Re: RFC: Make it practical to ship EVM signatures

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

On Fri, 2017-09-29 at 11:09 -0700, Matthew Garrett wrote:
> On Thu, Sep 28, 2017 at 5:53 PM, Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:
> > Without the inode included in the HMAC calculation, the same file
> > could exist as a different file name on the same file system.  No need
> > for the two files to be on different file systems.
> 
> But if I create a hardlink to an existing file then I get the same
> file with the same inode number and two different names on the same
> filesystem, and so security.evm would still match?

True, with a hard link that would be the case, but by the same file, I
meant a copy of the original file, not a hard link to the file.

> >> One of the reasons we're interested in allowing the use of signatures
> >> rather than HMACs is to avoid the case where a machine being
> >> compromised would allow an attacker to obtain the symmetric key and
> >> drop new appropriately HMACed binaries on the system that would
> >> persist even if the kernel was updated to fix the vulnerability.
> >
> > Assuming you're using a trusted key (TPM based key) to encrypt/decrypt
> > the EVM key (trusted key), then such an attack would require root
> > privileges with the ability to read kernel memory.  The EVM key is
> > never exposed to userspace in the clear.
> 
> That's a case that we need to be worried about. Trusted boot means we
> can ensure that a system boots an updated kernel, but if the attacker
> has been able to drop a modified sshd with a valid hmac onto the
> system then we have fewer guarantees about the integrity. We could
> continue using signatures for security.ima to avoid that, but then we
> lose the performance benefits of the hmac and also don't have the same
> level of guarantees around the other security metadata.

I think you mean "secure boot", not "trusted boot", in this case.

The original understanding was that IMA/EVM would enforce integrity
and not enforce mandatory access control (MAC).  The LSMs (eg.
SELinux) would be responsible for MAC.  Separation of duties.

With that understanding, if the LSM allows a file to be "dropped" onto
 the file system of a running system, IMA/EVM will hash and hmac the
permitted file.

I don't understand where you're going with this train of thought.  If
you're trying to make a case for EVM to run with only security.evm
signatures, then you wouldn't refer to the HMAC benefits.  If you're
trying to make a case for EVM signatures, with the inode they're not
portable, without the inode, they are susceptible to a cut and paste
attack.

Mickhail's proposed patches resolves this by having a portable EVM
signature that is never written to disk, but converted to an HMAC.

Mimi