SELinux Archive on lore.kernel.org
 help / color / Atom feed
From: Ondrej Mosnacek <omosnace@redhat.com>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: selinux@vger.kernel.org, Casey Schaufler <casey@schaufler-ca.com>,
	Linux Security Module list 
	<linux-security-module@vger.kernel.org>,
	Tejun Heo <tj@kernel.org>,
	linux-fsdevel@vger.kernel.org, cgroups@vger.kernel.org
Subject: Re: [PATCH v2 0/3] Allow initializing the kernfs node's secctx based on its parent
Date: Tue, 22 Jan 2019 09:49:01 +0100
Message-ID: <CAFqZXNsm2+79gMTqhxoZuPgibxfhSjJXu8742o7Ho7Yrf=2QCw@mail.gmail.com> (raw)
In-Reply-To: <CAFqZXNsxfjwDaCWDrqxP736y_3Jm-r=twaHtkkTDtMuym774Jw@mail.gmail.com>

On Mon, Jan 14, 2019 at 10:01 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> On Thu, Jan 10, 2019 at 6:55 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> > Resending after email configuration repair.
> >
> > On 1/10/2019 6:15 AM, Stephen Smalley wrote:
> > > On 1/9/19 5:03 PM, Casey Schaufler wrote:
> > >> On 1/9/2019 12:37 PM, Stephen Smalley wrote:
> > >>> On 1/9/19 12:19 PM, Casey Schaufler wrote:
> > >>>> On 1/9/2019 8:28 AM, Ondrej Mosnacek wrote:
> > >>>>> Changes in v2:
> > >>>>> - add docstring for the new hook in union security_list_options
> > >>>>> - initialize *ctx to NULL and *ctxlen to 0 in case the hook is not
> > >>>>>     implemented
> > >>>>> v1: https://lore.kernel.org/selinux/20190109091028.24485-1-omosnace@redhat.com/T/
> > >>>>>
> > >>>>> This series adds a new security hook that allows to initialize the security
> > >>>>> context of kernfs properly, taking into account the parent context. Kernfs
> > >>>>> nodes require special handling here, since they are not bound to specific
> > >>>>> inodes/superblocks, but instead represent the backing tree structure that
> > >>>>> is used to build the VFS tree when the kernfs tree is mounted.
> > >>>>>
> > >>>>> The kernfs nodes initially do not store any security context and rely on
> > >>>>> the LSM to assign some default context to inodes created over them.
> > >>>>
> > >>>> This seems like a bug in kernfs. Why doesn't kernfs adhere to the usual
> > >>>> and expected filesystem behavior?
> > >>>
> > >>> sysfs / kernfs didn't support xattrs at all when we first added support for setting security contexts to it, so originally all sysfs / kernfs inodes had a single security context, and we only required separate storage for the inodes that were explicitly labeled by userspace.
> > >>>
> > >>> Later kernfs grew support for trusted.* xattrs using simple_xattrs but the existing security.* support was left mostly unchanged.
> > >>
> > >> OK, so as I said, this seems like a bug in kernfs.
> > >>
> > >>>
> > >>>>
> > >>>>> Kernfs
> > >>>>> inodes, however, allow setting an explicit context via the *setxattr(2)
> > >>>>> syscalls, in which case the context is stored inside the kernfs node's
> > >>>>> metadata.
> > >>>>>
> > >>>>> SELinux (and possibly other LSMs) initialize the context of newly created
> > >>>>> FS objects based on the parent object's context (usually the child inherits
> > >>>>> the parent's context, unless the policy dictates otherwise).
> > >>>>
> > >>>> An LSM might use information about the parent other than the "context".
> > >>>> Smack, for example, uses an attribute SMACK64TRANSMUTE from the parent
> > >>>> to determine whether the Smack label of the new object should be taken
> > >>>> from the parent or the process. Passing the "context" of the parent is
> > >>>> insufficient for Smack.
> > >>>
> > >>> IIUC, this would involve switching the handling of security.* xattrs in kernfs over to use simple_xattrs too (so that we can store multiple such attributes), and then pass the entire simple_xattrs list or at least anything with a security.* prefix when initializing a new node or refreshing an existing inode.  Then the security module could extract any security.* attributes of interest for use in determining the label of new inodes and in refreshing the label of an inode.
>
> I actually had a patch to do just that at one point because I thought
> for a while that it would be required to call
> security_inode_init_security() (which I had tried to somehow force
> into the kernfs node creation at some point), but then I realized it
> is not actually needed (although would make thing a bit nicer) and put
> it away... I will try to dig it out and reuse here.

