[PATCH 5/5] evm: Enforce signatures on unsupported filesystem for EVM_INIT_X509

[Amir Goldstein]

On Tue, Jan 30, 2024 at 11:46 PM Stefan Berger <stefanb at linux.ibm.com> wrote:
>
> Unsupported filesystems currently do not enforce any signatures. Add
> support for signature enforcement of the "original" and "portable &
> immutable" signatures when EVM_INIT_X509 is enabled.
>
> The "original" signature type contains filesystem specific metadata.
> Thus it cannot be copied up and verified. However with EVM_INIT_X509
> and EVM_ALLOW_METADATA_WRITES enabled, the "original" file signature
> may be written.
>
> When EVM_ALLOW_METADATA_WRITES is not set or once it is removed from
> /sys/kernel/security/evm by setting EVM_INIT_HMAC for example, it is not
> possible to write or remove xattrs on the overlay filesystem.
>
> This change still prevents EVM from writing HMAC signatures on
> unsupported filesystem when EVM_INIT_HMAC is enabled.
>
> Signed-off-by: Stefan Berger <stefanb at linux.ibm.com>
> ---
>  security/integrity/evm/evm_main.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index e96d127b48a2..f49609dfcbc7 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -192,7 +192,11 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
>                      iint->evm_status == INTEGRITY_PASS_IMMUTABLE))
>                 return iint->evm_status;
>
> -       if (is_unsupported_fs(dentry))
> +       /*
> +        * On unsupported filesystems with EVM_INIT_X509 not enabled, skip
> +        * signature verification.
> +        */
> +       if (!(evm_initialized & EVM_INIT_X509) && is_unsupported_fs(dentry))
>                 return INTEGRITY_UNKNOWN;
>

Are the names is_unsupported_fs() and SB_I_EVM_UNSUPPORTED still
a good description of what overlayfs is after this change?
Is EVM really not supported on overlayfs after this change?

Would you consider a better descriptive name, for the helper and flag,
at least as descriptive as SB_I_IMA_UNVERIFIABLE_SIGNATURE?

Thanks,
Amir.