From: Chuck Lever <chuck.lever@oracle.com>
To: Mimi Zohar <zohar@linux.ibm.com>
Cc: Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
linux-integrity@vger.kernel.org
Subject: Re: [PATCH RFC 0/4] IMA on NFS prototype
Date: Thu, 21 Feb 2019 09:49:16 -0500 [thread overview]
Message-ID: <71608AB6-ED45-43BA-A520-0DC2DA7D1C44@oracle.com> (raw)
In-Reply-To: <1550665596.17768.32.camel@linux.ibm.com>
> On Feb 20, 2019, at 7:26 AM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>
> On Tue, 2019-02-19 at 22:51 -0500, Chuck Lever wrote:
>>> On Feb 19, 2019, at 7:36 PM, Mimi Zohar <zohar@linux.ibm.com> wrote:
>>>
>>> Hi Chuck,
>>>
>>>> EVM is not supported in this prototype. NFS does not support several
>>>> of the xattrs that are protected by EVM: SMACK64, Posix ACLs, and
>>>> Linux file capabilities are not supported, which makes EVM more
>>>> difficult to support on NFS mounts.
>>>
>>> There's no requirement for all of these xattrs to exist. If an xattr
>>> does exist, then it is included in the security.evm hmac/signature.
>>
>> Understood. The issue is that if they exist on a file residing on an NFS server,
>> such xattrs would not be visible to clients. My understanding is that then EVM
>> verification would fail on such files on NFS clients.
>>
>> We could possibly make EVM work in limited scenarios until such time that
>> the NFS protocol can make those xattrs available to NFS clients. I hope that
>> having only security.ima is useful at least for experimenting and maybe more.
>>
>> However, if folks think having security.evm also is needed, that is straight-
>> forward... just saying that there are currently other limits in NFS that make a
>> full EVM implementation problematic.
>
> Thank you for the explanation. Yes, I think there is a benefit of
> having a file signature, without EVM.
It's been pointed out to me that a malicious actor inserted between
an NFS server and an NFS client can concurrently substitute the IMA
signature and a file's content with that of another file on the same
NFS share.
This could be used to substitute /etc/group for /etc/passwd, for
example. Both files are unchanged and have verifiable IMA signatures.
The /etc/group file contains a passwd-like entry for root in it, but
without a password field. That would allow the actor to gain root
access on the NFS client.
NFS can mitigate this substitution by using Kerberos 5 integrity to
protect wire traffic from tampering. However, a malicious NFS server
could also perform this substitution, and krb5i would not be able to
detect it.
I'm wondering if there's a mechanism within IMA's toolset to detect
such a substitution on an NFS client.
--
Chuck Lever
next prev parent reply other threads:[~2019-02-21 14:49 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-14 20:43 [PATCH RFC 0/4] IMA on NFS prototype Chuck Lever
2019-02-14 20:43 ` [PATCH RFC 1/4] NFS: Define common IMA-related protocol elements Chuck Lever
2019-02-14 20:43 ` [PATCH RFC 2/4] NFS: Rename security xattr handler Chuck Lever
2019-02-14 20:43 ` [PATCH RFC 3/4] NFS: Prototype support for IMA on NFS (client) Chuck Lever
2019-02-14 20:43 ` [PATCH RFC 4/4] NFSD: Prototype support for IMA on NFS (server) Chuck Lever
2019-02-18 19:32 ` J. Bruce Fields
2019-02-18 19:41 ` Chuck Lever
2019-03-01 15:04 ` Bruce Fields
2019-03-01 16:01 ` Chuck Lever
2019-03-01 16:10 ` Bruce Fields
2019-02-20 0:36 ` [PATCH RFC 0/4] IMA on NFS prototype Mimi Zohar
2019-02-20 3:51 ` Chuck Lever
2019-02-20 12:26 ` Mimi Zohar
2019-02-21 14:49 ` Chuck Lever [this message]
2019-02-21 15:51 ` Mimi Zohar
2019-02-21 15:58 ` Chuck Lever
2019-02-22 20:16 ` J. Bruce Fields
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