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

[Jaskaran Singh Khurana <jaskarankhurana@linux.microsoft.com>]

Hello Eric,
On Thu, 27 Jun 2019, Eric Biggers wrote:

> On Wed, Jun 19, 2019 at 12:10:47PM -0700, Jaskaran Khurana wrote:
>> This patch set adds in-kernel pkcs7 signature checking for the roothash of
>> the dm-verity hash tree.
>> The verification is to support cases where the roothash is not secured by
>> Trusted Boot, UEFI Secureboot or similar technologies.
>> One of the use cases for this is for dm-verity volumes mounted after boot,
>> the root hash provided during the creation of the dm-verity volume has to
>> be secure and thus in-kernel validation implemented here will be used
>> before we trust the root hash and allow the block device to be created.
>>
>> Why we are doing validation in the Kernel?
>>
>> The reason is to still be secure in cases where the attacker is able to
>> compromise the user mode application in which case the user mode validation
>> could not have been trusted.
>> The root hash signature validation in the kernel along with existing
>> dm-verity implementation gives a higher level of confidence in the
>> executable code or the protected data. Before allowing the creation of
>> the device mapper block device the kernel code will check that the detached
>> pkcs7 signature passed to it validates the roothash and the signature is
>> trusted by builtin keys set at kernel creation. The kernel should be
>> secured using Verified boot, UEFI Secure Boot or similar technologies so we
>> can trust it.
>>
>> What about attacker mounting non dm-verity volumes to run executable
>> code?
>>
>> This verification can be used to have a security architecture where a LSM
>> can enforce this verification for all the volumes and by doing this it can
>> ensure that all executable code runs from signed and trusted dm-verity
>> volumes.
>>
>> Further patches will be posted that build on this and enforce this
>> verification based on policy for all the volumes on the system.
>>
>
> 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.
>
> - Eric
>

In a datacenter like environment, this will protect the system from below 
attacks:

1.Prevents attacker from deploying scripts that run arbitrary executables on the system.
2.Prevents physically present malicious admin to run arbitrary code on the
   machine.

Regards,
Jaskaran