[PATCH v4 6/7] Smack: Add support for unprivileged mounts from user namespaces

[Eric W. Biederman]

Seth Forshee <seth.forshee at canonical.com> writes:

> Security labels from unprivileged mounts cannot be trusted.
> Ideally for these mounts we would assign the objects in the
> filesystem the same label as the inode for the backing device
> passed to mount. Unfortunately it's currently impossible to
> determine which inode this is from the LSM mount hooks, so we
> settle for the label of the process doing the mount.
>
> This label is assigned to s_root, and also to smk_default to
> ensure that new inodes receive this label. The transmute property
> is also set on s_root to make this behavior more explicit, even
> though it is technically not necessary.
>
> If a filesystem has existing security labels, access to inodes is
> permitted if the label is the same as smk_root, otherwise access
> is denied. The SMACK64EXEC xattr is completely ignored.
>
> Explicit setting of security labels continues to require
> CAP_MAC_ADMIN in init_user_ns.
>
> Altogether, this ensures that filesystem objects are not
> accessible to subjects which cannot already access the backing
> store, that MAC is not violated for any objects in the fileystem
> which are already labeled, and that a user cannot use an
> unprivileged mount to gain elevated MAC privileges.
>
> sysfs, tmpfs, and ramfs are already mountable from user
> namespaces and support security labels. We can't rule out the
> possibility that these filesystems may already be used in mounts
> from user namespaces with security lables set from the init
> namespace, so failing to trust lables in these filesystems may
> introduce regressions. It is safe to trust labels from these
> filesystems, since the unprivileged user does not control the
> backing store and thus cannot supply security labels, so an
> explicit exception is made to trust labels from these
> filesystems.

Hmm.

>
> Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
> ---
>  security/smack/smack.h     |  8 +++++++-
>  security/smack/smack_lsm.c | 41 ++++++++++++++++++++++++++++++-----------
>  2 files changed, 37 insertions(+), 12 deletions(-)

[snip]

> @@ -3475,14 +3492,16 @@ static void smack_d_instantiate(struct dentry *opt_dentry, struct inode *inode)
>  			if (rc >= 0)
>  				transflag = SMK_INODE_TRANSMUTE;
>  		}
> -		/*
> -		 * Don't let the exec or mmap label be "*" or "@".
> -		 */
> -		skp = smk_fetch(XATTR_NAME_SMACKEXEC, inode, dp);
> -		if (IS_ERR(skp) || skp == &smack_known_star ||
> -		    skp == &smack_known_web)
> -			skp = NULL;
> -		isp->smk_task = skp;
> +		if (!(sbsp->smk_flags & SMK_SB_UNTRUSTED)) {
> +			/*
> +			 * Don't let the exec or mmap label be "*" or "@".
> +			 */
> +			skp = smk_fetch(XATTR_NAME_SMACKEXEC, inode, dp);
> +			if (IS_ERR(skp) || skp == &smack_known_star ||
> +			    skp == &smack_known_web)
> +				skp = NULL;
> +			isp->smk_task = skp;

I have to stop and ask is this really what we want to do?

If I have permission I can get around this by explicitly setting the
XATTR_NAME_SMACKEXEC.  Perhaps that does not matter but I think it is
siginficant.

We don't do any filtering on the the smk_mmap label.

Given the policy as I understand it is to only honor labels that match
smk_root would we not be better off allowing anything to be set and
filtering the labels at use when SMK_SB_UNTRUSTED is set?

Having three different policies depending on the kind of label concerns
me.

> +		}
>  
>  		skp = smk_fetch(XATTR_NAME_SMACKMMAP, inode, dp);
>  		if (IS_ERR(skp) || skp == &smack_known_star ||

Eric