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: Roberto Sassu <roberto.sassu@huaweicloud.com>
Cc: Casey Schaufler <casey@schaufler-ca.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 15:59:36 -0400	[thread overview]
Message-ID: <CAHC9VhTsSUM6_g5+ZOqZ=P6307hCAJW+-xEc4fKQcymPs5pYjQ@mail.gmail.com> (raw)
In-Reply-To: <7549b624-421e-30b9-ca99-de42929354c7@huaweicloud.com>

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?  I'm hoping
I'm missing something really obvious, but I can only see a single
Smack xattr being returned ...

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