linux-kernel - Re: [RFC PATCH v5 0/1] Add dm verity root hash pkcs7 sig validation.

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Date:   Fri, 28 Jun 2019 21:01:25 -0700 (PDT)
From:   James Morris <jmorris@...ei.org>
To:     Eric Biggers <ebiggers@...nel.org>
cc:     Jaskaran Khurana <jaskarankhurana@...ux.microsoft.com>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, agk@...hat.com, snitzer@...hat.com,
        dm-devel@...hat.com, scottsh@...rosoft.com, mpatocka@...hat.com,
        gmazyland@...il.com
Subject: Re: [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
<jmorris@...ei.org>