Re: [PATCH bpf-next v3 04/10] bpf: lsm: Add mutable hooks list for the BPF LSM

[KP Singh <kpsingh@chromium.org>]

On 23-Jan 15:50, Casey Schaufler wrote:
> On 1/23/2020 2:24 PM, KP Singh wrote:
> > On 23-Jan 11:09, Casey Schaufler wrote:
> >> On 1/23/2020 9:59 AM, KP Singh wrote:
> >>> On 23-Jan 09:03, Casey Schaufler wrote:
> >>>> On 1/23/2020 7:24 AM, KP Singh wrote:
> >>>>> From: KP Singh <kpsingh@google.com>
> >>>>>
> >>>>> - The list of hooks registered by an LSM is currently immutable as they
> >>>>>   are declared with __lsm_ro_after_init and they are attached to a
> >>>>>   security_hook_heads struct.
> >>>>> - For the BPF LSM we need to de/register the hooks at runtime. Making
> >>>>>   the existing security_hook_heads mutable broadens an
> >>>>>   attack vector, so a separate security_hook_heads is added for only
> >>>>>   those that ~must~ be mutable.
> >>>>> - These mutable hooks are run only after all the static hooks have
> >>>>>   successfully executed.
> >>>>>
> >>>>> This is based on the ideas discussed in:
> >>>>>
> >>>>>   https://lore.kernel.org/lkml/20180408065916.GA2832@ircssh-2.c.rugged-nimbus-611.internal
> >>>>>
> >>>>> Reviewed-by: Brendan Jackman <jackmanb@google.com>
> >>>>> Reviewed-by: Florent Revest <revest@google.com>
> >>>>> Reviewed-by: Thomas Garnier <thgarnie@google.com>
> >>>>> Signed-off-by: KP Singh <kpsingh@google.com>
> >>>>> ---
> >>>>>  MAINTAINERS             |  1 +
> >>>>>  include/linux/bpf_lsm.h | 72 +++++++++++++++++++++++++++++++++++++++++
> >>>>>  security/bpf/Kconfig    |  1 +
> >>>>>  security/bpf/Makefile   |  2 +-
> >>>>>  security/bpf/hooks.c    | 20 ++++++++++++
> >>>>>  security/bpf/lsm.c      |  7 ++++
> >>>>>  security/security.c     | 25 +++++++-------
> >>>>>  7 files changed, 116 insertions(+), 12 deletions(-)
> >>>>>  create mode 100644 include/linux/bpf_lsm.h
> >>>>>  create mode 100644 security/bpf/hooks.c
> >>>>>
> >>>>> diff --git a/MAINTAINERS b/MAINTAINERS
> >>>>> index e2b7f76a1a70..c606b3d89992 100644
> >>>>> --- a/MAINTAINERS
> >>>>> +++ b/MAINTAINERS
> >>>>> @@ -3209,6 +3209,7 @@ L:	linux-security-module@vger.kernel.org
> >>>>>  L:	bpf@vger.kernel.org
> >>>>>  S:	Maintained
> >>>>>  F:	security/bpf/
> >>>>> +F:	include/linux/bpf_lsm.h
> >>>>>  
> >>>>>  BROADCOM B44 10/100 ETHERNET DRIVER
> >>>>>  M:	Michael Chan <michael.chan@broadcom.com>
> >>>>> diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
> >>>>> new file mode 100644
> >>>>> index 000000000000..57c20b2cd2f4
> >>>>> --- /dev/null
> >>>>> +++ b/include/linux/bpf_lsm.h
> >>>>> @@ -0,0 +1,72 @@
> >>>>> +/* SPDX-License-Identifier: GPL-2.0 */
> >>>>> +
> >>>>> +/*
> >>>>> + * Copyright 2019 Google LLC.
> >>>>> + */
> >>>>> +
> >>>>> +#ifndef _LINUX_BPF_LSM_H
> >>>>> +#define _LINUX_BPF_LSM_H
> >>>>> +
> >>>>> +#include <linux/bpf.h>
> >>>>> +#include <linux/lsm_hooks.h>
> >>>>> +
> >>>>> +#ifdef CONFIG_SECURITY_BPF
> >>>>> +
> >>>>> +/* Mutable hooks defined at runtime and executed after all the statically
> >>>>> + * defined LSM hooks.