Okay, now that I tried to do this with full xattr support I ran into a
problem. Along with converting kernfs to use simple_xattrs for
security attributes, I removed the call to
security_inode_notifysecctx() from kernfs_refresh_inode(), as it no
longer makes sense (kernfs doesn't know which attribute contains the
context; the LSM should now be able to pull it out via
vfs_getxattr()). However, SELinux now doesn't set the right security
context in the selinux_d_instantiate() hook, because the policy tells
it to use genfs, not xattr.

So... I'm not sure how to fix this. Setting fs_use_xattr for cgroupfs
in the policy won't work, because then all nodes will be unlabeled_t
by default. Maybe we could patch the genfs case in
inode_doinit_with_dentry() to try fetching the xattr first? I'm not
very confident about touching that part of the code, so I would
welcome some advice here.

This is the code I have so far, in case it helps:
https://gitlab.com/omos/linux-public/compare/selinux-next...selinux-fix-cgroupfs-v8

Thanks,

-- 
Ondrej Mosnacek <omosnace at redhat dot com>
Associate Software Engineer, Security Technologies
Red Hat, Inc.

  reply index

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-09 16:28 Ondrej Mosnacek
2019-01-09 16:28 ` [PATCH v2 1/3] LSM: Add new hook for generic node initialization Ondrej Mosnacek
2019-01-09 17:08   ` Casey Schaufler
2019-01-11  1:57     ` Paul Moore
2019-01-11 18:30       ` Casey Schaufler
2019-01-14  9:01       ` Ondrej Mosnacek
2019-01-09 16:28 ` [PATCH v2 2/3] selinux: Implement the object_init_security hook Ondrej Mosnacek
2019-01-09 16:28 ` [PATCH v2 3/3] kernfs: Initialize security of newly created nodes Ondrej Mosnacek
2019-01-11 20:52   ` Tejun Heo
2019-01-09 17:19 ` [PATCH v2 0/3] Allow initializing the kernfs node's secctx based on its parent Casey Schaufler
2019-01-09 20:37   ` Stephen Smalley
2019-01-09 22:03     ` Casey Schaufler
2019-01-10 14:15       ` Stephen Smalley
2019-01-10 17:54         ` Casey Schaufler
2019-01-10 19:37           ` Stephen Smalley
2019-01-11  2:20             ` Paul Moore
2019-01-14  9:01               ` Ondrej Mosnacek
2019-01-11 18:22             ` Casey Schaufler
2019-01-14  9:01           ` Ondrej Mosnacek
2019-01-22  8:49             ` Ondrej Mosnacek [this message]
2019-01-22 14:17               ` Stephen Smalley
2019-01-22 15:26                 ` Stephen Smalley

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CAFqZXNsm2+79gMTqhxoZuPgibxfhSjJXu8742o7Ho7Yrf=2QCw@mail.gmail.com' \
    --to=omosnace@redhat.com \
    --cc=casey@schaufler-ca.com \
    --cc=cgroups@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-security-module@vger.kernel.org \
    --cc=sds@tycho.nsa.gov \
    --cc=selinux@vger.kernel.org \
    --cc=tj@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

SELinux Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/selinux/0 selinux/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 selinux selinux/ https://lore.kernel.org/selinux \
		selinux@vger.kernel.org
	public-inbox-index selinux

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.selinux


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git