Re: [PATCH bpf-next v5 5/7] bpf: lsm: Initialize the BPF LSM hooks

Casey Schaufler
2020-03-23T20:47:36Z

On 3/23/2020 12:44 PM, Kees Cook wrote:
> On Mon, Mar 23, 2020 at 05:44:13PM +0100, KP Singh wrote:
>> From: KP Singh <kpsingh@google.com>
>>
>> The bpf_lsm_ nops are initialized into the LSM framework like any other
>> LSM. Some LSM hooks do not have 0 as their default return value. The
>> __weak symbol for these hooks is overridden by a corresponding
>> definition in security/bpf/hooks.c
>>
>> The LSM can be enabled / disabled with CONFIG_LSM.
>>
>> Signed-off-by: KP Singh <kpsingh@google.com>
> Nice! This is super clean on the LSM side of things. :)
>
> One note below...
>
>> Reviewed-by: Brendan Jackman <jackmanb@google.com>
>> Reviewed-by: Florent Revest <revest@google.com>
>> ---
>> security/Kconfig | 10 ++++----
>> security/Makefile | 2 ++
>> security/bpf/Makefile | 5 ++++
>> security/bpf/hooks.c | 55 +++++++++++++++++++++++++++++++++++++++++++
>> 4 files changed, 67 insertions(+), 5 deletions(-)
>> create mode 100644 security/bpf/Makefile
>> create mode 100644 security/bpf/hooks.c
>>
>> diff --git a/security/Kconfig b/security/Kconfig
>> index 2a1a2d396228..cd3cc7da3a55 100644
>> --- a/security/Kconfig
>> +++ b/security/Kconfig
>> @@ -277,11 +277,11 @@ endchoice
>>
>> config LSM
>> string "Ordered list of enabled LSMs"
>> - default "lockdown,yama,loadpin,safesetid,integrity,smack,selinux,tomoyo,apparmor" if DEFAULT_SECURITY_SMACK
>> - default "lockdown,yama,loadpin,safesetid,integrity,apparmor,selinux,smack,tomoyo" if DEFAULT_SECURITY_APPARMOR
>> - default "lockdown,yama,loadpin,safesetid,integrity,tomoyo" if DEFAULT_SECURITY_TOMOYO
>> - default "lockdown,yama,loadpin,safesetid,integrity" if DEFAULT_SECURITY_DAC
>> - default "lockdown,yama,loadpin,safesetid,integrity,selinux,smack,tomoyo,apparmor"
>> + default "lockdown,yama,loadpin,safesetid,integrity,smack,selinux,tomoyo,apparmor,bpf" if DEFAULT_SECURITY_SMACK
>> + default "lockdown,yama,loadpin,safesetid,integrity,apparmor,selinux,smack,tomoyo,bpf" if DEFAULT_SECURITY_APPARMOR
>> + default "lockdown,yama,loadpin,safesetid,integrity,tomoyo,bpf" if DEFAULT_SECURITY_TOMOYO
>> + default "lockdown,yama,loadpin,safesetid,integrity,bpf" if DEFAULT_SECURITY_DAC
>> + default "lockdown,yama,loadpin,safesetid,integrity,selinux,smack,tomoyo,apparmor,bpf"
>> help
>> A comma-separated list of LSMs, in initialization order.
>> Any LSMs left off this list will be ignored. This can be
>> diff --git a/security/Makefile b/security/Makefile
>> index 746438499029..22e73a3482bd 100644
>> --- a/security/Makefile
>> +++ b/security/Makefile
>> @@ -12,6 +12,7 @@ subdir-$(CONFIG_SECURITY_YAMA) += yama
>> subdir-$(CONFIG_SECURITY_LOADPIN) += loadpin
>> subdir-$(CONFIG_SECURITY_SAFESETID) += safesetid
>> subdir-$(CONFIG_SECURITY_LOCKDOWN_LSM) += lockdown
>> +subdir-$(CONFIG_BPF_LSM) += bpf
>>
>> # always enable default capabilities
>> obj-y += commoncap.o
>> @@ -30,6 +31,7 @@ obj-$(CONFIG_SECURITY_LOADPIN) += loadpin/
>> obj-$(CONFIG_SECURITY_SAFESETID) += safesetid/
>> obj-$(CONFIG_SECURITY_LOCKDOWN_LSM) += lockdown/
>> obj-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
>> +obj-$(CONFIG_BPF_LSM) += bpf/
>>
>> # Object integrity file lists
>> subdir-$(CONFIG_INTEGRITY) += integrity
>> diff --git a/security/bpf/Makefile b/security/bpf/Makefile
>> new file mode 100644
>> index 000000000000..c7a89a962084
>> --- /dev/null
>> +++ b/security/bpf/Makefile
>> @@ -0,0 +1,5 @@
>> +# SPDX-License-Identifier: GPL-2.0
>> +#
>> +# Copyright (C) 2020 Google LLC.
>> +
>> +obj-$(CONFIG_BPF_LSM) := hooks.o
>> diff --git a/security/bpf/hooks.c b/security/bpf/hooks.c
>> new file mode 100644
>> index 000000000000..68e5824868f9
>> --- /dev/null
>> +++ b/security/bpf/hooks.c
>> @@ -0,0 +1,55 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +/*
>> + * Copyright (C) 2020 Google LLC.
>> + */
>> +#include <linux/lsm_hooks.h>
>> +#include <linux/bpf_lsm.h>
>> +
>> +/* Some LSM hooks do not have 0 as their default return values. Override the
>> + * __weak definitons generated by default for these hooks
> If you wanted to avoid this, couldn't you make the default return value
> part of lsm_hooks.h?
>
> e.g.:
>
> LSM_HOOK(int, -EOPNOTSUPP, inode_getsecurity, struct inode *inode,
> const char *name, void **buffer, bool alloc)

