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

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Date:   Thu, 6 Apr 2023 11:14:34 +0200
From:   Roberto Sassu <roberto.sassu@...weicloud.com>
To:     Casey Schaufler <casey@...aufler-ca.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>
Subject: Re: [PATCH v10 2/4] security: Allow all LSMs to provide xattrs for
 inode_init_security hook

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(), 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