[dm-crypt] Re: [DM-Verity][HELP] Unlock Verity Target during runtime

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Mar 17, 2021 at 04:09:46PM -0700, Aditya Prakash wrote:
> Hi,
> I am trying to implement DM-Verity for the rootfs. However, I am not sure
> what the scope of DM Verity. I can see that given a root/sudo user, I can
> unload the verity target, write to rootfs and format and load it again, is
> it possible that this is normal that can be carried out using rootkits or
> any privileged app?
> Also, how can DM-Verity ensure that the system boots to correct rootfs if
> rootfs was changed after an OTA?
> Please help me to understand the actual scope of DM Verity
> 
> Thanks,
> AP

dm-verity's only job is to verify the contents of a block device against a root
hash which is provided by userspace when setting up the dm-verity target.

It's up to the userspace code setting up dm-verity to provide the "correct" root
hash -- for example, by verifying that the root hash is digitally signed by a
trusted source.

Take a look at how dm-verity is used in Verified Boot in Android and Chrome OS,
for example.  There are multiple components, and dm-verity is just one of them.

- Eric
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