Re: [PATCH bpf-next 0/2] lsm: give BPF programs a way to query locked_down state

[Justin Suess <utilityemal77@gmail.com>]

On Tue, Aug 18, 2026 at 11:47:55AM +0800, Xiujianfeng wrote:
> +cc Nicolas
> 
> On 8/15/2026 7:20 PM, Justin Suess wrote:
> > Howdy,
> > 
> > BPF programs can attach to the locked_down LSM hook and contribute a
> > verdict, but they have never been able to ask the locked_down question
> > themselves: there is no way for a program to invoke the hook and learn
> > whether a given operation is locked down. (i.e be a caller of
> > security_locked_down rather than a consumer).
> > 
> > Today the state has to be fed in out of band, for example userspace
> > reading /sys/kernel/security/lockdown and writing the result into a
> > map. That is a time-of-check/time-of-use race: a security_locked_down
> > verdict can be raised at runtime, so the cached answer can be stale
> > by the time the program acts on it.
> 
> Adding the bpf_security_locked_down() kfunc does not actually solve the
> TOCTOU issue you mentioned. Even after bpf_security_locked_down() is
> called, userspace can still change the lockdown state via /sys/kernel/
> security/lockdown.
>
This would put it at the same raciness level as any kernel caller of
security_locked_down.

More importantly, the sysfs file is not equivalent to calling the
hook. /sys/kernel/security/lockdown reflects only the lockdown LSM's
static level.

security_locked_down() is a generic LSM hook and other
implementers of locked_down: including BPF LSM programs attached to
that hook (which is already supported) can contribute per-reason,
dynamic verdicts that never appear in sysfs. The composite answer is
only obtainable by invoking the hook, which is what this kfunc exposes.

> I fail to see the necessity for a BPF program to know whether a specific
> operation is locked down. The test case provided in patch 2 does not
> clearly demonstrate a scenario where this is required. In my view, the
> verdict should happen exactly where security_locked_down() is currently
> invoked.
>
Elaborating a bit more on the use cases:

1. Being able to know if features LOCKDOWN_BPF_READ / LOCKDOWN_KPROBES is
   locked down without a userspace hop / polling an LSM-specific interface
   for error handling.

2. Use with the BPF LSM implmentations of security_locked_down, without
   Lockdown even in the picture.

3. A BPF LSM can restrict access to arbritrary sensitive resources based
   on the security_locked_down verdict.

> Furthermore, based on the current implementation of Lockdown, integrity
> is the prerequisite for confidentiality. It is designed to be coarse-
> grained and does not support per-operation lockdown. The fact that LSM
> BPF can already hook into locked_down seems to violate this foundational
> model, this is is analogous to the bitmap implementation [1], I’m
> considering whether we should restrict BPF from attaching to the
> locked_down hook. Nicolas, what are your thoughts on this?

You describe a Lockdown-specific model, why should other security models /
implementations of the same hook be made to have the same semantics?

The point of the LSM hooks are to be generic.

(I have use cases that do rely on that hook; though I'm embarrased to
speak on it and it is probably abuse: I use this hook to temporarily
disable hibernation in BPF)

Justin
> 
> [1]
> https://lore.kernel.org/all/20250728111517.134116-1-nik.borisov@suse.com/
> 
> > 
> > Add a bpf_security_locked_down() kfunc that calls
> > security_locked_down() and returns its verdict, letting LSM and
> > syscall programs query locked_down state at decision time. Out-of-range
> > reasons are rejected with -EINVAL before dispatching the hook, and the
> > kfunc is refused to programs attached to the locked_down hook itself,
> > which would recurse into the dispatch. (how the obvious recursion issue
> > is addressed).
> > 
> > As this is the first pure-lsm-hook kfunc, add a new file security/lsm_kfuncs.c
> > to host it.
> > 
> > This kfunc has no reliance on / relation to the Lockdown LSM, despite the
> > similar naming. It is an LSM-agnostic caller of security_locked_down, and
> > Lockdown just happens to be the only in-tree subscriber to this hook at the
> > moment.
> > 
> > In fact, the test environment does not rely on CONFIG_SECURITY_LOCKDOWN at
> > all, and uses a BPF implementation of security_locked_down.
> > 
> > This kfunc can cause notices to be printed with kmsg if the Lockdown LSM is
> > enabled due to this line in security/lockdown/lockdown.c:
> > 
> >    pr_notice_ratelimited("Lockdown: %s: %s is restricted; see man kernel_lockdown.7\n",
> > 				                 current->comm, lockdown_reasons[what]);
> > 
> > Patch 1 adds the kfunc, patch 2 the selftests. This is based on bpf-next/master, but
> > applies cleanly to the lsm tree.
> > 
> > Justin
> > 
> > Justin Suess (2):
> >    lsm: add bpf_security_locked_down() kfunc
> >    selftests/bpf: Test bpf_security_locked_down kfunc
> > 
> >   security/Makefile                             |  1 +
> >   security/lsm_kfuncs.c                         | 84 +++++++++++++++++++
> >   .../selftests/bpf/prog_tests/lsm_kfuncs.c     | 28 +++++++
> >   .../testing/selftests/bpf/progs/lsm_kfuncs.c  | 34 ++++++++
> >   .../selftests/bpf/progs/lsm_kfuncs_fail.c     | 26 ++++++
> >   5 files changed, 173 insertions(+)
> >   create mode 100644 security/lsm_kfuncs.c
> >   create mode 100644 tools/testing/selftests/bpf/prog_tests/lsm_kfuncs.c
> >   create mode 100644 tools/testing/selftests/bpf/progs/lsm_kfuncs.c
> >   create mode 100644 tools/testing/selftests/bpf/progs/lsm_kfuncs_fail.c
> > 
> > 
> > base-commit: d82ebfc685c91e7f5623a8be949da1ddb767420b
>