Messages in this thread | | | Subject | Re: [PATCH v2 2/3] ima: don't ignore INTEGRITY_UNKNOWN EVM status | From | Roberto Sassu <> | Date | Mon, 3 Jun 2019 16:44:12 +0200 |
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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
-- HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063 Managing Director: Bo PENG, Jian LI, Yanli SHI
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