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

[Casey Schaufler]

On 4/5/2023 12:59 PM, Paul Moore wrote:
> On Wed, Apr 5, 2023 at 5:44 AM Roberto Sassu
> <roberto.sassu@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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.