[PATCH bpf-next v3 01/11] bpf, lsm: Annotate lsm hook return value range

[Casey Schaufler]

On 6/8/2024 6:54 AM, Alexei Starovoitov wrote:
> On Sat, Jun 8, 2024 at 1:04 AM Xu Kuohai <xukuohai at huaweicloud.com> wrote:
>> On 6/7/2024 5:53 AM, Paul Moore wrote:
>>> On Thu, Apr 11, 2024 at 8:24 AM Xu Kuohai <xukuohai at huaweicloud.com> wrote:
>>>> From: Xu Kuohai <xukuohai at huawei.com>
>>>>
>>>> Add macro LSM_RET_INT to annotate lsm hook return integer type and the
>>>> default return value, and the expected return range.
>>>>
>>>> The LSM_RET_INT is declared as:
>>>>
>>>> LSM_RET_INT(defval, min, max)
>>>>
>>>> where
>>>>
>>>> - defval is the default return value
>>>>
>>>> - min and max indicate the expected return range is [min, max]
>>>>
>>>> The return value range for each lsm hook is taken from the description
>>>> in security/security.c.
>>>>
>>>> The expanded result of LSM_RET_INT is not changed, and the compiled
>>>> product is not changed.
>>>>
>>>> Signed-off-by: Xu Kuohai <xukuohai at huawei.com>
>>>> ---
>>>>   include/linux/lsm_hook_defs.h | 591 +++++++++++++++++-----------------
>>>>   include/linux/lsm_hooks.h     |   6 -
>>>>   kernel/bpf/bpf_lsm.c          |  10 +
>>>>   security/security.c           |   1 +
>>>>   4 files changed, 313 insertions(+), 295 deletions(-)
>>> ...
>>>
>>>> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
>>>> index 334e00efbde4..708f515ffbf3 100644
>>>> --- a/include/linux/lsm_hook_defs.h
>>>> +++ b/include/linux/lsm_hook_defs.h
>>>> @@ -18,435 +18,448 @@
>>>>    * The macro LSM_HOOK is used to define the data structures required by
>>>>    * the LSM framework using the pattern:
>>>>    *
>>>> - *     LSM_HOOK(<return_type>, <default_value>, <hook_name>, args...)
>>>> + *     LSM_HOOK(<return_type>, <return_description>, <hook_name>, args...)
>>>>    *
>>>>    * struct security_hook_heads {
>>>> - *   #define LSM_HOOK(RET, DEFAULT, NAME, ...) struct hlist_head NAME;
>>>> + *   #define LSM_HOOK(RET, RETVAL_DESC, NAME, ...) struct hlist_head NAME;
>>>>    *   #include <linux/lsm_hook_defs.h>
>>>>    *   #undef LSM_HOOK
>>>>    * };
>>>>    */
>>>> -LSM_HOOK(int, 0, binder_set_context_mgr, const struct cred *mgr)
>>>> -LSM_HOOK(int, 0, binder_transaction, const struct cred *from,
>>>> +LSM_HOOK(int, LSM_RET_INT(0, -MAX_ERRNO, 0), binder_set_context_mgr, const struct cred *mgr)
>>>> +LSM_HOOK(int, LSM_RET_INT(0, -MAX_ERRNO, 0), binder_transaction, const struct cred *from,
>>>>           const struct cred *to)
>>>> -LSM_HOOK(int, 0, binder_transfer_binder, const struct cred *from,
>>>> +LSM_HOOK(int, LSM_RET_INT(0, -MAX_ERRNO, 0), binder_transfer_binder, const struct cred *from,
>>>>           const struct cred *to)
>>>> -LSM_HOOK(int, 0, binder_transfer_file, const struct cred *from,
>>>> +LSM_HOOK(int, LSM_RET_INT(0, -MAX_ERRNO, 0), binder_transfer_file, const struct cred *from,
>>>>           const struct cred *to, const struct file *file)
>>> I'm not overly excited about injecting these additional return value
>>> range annotations into the LSM hook definitions, especially since the
>>> vast majority of the hooks "returns 0 on success, negative values on
>>> error".  I'd rather see some effort put into looking at the
>>> feasibility of converting some (all?) of the LSM hook return value
>>> exceptions into the more conventional 0/-ERRNO format.  Unfortunately,
>>> I haven't had the time to look into that myself, but if you wanted to
>>> do that I think it would be a good thing.
>>>
>> I agree that keeping all hooks return a consistent range of 0/-ERRNO
>> is more elegant than adding return value range annotations. However, there
>> are two issues that might need to be addressed first:
>>
>> 1. Compatibility
>>
>> For instance, security_vm_enough_memory_mm() determines whether to
>> set cap_sys_admin by checking if the hook vm_enough_memory returns
>> a positive number. If we were to change the hook vm_enough_memory
>> to return 0 to indicate the need for cap_sys_admin, then for the
>> LSM BPF program currently returning 0, the interpretation of its
>> return value would be reversed after the modification.
> This is not an issue. bpf lsm progs are no different from other lsm-s.
> If the meaning of return value or arguments to lsm hook change
> all lsm-s need to adjust as well. Regardless of whether they are
> written as in-kernel lsm-s, bpf-lsm, or out-of-tree lsm-s.
>
>> 2. Expressing multiple non-error states using 0/-ERRNO
>>
>> IIUC, although 0/-ERRNO can be used to express different errors,
>> only 0 can be used for non-error state. If there are multiple
>> non-error states, they cannot be distinguished. For example,
>> security_inode_need_killpriv() returns < 0 on error, 0 if
>> security_inode_killpriv() doesn't need to be called, and > 0
>> if security_inode_killpriv() does need to be called.
> This looks like a problem indeed.

Hang on. There aren't really three states here. security_inode_killpriv()
is called only on the security_inode_need_killpriv() > 0 case. I'm not
looking at the code this instant, but adjusting the return to something
like -ENOSYS (OK, maybe not a great choice, but you get the idea) instead
of 0 in the don't call case and switching the positive value to 0 should
work just fine.

We're working on getting the LSM interfaces to be more consistent. This
particular pair of hooks is an example of why we need to do that. 

>  Converting all hooks to 0/-errno
> doesn't look practical.
>