> >>>>> + */
> >>>>> +extern struct security_hook_heads bpf_lsm_hook_heads;
> >>>>> +
> >>>>> +int bpf_lsm_srcu_read_lock(void);
> >>>>> +void bpf_lsm_srcu_read_unlock(int idx);
> >>>>> +
> >>>>> +#define CALL_BPF_LSM_VOID_HOOKS(FUNC, ...)			\
> >>>>> +	do {							\
> >>>>> +		struct security_hook_list *P;			\
> >>>>> +		int _idx;					\
> >>>>> +								\
> >>>>> +		if (hlist_empty(&bpf_lsm_hook_heads.FUNC))	\
> >>>>> +			break;					\
> >>>>> +								\
> >>>>> +		_idx = bpf_lsm_srcu_read_lock();		\
> >>>>> +		hlist_for_each_entry(P, &bpf_lsm_hook_heads.FUNC, list) \
> >>>>> +			P->hook.FUNC(__VA_ARGS__);		\
> >>>>> +		bpf_lsm_srcu_read_unlock(_idx);			\
> >>>>> +	} while (0)
> >>>>> +
> >>>>> +#define CALL_BPF_LSM_INT_HOOKS(FUNC, ...) ({			\
> >>>>> +	int _ret = 0;						\
> >>>>> +	do {							\
> >>>>> +		struct security_hook_list *P;			\
> >>>>> +		int _idx;					\
> >>>>> +								\
> >>>>> +		if (hlist_empty(&bpf_lsm_hook_heads.FUNC))	\
> >>>>> +			break;					\
> >>>>> +								\
> >>>>> +		_idx = bpf_lsm_srcu_read_lock();		\
> >>>>> +								\
> >>>>> +		hlist_for_each_entry(P,				\
> >>>>> +			&bpf_lsm_hook_heads.FUNC, list) {	\
> >>>>> +			_ret = P->hook.FUNC(__VA_ARGS__);		\
> >>>>> +			if (_ret && IS_ENABLED(CONFIG_SECURITY_BPF_ENFORCE)) \
> >>>>> +				break;				\
> >>>>> +		}						\
> >>>>> +		bpf_lsm_srcu_read_unlock(_idx);			\
> >>>>> +	} while (0);						\
> >>>>> +	IS_ENABLED(CONFIG_SECURITY_BPF_ENFORCE) ? _ret : 0;	\
> >>>>> +})
> >>>>> +
> >>>>> +#else /* !CONFIG_SECURITY_BPF */
> >>>>> +
> >>>>> +#define CALL_BPF_LSM_INT_HOOKS(FUNC, ...) (0)
> >>>>> +#define CALL_BPF_LSM_VOID_HOOKS(...)
> >>>>> +
> >>>>> +static inline int bpf_lsm_srcu_read_lock(void)
> >>>>> +{
> >>>>> +	return 0;
> >>>>> +}
> >>>>> +static inline void bpf_lsm_srcu_read_unlock(int idx) {}
> >>>>> +
> >>>>> +#endif /* CONFIG_SECURITY_BPF */
> >>>>> +
> >>>>> +#endif /* _LINUX_BPF_LSM_H */
> >>>>> diff --git a/security/bpf/Kconfig b/security/bpf/Kconfig
> >>>>> index a5f6c67ae526..595e4ad597ae 100644
> >>>>> --- a/security/bpf/Kconfig
> >>>>> +++ b/security/bpf/Kconfig
> >>>>> @@ -6,6 +6,7 @@ config SECURITY_BPF
> >>>>>  	bool "BPF-based MAC and audit policy"
> >>>>>  	depends on SECURITY
> >>>>>  	depends on BPF_SYSCALL
> >>>>> +	depends on SRCU
> >>>>>  	help
> >>>>>  	  This enables instrumentation of the security hooks with
> >>>>>  	  eBPF programs.
> >>>>> diff --git a/security/bpf/Makefile b/security/bpf/Makefile
> >>>>> index c78a8a056e7e..c526927c337d 100644
> >>>>> --- a/security/bpf/Makefile
> >>>>> +++ b/security/bpf/Makefile
> >>>>> @@ -2,4 +2,4 @@
> >>>>>  #
> >>>>>  # Copyright 2019 Google LLC.
> >>>>>  
> >>>>> -obj-$(CONFIG_SECURITY_BPF) := lsm.o ops.o
> >>>>> +obj-$(CONFIG_SECURITY_BPF) := lsm.o ops.o hooks.o
> >>>>> diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
> >>>>> new file mode 100644
> >>>>> index 000000000000..b123d9cb4cd4
> >>>>> --- /dev/null
> >>>>> +++ b/security/bpf/hooks.c
> >>>>> @@ -0,0 +1,20 @@
> >>>>> +// SPDX-License-Identifier: GPL-2.0
> >>>>> +
> >>>>> +/*
> >>>>> + * Copyright 2019 Google LLC.