If you're going to do that you'll have to keep lsm_hooks.h and security.c
default values in sync somehow. Note that the four functions you've called
out won't be using call_int_hook() after the next round of stacking. I'm not
nixing the idea, I just don't want the default return for the security_
functions defined in two places.

>
> ...
>
> #define LSM_HOOK(RET, DEFAULT, NAME, ...) \
> LSM_HOOK_##RET(NAME, DEFAULT, __VA_ARGS__)
> ...
> #define LSM_HOOK_int(NAME, DEFAULT, ...) \
> noinline int bpf_lsm_##NAME(__VA_ARGS__) \
> { \
> return (DEFAULT); \
> }
>
> Then all the __weak stuff is gone, and the following 4 functions don't
> need to be written out, and the information is available to the macros
> if anyone else might ever want it.
>
> -Kees
>
>> + */
>> +noinline int bpf_lsm_inode_getsecurity(struct inode *inode, const char *name,
>> + void **buffer, bool alloc)
>> +{
>> + return -EOPNOTSUPP;
>> +}
>> +
>> +noinline int bpf_lsm_inode_setsecurity(struct inode *inode, const char *name,
>> + const void *value, size_t size,
>> + int flags)
>> +{
>> + return -EOPNOTSUPP;
>> +}
>> +
>> +noinline int bpf_lsm_task_prctl(int option, unsigned long arg2,
>> + unsigned long arg3, unsigned long arg4,
>> + unsigned long arg5)
>> +{
>> + return -ENOSYS;
>> +}
>> +
>> +noinline int bpf_lsm_xfrm_state_pol_flow_match(struct xfrm_state *x,
>> + struct xfrm_policy *xp,
>> + const struct flowi *fl)
>> +{
>> + return 1;
>> +}
>> +
>> +static struct security_hook_list bpf_lsm_hooks[] __lsm_ro_after_init = {
>> + #define LSM_HOOK(RET, NAME, ...) LSM_HOOK_INIT(NAME, bpf_lsm_##NAME),
>> + #include <linux/lsm_hook_names.h>
>> + #undef LSM_HOOK
>> +};
>> +
>> +static int __init bpf_lsm_init(void)
>> +{
>> + security_add_hooks(bpf_lsm_hooks, ARRAY_SIZE(bpf_lsm_hooks), "bpf");
>> + pr_info("LSM support for eBPF active\n");
>> + return 0;
>> +}
>> +
>> +DEFINE_LSM(bpf) = {
>> + .name = "bpf",
>> + .init = bpf_lsm_init,
>> +};
>> --
>> 2.20.1
>>