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

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Date:   Fri, 14 Feb 2020 15:24:43 -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 1:08 PM, Stephen Smalley wrote:
> 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.

I would tentatively argue for inheriting both sclass and SID from the 
context_inode for the sake of sane policy writing.  In the userfaultfd 
case, that will still end up using the new SECCLASS_ANONINODE or 
whatever since the sclass will be initially set to that value for the 
transition SID case and then inherited on fork.  But for /dev/kvm, it 
would be the class from the /dev/kvm inode, i.e. SECCLASS_CHR_FILE.