> >>>>> + */
> >>>>> +
> >>>>> +#include <linux/bpf_lsm.h>
> >>>>> +#include <linux/srcu.h>
> >>>>> +
> >>>>> +DEFINE_STATIC_SRCU(security_hook_srcu);
> >>>>> +
> >>>>> +int bpf_lsm_srcu_read_lock(void)
> >>>>> +{
> >>>>> +	return srcu_read_lock(&security_hook_srcu);
> >>>>> +}
> >>>>> +
> >>>>> +void bpf_lsm_srcu_read_unlock(int idx)
> >>>>> +{
> >>>>> +	return srcu_read_unlock(&security_hook_srcu, idx);
> >>>>> +}
> >>>>> diff --git a/security/bpf/lsm.c b/security/bpf/lsm.c
> >>>>> index dc9ac03c7aa0..a25a068e1781 100644
> >>>>> --- a/security/bpf/lsm.c
> >>>>> +++ b/security/bpf/lsm.c
> >>>>> @@ -4,6 +4,7 @@
> >>>>>   * Copyright 2019 Google LLC.
> >>>>>   */
> >>>>>  
> >>>>> +#include <linux/bpf_lsm.h>
> >>>>>  #include <linux/lsm_hooks.h>
> >>>>>  
> >>>>>  /* This is only for internal hooks, always statically shipped as part of the
> >>>>> @@ -12,6 +13,12 @@
> >>>>>   */
> >>>>>  static struct security_hook_list bpf_lsm_hooks[] __lsm_ro_after_init = {};
> >>>>>  
> >>>>> +/* Security hooks registered dynamically by the BPF LSM and must be accessed
> >>>>> + * by holding bpf_lsm_srcu_read_lock and bpf_lsm_srcu_read_unlock. The mutable
> >>>>> + * hooks dynamically allocated by the BPF LSM are appeneded here.
> >>>>> + */
> >>>>> +struct security_hook_heads bpf_lsm_hook_heads;
> >>>>> +
> >>>>>  static int __init bpf_lsm_init(void)
> >>>>>  {
> >>>>>  	security_add_hooks(bpf_lsm_hooks, ARRAY_SIZE(bpf_lsm_hooks), "bpf");
> >>>>> diff --git a/security/security.c b/security/security.c
> >>>>> index 30a8aa700557..95a46ca25dcd 100644
> >>>>> --- a/security/security.c
> >>>>> +++ b/security/security.c
> >>>>> @@ -27,6 +27,7 @@
> >>>>>  #include <linux/backing-dev.h>
> >>>>>  #include <linux/string.h>
> >>>>>  #include <linux/msg.h>
> >>>>> +#include <linux/bpf_lsm.h>
> >>>>>  #include <net/flow.h>
> >>>>>  
> >>>>>  #define MAX_LSM_EVM_XATTR	2
> >>>>> @@ -657,20 +658,22 @@ static void __init lsm_early_task(struct task_struct *task)
> >>>>>  								\
> >>>>>  		hlist_for_each_entry(P, &security_hook_heads.FUNC, list) \
> >>>>>  			P->hook.FUNC(__VA_ARGS__);		\
> >>>>> +		CALL_BPF_LSM_VOID_HOOKS(FUNC, __VA_ARGS__);	\
> >>>> I'm sorry if I wasn't clear on the v2 review.
> >>>> This does not belong in the infrastructure. You should be
> >>>> doing all the bpf_lsm hook processing in you module.
> >>>> bpf_lsm_task_alloc() should loop though all the bpf
> >>>> task_alloc hooks if they have to be handled differently
> >>>> from "normal" LSM hooks.
> >>> The BPF LSM does not define static hooks (the ones registered to
> >>> security_hook_heads in security.c with __lsm_ro_after_init) for each
> >>> LSM hook. If it tries to do that one ends with what was in v1:
> >>>
> >>>   https://lore.kernel.org/bpf/20191220154208.15895-7-kpsingh@chromium.org
> >>>
> >>> This gets quite ugly (security/bpf/hooks.h from v1) and was noted by
> >>> the BPF maintainers:
> >>>
> >>>   https://lore.kernel.org/bpf/20191222012722.gdqhppxpfmqfqbld@ast-mbp.dhcp.thefacebook.com/
> >>>
> >>> As I mentioned, some of the ideas we used here are based on:
> >>>
> >>>   https://lore.kernel.org/lkml/20180408065916.GA2832@ircssh-2.c.rugged-nimbus-611.internal
> >>>
> >>> Which gave each LSM the ability to add mutable hooks at runtime. If
> >>> you prefer we can make this generic and allow the LSMs to register
> >>> mutable hooks with the BPF LSM be the only LSM that uses it (and
> >>> enforce it with a whitelist).
