linux-kernel - Re: [PATCH v10 2/4] security: Allow all LSMs to provide xattrs for inode_init

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Message-ID: <733a7896-0b0e-f86f-0068-0285d646d563@schaufler-ca.com>
Date:   Thu, 6 Apr 2023 09:17:05 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Roberto Sassu <roberto.sassu@...weicloud.com>,
        Paul Moore <paul@...l-moore.com>
Cc:     zohar@...ux.ibm.com, dmitry.kasatkin@...il.com, jmorris@...ei.org,
        serge@...lyn.com, stephen.smalley.work@...il.com,
        eparis@...isplace.org, reiserfs-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        bpf@...r.kernel.org, kpsingh@...nel.org, keescook@...omium.org,
        nicolas.bouchinet@...p-os.org,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v10 2/4] security: Allow all LSMs to provide xattrs for
 inode_init_security hook

On 4/6/2023 2:14 AM, Roberto Sassu wrote:
> On 4/5/2023 11:07 PM, Casey Schaufler wrote:
>> On 4/5/2023 1:49 PM, Paul Moore wrote:
>>> On Wed, Apr 5, 2023 at 4:43 PM Casey Schaufler
>>> <casey@...aufler-ca.com> wrote:
>>>> On 4/5/2023 12:59 PM, Paul Moore wrote:
>>>>> On Wed, Apr 5, 2023 at 5:44 AM Roberto Sassu
>>>>> <roberto.sassu@...weicloud.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@...weicloud.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 didn't think so.
>>>
>>> To be really specific, that's what we're talking about with this
>>> patch: the number of xattrs that smack_inode_init_security() can
>>> return to the LSM hook (and EVM, and the caller ...).  If it's only
>>> ever going to be one, I think we can adjust the
>>> 'SMACK_INODE_INIT_XATTRS' down to '1' and save ourselves some
>>> allocation space.
>>
>> Does evm have an expectation that mumble_inode_init_security() is
>> going to report all the relevant attributes? It has to be getting
>> them somehow, which leads me to wonder if we might want to extend
>> smack_inode_init_security() to do so. Even if we did, the maximum
>> value would be '2', SMACK64 and SMACK64_TRANSMUTE. Now that would
>> require a whole lot of work in the calling filesystems, as setting
>> the transmute attribute would be moving out of smack_d_instantiate()
>> and into the callers. Or something like that.
>
> After changing the inode_init_security hook definition to pass the
> full xattr array, this is not going to be a problem. EVM sees all
> xattrs that are going to be set when an inode is created, and adds its
> own too.
>
> If you have enough information to set security.SMACK_TRANSMUTE64 in
> smack_inode_init_security(),

I think there's enough information to do that. I'm going to have to look
at your patch more closely.

> this patch sets already allows to set both xattrs at the same time. We
> would just need to call lsm_get_xattr_slot() another time, assuming
> that we reserve two xattrs.
>
> Roberto
>