[PATCH v10 2/4] security: Allow all LSMs to provide xattrs for inode_init_security hook

[Roberto Sassu]

On 4/5/2023 10:43 PM, Casey Schaufler wrote:
> On 4/5/2023 12:59 PM, Paul Moore wrote:
>> On Wed, Apr 5, 2023 at 5:44 AM Roberto Sassu
>> <roberto.sassu at huaweicloud.com> wrote:
>>> On 4/5/2023 4:08 AM, Casey Schaufler wrote:
>>>> On 4/4/2023 11:54 AM, Paul Moore wrote:
>>>>> On Fri, Mar 31, 2023 at 8:33 AM Roberto Sassu
>>>>> <roberto.sassu at huaweicloud.com> wrote:
>> ..
>>
>>>>>> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
>>>>>> index cfcbb748da2..8392983334b 100644
>>>>>> --- a/security/smack/smack_lsm.c
>>>>>> +++ b/security/smack/smack_lsm.c
>>>>>> @@ -52,6 +52,15 @@
>>>>>>    #define SMK_RECEIVING  1
>>>>>>    #define SMK_SENDING    2
>>>>>>
>>>>>> +/*
>>>>>> + * Smack uses multiple xattrs.
>>>>>> + * SMACK64 - for access control, SMACK64EXEC - label for the program,
>>>>> I think it would be good to move SMACK64EXEC to its own line; it took
>>>>> me a minute to figure out why SMACK_INODE_INIT_XATTRS was set to '4'
>>>>> when I only say three comment lines ... ;)
>>>>>
>>>>>> + * SMACK64MMAP - controls library loading,
>>>>>> + * SMACK64TRANSMUTE - label initialization,
>>>>>> + * Not saved on files - SMACK64IPIN and SMACK64IPOUT
>>>>>> + */
>>>>>> +#define SMACK_INODE_INIT_XATTRS 4
>>>>> If smack_inode_init_security() only ever populates a single xattr, and
>>>>> that is the only current user of SMACK_INODE_INIT_XATTRS, can we make
>>>>> this '1' and shrink the xattr allocation a bit?
>>>> If the parent directory is marked with SMACK64_TRANSMUTE, the access
>>>> rule allowing the access has the "t" mode, and the object being initialized
>>>> is a directory, the new inode should get the SMACK64_TRANSMUTE attribute.
>>>> The callers of security_inode_init_security() don't seem to care.
>>>> I can't say if the evm code is getting SMACK64_TRANSMUTE or, for that
>>>> matter, SMACK64_EXEC and SMACK64_MMAP, some other way. The older system
>>>> allowed for multiple Smack xattrs, but I'm not clear on exactly how.
>>> If you like to set an additional xattr, that would be possible now.
>>> Since we reserve multiple xattrs, we can call lsm_get_xattr_slot()
>>> another time and set SMACK64_TRANSMUTE.
>>>
>>> I think, if the kernel config has CONFIG_EVM_EXTRA_SMACK_XATTRS set,
>>> EVM would protect SMACK64_TRANSMUTE too.
>> Ooookay, but can someone explain to me how either the current, or
>> patched, smack_inode_init_security() function can return multiple
>> xattrs via the security_inode_init_security() LSM hook?
> 
> It can't.
> 
>>    I'm hoping
>> I'm missing something really obvious, but I can only see a single
>> Smack xattr being returned ...
> 
> Smack is setting the transmute attribute in smack_d_instantiate().
> The exec and mmap attributes are always set explicitly.
> 
> I don't know how the "extra" Smack attributes were obtained by evm
> before, and I haven't been looking at how they're doing it now.
> I have assumed that CONFIG_EVM_EXTRA_SMACK_XATTRS does something
> meaningful.

EVM has a list of xattrs to protect. Without 
CONFIG_EVM_EXTRA_SMACK_XATTRS, EVM protects only security.SMACK64. With 
it, also the other Smack xattrs.

EVM calculates the HMAC of xattrs from that list at inode creation time 
(during the execution of security_inode_init_security(), after other 
LSMs) and during set/remove xattrs operations on the VFS.

Currently, Smack provides only security.SMACK64, so I agree with Paul 
that we should reserve as many xattr as we use. If in the future, we add 
security.SMACK_TRANSMUTE64, we can increase the number of reserved 
xattrs to 2.

Roberto