linux-kernel - Re: [PATCH 2/3] Teach SELinux about anonymous inodes

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Date:   Fri, 14 Feb 2020 13:08:48 -0500
From:   Stephen Smalley <sds@...ho.nsa.gov>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Tim Murray <timmurray@...gle.com>,
        SElinux list <selinux@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>, paul@...l-moore.com,
        Nick Kralevich <nnk@...gle.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>
Subject: Re: [PATCH 2/3] Teach SELinux about anonymous inodes

On 2/14/20 1:02 PM, Stephen Smalley wrote:
> It shouldn't fire for non-anon inodes because on a (non-anon) file 
> creation, security_transition_sid() is passed the parent directory SID 
> as the second argument and we only assign task SIDs to /proc/pid 
> directories, which don't support (userspace) file creation anyway.
> 
> However, in the absence of a matching type_transition rule, we'll end up 
> defaulting to the task SID on the anon inode, and without a separate 
> class we won't be able to distinguish it from a /proc/pid inode.  So 
> that might justify a separate anoninode or similar class.
> 
> This however reminded me that for the context_inode case, we not only 
> want to inherit the SID but also the sclass from the context_inode. That 
> is so that anon inodes created via device node ioctls inherit the same 
> SID/class pair as the device node and a single allowx rule can govern 
> all ioctl commands on that device.

At least that's the way our patch worked with the /dev/kvm example. 
However, if we are introducing a separate anoninode class for the 
type_transition case, maybe we should apply that to all anon inodes 
regardless of how they are labeled (based on context_inode or 
transition) and then we'd need to write two allowx rules, one for ioctls 
on the original device node and one for those on anon inodes created 
from it.  Not sure how Android wants to handle that as the original 
developer and primary user of SELinux ioctl whitelisting.