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

[James Bottomley]

On Mon, 2019-06-03 at 16:29 +0200, Roberto Sassu wrote:
> 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?

We don't.  We only guarantee immutability for unprivileged containers,
so root can't be inside.

James