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

From: Roberto Sassu
Date: Mon Jun 03 2019 - 05:29:02 EST

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.ÂÂIMA
and EVM are intentionally independent of each other and can be
configured independently of each other. ÂThe intersection of the two
is the call to evm_verifyxattr(). ÂINTEGRITY_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 orÂhardening.

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.

This patch mitigates the issue by selecting signature verification as the
only method allowed for appraisal when EVM is not initialized. Since the
new behavior might break user space, it must be turned on by adding the
'-evm' suffix to the value of the ima_appraise= kernel option.

Fixes: 2fe5d6def1672 ("ima: integrity appraisal extension")
Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Documentation/admin-guide/kernel-parameters.txt | 3 ++-
security/integrity/ima/ima_appraise.c | 8 ++++++++
2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 138f6664b2e2..d84a2e612b93 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1585,7 +1585,8 @@
Set number of hash buckets for inode cache.
ima_appraise= [IMA] appraise integrity measurements
- Format: { "off" | "enforce" | "fix" | "log" }
+ Format: { "off" | "enforce" | "fix" | "log" |
+ "enforce-evm" | "log-evm" }

Is it necessary to define both "enforce-evm" and "log-evm"? ÂPerhaps
defining "require-evm" is sufficient.

ima_appraise= accepts as values modes of operation. I consider the -evm
suffix as a modifier of already defined modes.


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