Messages in this thread | | | Date | Fri, 28 Jun 2019 21:01:25 -0700 (PDT) | From | James Morris <> | Subject | Re: [RFC PATCH v5 0/1] Add dm verity root hash pkcs7 sig validation. |
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On Thu, 27 Jun 2019, Eric Biggers wrote:
> I don't understand your justification for this feature. > > If userspace has already been pwned severely enough for the attacker to be > executing arbitrary code with CAP_SYS_ADMIN (which is what the device mapper > ioctls need), what good are restrictions on loading more binaries from disk? > > Please explain your security model.
Let's say the system has a policy where all code must be signed with a valid key, and that one mechanism for enforcing this is via signed dm-verity volumes. Validating the signature within the kernel provides stronger assurance than userspace validation. The kernel validates and executes the code, using kernel-resident keys, and does not need to rely on validation which has occurred across a trust boundary.
You don't need arbitrary CAP_SYS_ADMIN code execution, you just need a flaw in the app (or its dependent libraries, or configuration) which allows signature validation to be bypassed.
The attacker now needs a kernel rather than a userspace vulnerability to bypass the signed code policy.
-- James Morris <jmorris@namei.org>
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