Re: [PATCH v2 2/3] ima: don't ignore INTEGRITY_UNKNOWN EVM status

From: Roberto Sassu
Date: Mon Jun 03 2019 - 10:33:32 EST

On 6/3/2019 3:43 PM, James Bottomley wrote:
On Mon, 2019-06-03 at 11:25 +0200, Roberto Sassu wrote:
On 5/30/2019 2:00 PM, Mimi Zohar wrote:
On Wed, 2019-05-29 at 15:30 +0200, Roberto Sassu wrote:
Currently, ima_appraise_measurement() ignores the EVM status when
evm_verifyxattr() returns INTEGRITY_UNKNOWN. If a file has a
valid security.ima xattr with type IMA_XATTR_DIGEST or
IMA_XATTR_DIGEST_NG, ima_appraise_measurement() returns
INTEGRITY_PASS regardless of the EVM status. The problem is that
the EVM status is overwritten with the appraisal statu

Roberto, your framing of this problem is harsh and misleading. IMA
and EVM are intentionally independent of each other and can be
configured independently of each other. The intersection of the
two is the call to evm_verifyxattr(). INTEGRITY_UNKNOWN is
returned for a number of reasons - when EVM is not configured, the
EVM hmac key has not yet been loaded, the protected security
attribute is unknown, or the file is not in policy.

This patch does not differentiate between any of the above cases,
requiring mutable files to always be protected by EVM, when
specified as an "ima_appraise=" option on the boot command line.

IMA could be extended to require EVM on a per IMA policy rule
basis. Instead of framing allowing IMA file hashes without EVM as a
bug that has existed from the very beginning, now that IMA/EVM have
matured and is being used, you could frame it as extending IMA
or hardening.

I'm seeing it from the perspective of an administrator that manages
an already hardened system, and expects that the system only grants
access to files with a valid signature/HMAC. That system would not
enforce this behavior if EVM keys are removed and the digest in
security.ima is set to the actual file digest.

Framing it as a bug rather than an extension would in my opinion help
to convince people about the necessity to switch to the safe mode, if
their system is already hardened.

I have a use case for IMA where I use it to enforce immutability of
containers. In this use case, the cluster admin places hashes on
executables as the image is unpacked so that if an executable file is
changed, IMA will cause an execution failure. For this use case, I
don't care about the EVM, in fact we don't use it, because the only
object is to fail execution if a binary is mutated.

How would you prevent root in the container from updating security.ima?


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