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

From: Paul Moore <paul@paul-moore.com>
To: Casey Schaufler <casey@schaufler-ca.com>
Cc: Roberto Sassu <roberto.sassu@huaweicloud.com>,
	zohar@linux.ibm.com, dmitry.kasatkin@gmail.com,
	jmorris@namei.org, serge@hallyn.com,
	stephen.smalley.work@gmail.com, eparis@parisplace.org,
	reiserfs-devel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org, selinux@vger.kernel.org,
	bpf@vger.kernel.org, kpsingh@kernel.org, keescook@chromium.org,
	nicolas.bouchinet@clip-os.org,
	Roberto Sassu <roberto.sassu@huawei.com>
Subject: Re: [PATCH v10 2/4] security: Allow all LSMs to provide xattrs for inode_init_security hook
Date: Wed, 5 Apr 2023 16:49:22 -0400	[thread overview]
Message-ID: <CAHC9VhTO02CGUt0DUUmx=TUYS7Q81fas_Qy5miOFonaye0NEmw@mail.gmail.com> (raw)
In-Reply-To: <83ddfcb9-b4a6-71b4-a20c-62f484c8e040@schaufler-ca.com>

On Wed, Apr 5, 2023 at 4:43 PM Casey Schaufler <casey@schaufler-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@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@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 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.

-- 
paul-moore.com