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SubjectRe: [RFC PATCH v5 0/1] Add dm verity root hash pkcs7 sig validation.
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

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