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

[Roberto Sassu]

On 6/3/2019 4:31 PM, James Bottomley wrote:
> 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.

Ok.

Regarding the new behavior, this must be explicitly enabled by adding
ima_appraise=enforce-evm or log-evm to the kernel command line.
Otherwise, the current behavior is preserved with this patch. Would this
be ok?

Roberto

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