On Thu, 2020-06-18 at 18:04 +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.
Appended signatures may also satisfy this requirement, but are not
applicable as ...
> IMA signatures
> ensure data source authentication for file content and prevent any change.
> EVM signatures instead ensure data source authentication for file metadata.
> Given that the digest or signature of the file content must be included in
> the metadata, EVM signatures provide at least the same guarantees of IMA
> signatures.
^provide the same file data guarantees of IMA signatures, as well as
providing file metadata guarantees.
>
> This patch lets systems protected with EVM signatures pass appraisal
> verification if the appraise_type=imasig requirement is specified in the
> policy. This facilitates deployment in the scenarios where only EVM
> signatures are available.
>
> 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)
>
> In addition, similarly to IMA signatures, this patch temporarily allows
> new files without or with incomplete metadata to be opened so that content
> can be written.
>
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
After addressing the comments above and below,
Reviewed-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
> ---
> security/integrity/ima/ima_appraise.c | 20 +++++++++++++-------
> 1 file changed, 13 insertions(+), 7 deletions(-)
>
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index 21bda264fc30..9505bb390d90 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)
> /*
> @@ -394,6 +398,8 @@ int ima_appraise_measurement(enum ima_hooks func,
> cause = "missing-HMAC";
> goto out;
> case INTEGRITY_FAIL_IMMUTABLE:
> + set_bit(IMA_DIGSIG, &iint->atomic_flags);
> + fallthrough;
> case INTEGRITY_FAIL: /* Invalid HMAC/signature. */
> cause = "invalid-HMAC";
> goto out;
> @@ -437,9 +443,9 @@ int ima_appraise_measurement(enum ima_hooks func,
> status = INTEGRITY_PASS;
> }
>
> - /* Permit new files with file signatures, but without data. */
> + /* Permit new files marked as immutable, but without data. */
This comment isn't quite right.
> if (inode->i_size == 0 && iint->flags & IMA_NEW_FILE &&
> - xattr_value && xattr_value->type == EVM_IMA_XATTR_DIGSIG) {
> + test_bit(IMA_DIGSIG, &iint->atomic_flags)) {
> status = INTEGRITY_PASS;
> }
>
