Messages in this thread | | | Date | Mon, 1 Jul 2019 10:33:00 -0700 (PDT) | From | Jaskaran Singh Khurana <> | Subject | Re: [RFC PATCH v5 0/1] Add dm verity root hash pkcs7 sig validation. |
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Hello Milan, On Mon, 1 Jul 2019, Milan Broz wrote:
> On 29/06/2019 06:01, James Morris wrote: >> 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. > > Yes, but as it is implemented in this patch, a certificate is provided as > a binary blob by the (super)user that activates the dm-verity device. > > Actually, I can put there anything that looks like a correct signature (self-signed > or so), and dm-verity code is happy because the root hash is now signed. > > Maybe could this concept be extended to support in-kernel compiled certificates? > > I like the idea of signed root hash, but the truth is that if you have access > to device activation, it brings nothing, you can just put any cert in the keyring > and use it. > > Milan >
The signature needs to be trusted by the .builtin_trusted_keys which is a read-only list of keys that were compiled into the kernel. The verify_pkcs7_signature verifies trust against the builtin keyring so I think what you are suggesting is covered here.
Regards, Jaskaran.
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