> >>>
> >>> Would this generic approach be something you would consider better
> >>> than just calling the BPF mutable hooks directly?
> >> What I think makes sense is for the BPF LSM to have a hook
> >> for each of the interfaces and for that hook to handle the
> >> mutable list for the interface. If BPF not included there
> >> will be no mutable hooks. 
> >>
> >> Yes, your v1 got this right.
> > BPF LSM does provide mutable LSM hooks and it ends up being simpler
> > to implement/maintain when they are treated as such.
> 
> If you want to put mutable hook handling in the infrastructure
> you need to make it general mutable hook handling as opposed to
> BPF hook handling. I don't know if that would be acceptable for
> all the reasons called out about dynamic module loading.

We can have generic mutable hook handling and if an LSM doesn't
provide a mutable security_hook_heads, it would not allow dynamic
hooks / dynamic module loading.

So, in practice it will just be the BPF LSM that allows mutable hooks
and the other existing LSMs won't. I guess it will be cleaner than
calling the BPF hooks directly from the LSM code (i.e in security.c)

- KP

> 
> >
> >  The other approaches which we have considered are:
> >
> > - Using macro magic to allocate static hook bodies which call eBPF
> >   programs as implemented in v1. This entails maintaining a
> >   separate list of LSM hooks in the BPF LSM which is evident from the
> >   giant security/bpf/include/hooks.h in:
> >
> >   https://lore.kernel.org/bpf/20191220154208.15895-7-kpsingh@chromium.org
> 
> I haven't put much though into how you might make that cleaner,
> but I don't see how you can expect any approach to turn out
> smaller than or less ugly than security.c.
> 
> >
> > - Another approach one can think of is to allocate all the trampoline
> >   images (one page each) at __init and update these images to invoke
> >   BPF programs when they are attached.
> >
> > Both these approaches seem to suffer from the downside of doing more
> > work when it's not really needed (i.e. doing prep work for hooks which
> > have no eBPF programs attached) and they appear to to mask the fact
> > that what the BPF LSM provides is actually mutable LSM hooks by
> > allocating static wrappers around mutable callbacks.
> 
> That's a "feature" of the LSM infrastructure. If you're not using a hook
> you just don't provide one. It is a artifact of your intent of providing
> a general extension that requires you provide a hook which may do nothing
> for every interface.
> 
> >
> > Are there other downsides apart from the fact we have an explicit call
> > to the mutable hooks in the LSM code? (Note that we want to have these
> > mutable hooks run after all the static LSM hooks so ordering
> > would still end up being LSM_ORDER_LAST)
> 
> My intention when I suggested using LSM_ORDER_LAST was to
> remove the explicit calls to BPF in the infrastructure.
> 
> >
> > It would be great to hear the maintainers' perspective based on the
> > trade-offs involved with the different approaches discussed.
> 
> Please bear in mind that the maintainer (James Morris) didn't
> develop the hook list scheme.
> 
> > We are happy to adapt our approach based on the consensus we reach
> > here.
> >
> > - KP
> >
> >>> - KP
> >>>
> >>>>>  	} while (0)
> >>>>>  
> >>>>> -#define call_int_hook(FUNC, IRC, ...) ({			\
> >>>>> -	int RC = IRC;						\
> >>>>> -	do {							\
> >>>>> -		struct security_hook_list *P;			\
> >>>>> -								\
> >>>>> +#define call_int_hook(FUNC, IRC, ...) ({				\
> >>>>> +	int RC = IRC;							\
> >>>>> +	do {								\
> >>>>> +		struct security_hook_list *P;				\
> >>>>>  		hlist_for_each_entry(P, &security_hook_heads.FUNC, list) { \
> >>>>> -			RC = P->hook.FUNC(__VA_ARGS__);		\
> >>>>> -			if (RC != 0)				\
> >>>>> -				break;				\
> >>>>> -		}						\
> >>>>> -	} while (0);						\
> >>>>> -	RC;							\
> >>>>> +			RC = P->hook.FUNC(__VA_ARGS__);			\
> >>>>> +			if (RC != 0)					\
> >>>>> +				break;					\
> >>>>> +		}							\
> >>>>> +		if (RC == 0)						\
> >>>>> +			RC = CALL_BPF_LSM_INT_HOOKS(FUNC, __VA_ARGS__);	\
> >>>>> +	} while (0);							\
> >>>>> +	RC;								\
> >>>>>  })
> >>>>>  
> >>>>>  /* Security operations */
>