linux-kernel - Problem with setxattr on sockfs with Smack after 971df15bd54ad46e907046ff33750a137b2f0096

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Date:   Mon, 31 Oct 2016 14:55:19 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Andreas Gruenbacher <agruenba@...hat.com>,
        Al Viro <viro@...IV.linux.org.uk>
Cc:     LKLM <linux-kernel@...r.kernel.org>,
        LSM <linux-security-module@...r.kernel.org>,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Problem with setxattr on sockfs with Smack after
 971df15bd54ad46e907046ff33750a137b2f0096

Smack has always used extended attributes to identify
the security information used to make access control
decisions on packet delivery. The two attributes
security.SMACK64_IPIN and security.SMACK64_IPOUT
contain the label used for inbound and outbound
checks respectively. A process with CAP_MAC_ADMIN can
change these values using fsetxattr() to allow a
privileged service to communicate more openly than
is allowed under the strict Smack policy.

After the xattr rework the fsetxattr() call still
sets the Smack attribute correctly, because the
smack_inode_setxattr() hook is still getting called,
but it returns EOPNOTSUPP. I believe that this is
either a result of the attribute name being unknown
to sockfs (as mentioned in the commit message) or
one of the other changes made in the process of the
xattr rework. I haven't finished the bisect yet,
but I'm reasonably certain the issue arises here.

Should I add the Smack attributes to the list of
attributes sockfs acknowledges? Is there a better
approach?

Thank you.