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

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Message-ID: <c3751b2b-aa4b-2105-c238-29816bc85607@schaufler-ca.com>
Date:   Wed, 5 Apr 2023 14:07:28 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Roberto Sassu <roberto.sassu@...weicloud.com>, 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 4/5/2023 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.