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

[Xu Kuohai <xukuohai@huaweicloud.com>]

On 6/7/2024 5:53 AM, Paul Moore wrote:
> On Thu, Apr 11, 2024 at 8:24 AM Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>>
>> From: Xu Kuohai <xukuohai@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@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.

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.