[PATCH 2/2] KEYS: trusted: dcp: fix leak of blob encryption key

[Jarkko Sakkinen]

On Wed Jul 3, 2024 at 3:53 PM EEST, David Gstir wrote:
> Trusted keys unseal the key blob on load, but keep the sealed payload in
> the blob field so that every subsequent read (export) will simply
> convert this field to hex and send it to userspace.
>
> With DCP-based trusted keys, we decrypt the blob encryption key (BEK)
> in the Kernel due hardware limitations and then decrypt the blob payload.
> BEK decryption is done in-place which means that the trusted key blob
> field is modified and it consequently holds the BEK in plain text.
> Every subsequent read of that key thus send the plain text BEK instead
> of the encrypted BEK to userspace.
>
> This issue only occurs when importing a trusted DCP-based key and
> then exporting it again. This should rarely happen as the common use cases
> are to either create a new trusted key and export it, or import a key
> blob and then just use it without exporting it again.
>
> Fix this by performing BEK decryption and encryption in a dedicated
> buffer. Further always wipe the plain text BEK buffer to prevent leaking
> the key via uninitialized memory.
>
> Signed-off-by: David Gstir <david at sigma-star.at>
> Fixes: 2e8a0f40a39c ("KEYS: trusted: Introduce NXP DCP-backed trusted keys")

Similar comments, fixes before sob etc and CC to stable with "# v6.10+"

BR, Jarkko