[PATCH] ima: Allow imasig requirement to be satisfied by EVM portable signatures

[Mimi Zohar]

On Tue, 2020-04-21 at 11:24 +0200, Roberto Sassu wrote:
> System administrators can require that all accessed files have a signature
> by specifying appraise_type=imasig in a policy rule.
> 
> Currently, only IMA signatures satisfy this requirement. However, also EVM
> portable signatures can satisfy it. Metadata, including security.ima, are
> signed and cannot change.

Please expand this paragraph with a short comparison of the security
guarantees provided by EVM immutable, portable signatures versus ima-
sig.

> 
> This patch helps in the scenarios where system administrators want to
> enforce this restriction but only EVM portable signatures are available.

Yes, I agree it "helps", but we still need to address the ability of
setting/removing security.ima, which isn't possible with an IMA
signature.  This sounds like we need to define an immutable file hash.
 What do you think?

> The patch makes the following changes:
> 
> file xattr types:
> security.ima: IMA_XATTR_DIGEST/IMA_XATTR_DIGEST_NG
> security.evm: EVM_XATTR_PORTABLE_DIGSIG
> 
> execve(), mmap(), open() behavior (with appraise_type=imasig):
> before: denied (file without IMA signature, imasig requirement not met)
> after: allowed (file with EVM portable signature, imasig requirement met)
> 
> open(O_WRONLY) behavior (without appraise_type=imasig):
> before: allowed (file without IMA signature, not immutable)
> after: denied (file with EVM portable signature, immutable)
> 
> Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
> ---
>  security/integrity/ima/ima_appraise.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index a9649b04b9f1..69a6a958f811 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -219,12 +219,16 @@ static int xattr_verify(enum ima_hooks func, struct integrity_iint_cache *iint,
>  		hash_start = 1;
>  		/* fall through */
>  	case IMA_XATTR_DIGEST:
> -		if (iint->flags & IMA_DIGSIG_REQUIRED) {
> -			*cause = "IMA-signature-required";
> -			*status = INTEGRITY_FAIL;
> -			break;
> +		if (*status != INTEGRITY_PASS_IMMUTABLE) {
> +			if (iint->flags & IMA_DIGSIG_REQUIRED) {
> +				*cause = "IMA-signature-required";
> +				*status = INTEGRITY_FAIL;
> +				break;
> +			}
> +			clear_bit(IMA_DIGSIG, &iint->atomic_flags);
> +		} else {
> +			set_bit(IMA_DIGSIG, &iint->atomic_flags);
>  		}
> -		clear_bit(IMA_DIGSIG, &iint->atomic_flags);
>  		if (xattr_len - sizeof(xattr_value->type) - hash_start >=
>  				iint->ima_hash->length)
>  			/*

Nice!

Mimi