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@xxxxxxxxxx>
> ---
> 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
