wanted to make sure that we weren't
unintentionally side-stepping.

> However, a subsystem is free to add as much bpf support as it wants:
> none, some, flamethrower mode. Cgroupfs has traditionally been very bpf
> friendly. I maintain namespaces and rewrote the infra allowing me to
> manage them uniformly now. bpf literally just needs an attach point. I
> could also just add fmodret tracepoints and achieve the same result.
> 
> The same way you add bpf kfuncs to support access to functionality that
> put you way past what an in-tree use would be able do. The question is
> whether you want such capabilities to be bounded by in-tree users as
> well.
>
> Either a bpf lsm is an inextensible fixture bound to the scope of
> security_* or you allow subsystems open to it to add functionality just
> like adding a kfuncs is.

Adding these dedicated BPF LSM hooks is OK with me, especially knowing
that I have agreement from you that you'll also be maintaining their
call sites.