Re: [PATCH bpf-next v4 3/8] bpf: lsm: provide attachment points for BPF LSM programs

[Casey Schaufler <casey@schaufler-ca.com>]

On 2/20/2020 9:52 AM, KP Singh wrote:
> From: KP Singh <kpsingh@google.com>

Sorry about the heavy list pruning - the original set
blows thunderbird up.

>
> The BPF LSM programs are implemented as fexit trampolines to avoid the
> overhead of retpolines. These programs cannot be attached to security_*
> wrappers as there are quite a few security_* functions that do more than
> just calling the LSM callbacks.
>
> This was discussed on the lists in:
>
>   https://lore.kernel.org/bpf/20200123152440.28956-1-kpsingh@chromium.org/T/#m068becce588a0cdf01913f368a97aea4c62d8266
>
> Adding a NOP callback after all the static LSM callbacks are called has
> the following benefits:
>
> - The BPF programs run at the right stage of the security_* wrappers.
> - They run after all the static LSM hooks allowed the operation,
>   therefore cannot allow an action that was already denied.

I still say that the special call-out to BPF is unnecessary.
I remain unconvinced by the arguments. You aren't doing anything
so special that the general mechanism won't work.

>
> There are some hooks which do not call call_int_hooks or
> call_void_hooks. It's not possible to call the bpf_lsm_* functions
> without checking if there is BPF LSM program attached to these hooks.
> This is added further in a subsequent patch. For now, these hooks are
> marked as NO_BPF (i.e. attachment of BPF programs is not possible).
>
> Signed-off-by: KP Singh <kpsingh@google.com>
> ---
>  include/linux/bpf_lsm.h | 34 ++++++++++++++++++++++++++++++++++
>  kernel/bpf/bpf_lsm.c    | 16 ++++++++++++++++
>  security/security.c     |  3 +++
>  3 files changed, 53 insertions(+)
>  create mode 100644 include/linux/bpf_lsm.h
>
> diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
> new file mode 100644
> index 000000000000..f867f72f6aa9
> --- /dev/null
> +++ b/include/linux/bpf_lsm.h
> @@ -0,0 +1,34 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +/*
> + * Copyright 2019 Google LLC.
> + */
> +
> +#ifndef _LINUX_BPF_LSM_H
> +#define _LINUX_BPF_LSM_H
> +
> +#include <linux/bpf.h>
> +
> +#ifdef CONFIG_BPF_LSM
> +
> +#define LSM_HOOK(RET, NAME, ...) RET bpf_lsm_##NAME(__VA_ARGS__);
> +#include <linux/lsm_hook_names.h>
> +#undef LSM_HOOK
> +
> +#define RUN_BPF_LSM_VOID_PROGS(FUNC, ...) bpf_lsm_##FUNC(__VA_ARGS__)
> +#define RUN_BPF_LSM_INT_PROGS(RC, FUNC, ...) ({				\
> +	do {								\
> +		if (RC == 0)						\
> +			RC = bpf_lsm_##FUNC(__VA_ARGS__);		\
> +	} while (0);							\
> +	RC;								\
> +})
> +
> +#else /* !CONFIG_BPF_LSM */
> +
> +#define RUN_BPF_LSM_INT_PROGS(RC, FUNC, ...) (RC)
> +#define RUN_BPF_LSM_VOID_PROGS(FUNC, ...)
> +
> +#endif /* CONFIG_BPF_LSM */
> +
> +#endif /* _LINUX_BPF_LSM_H */
> diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> index affb6941622e..abc847c9b9a1 100644
> --- a/kernel/bpf/bpf_lsm.c
> +++ b/kernel/bpf/bpf_lsm.c
> @@ -7,6 +7,22 @@
>  #include <linux/filter.h>
>  #include <linux/bpf.h>
>  #include <linux/btf.h>
> +#include <linux/bpf_lsm.h>
> +
> +/* For every LSM hook  that allows attachment of BPF programs, declare a NOP
> + * function where a BPF program can be attached as an fexit trampoline.
> + */
> +#define LSM_HOOK(RET, NAME, ...) LSM_HOOK_##RET(NAME, __VA_ARGS__)
> +#define LSM_HOOK_int(NAME, ...) noinline int bpf_lsm_##NAME(__VA_ARGS__)  \
> +{									  \
> +	return 0;							  \
> +}
> +
> +#define LSM_HOOK_void(NAME, ...) \
> +	noinline void bpf_lsm_##NAME(__VA_ARGS__) {}
> +
> +#include <linux/lsm_hook_names.h>
> +#undef LSM_HOOK
>  
>  const struct bpf_prog_ops lsm_prog_ops = {
>  };
> diff --git a/security/security.c b/security/security.c
> index 565bc9b67276..aa111392a700 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -28,6 +28,7 @@
>  #include <linux/string.h>
>  #include <linux/msg.h>
>  #include <net/flow.h>
> +#include <linux/bpf_lsm.h>
>  
>  #define MAX_LSM_EVM_XATTR	2
>  
> @@ -684,6 +685,7 @@ 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__);		\
> +		RUN_BPF_LSM_VOID_PROGS(FUNC, __VA_ARGS__);	\

>  	} while (0)
>  
>  #define call_int_hook(FUNC, IRC, ...) ({			\
> @@ -696,6 +698,7 @@ static void __init lsm_early_task(struct task_struct *task)
>  			if (RC != 0)				\
>  				break;				\
>  		}						\
> +		RC = RUN_BPF_LSM_INT_PROGS(RC, FUNC, __VA_ARGS__); \
>  	} while (0);						\
>  	RC;							\
>  })