Re: [PATCH 05/11] evm: Allow xattr/attr operations for portable signatures if check fails

[Mimi Zohar]

On Thu, 2020-06-18 at 18:01 +0200, Roberto Sassu wrote:
> If files with portable signatures are copied from one location to another
> or are extracted from an archive, verification can temporarily fail until
> all xattrs/attrs are set in the destination. Portable signatures are the
> only ones that can be moved to different files, as they don't depend on
> system-specific information such as the inode generation.

^Only portable signatures may be moved or copied from one file to
another, as they ...  Instead portable signatures must include
"security.ima".

> Unlike other security.evm types, portable signatures

^, EVM portable signatures are also immutable.  They

>  can never be replaced
> even if an xattr/attr operation is granted, as once evm_update_evmxattr()
> detects this type, it returns without updating the HMAC. Thus, it wouldn't
> be a problem to allow those operations so that verification passes on the
> destination after all xattrs/attrs are copied.

This needs to be reworded a bit.
> 
> This patch first introduces a new integrity status called
> INTEGRITY_FAIL_IMMUTABLE, that allows callers of
> evm_verify_current_integrity() to detect that a portable signature didn't
> pass verification and then adds an exception in evm_protect_xattr() and
> evm_inode_setattr() for this status and returns 0 instead of -EPERM.
> 
> Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>

After this patch, nothing prevents modifying the xattrs after all of
them are in place and the signature verification would be successful. 
(Ok, that is being addressed in subsequent patches.)

> ---
>  include/linux/integrity.h             |  1 +
>  security/integrity/evm/evm_main.c     | 25 ++++++++++++++++++++-----
>  security/integrity/ima/ima_appraise.c |  1 +
>  3 files changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/integrity.h b/include/linux/integrity.h
> index 2271939c5c31..2ea0f2f65ab6 100644
> --- a/include/linux/integrity.h
> +++ b/include/linux/integrity.h
> @@ -13,6 +13,7 @@ enum integrity_status {
>  	INTEGRITY_PASS = 0,
>  	INTEGRITY_PASS_IMMUTABLE,
>  	INTEGRITY_FAIL,
> +	INTEGRITY_FAIL_IMMUTABLE,
>  	INTEGRITY_NOLABEL,
>  	INTEGRITY_NOXATTRS,
>  	INTEGRITY_UNKNOWN,
> diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
> index 4e9f5e8b21d5..30072030f05d 100644
> --- a/security/integrity/evm/evm_main.c
> +++ b/security/integrity/evm/evm_main.c
> @@ -27,7 +27,8 @@
>  int evm_initialized;
>  
>  static const char * const integrity_status_msg[] = {
> -	"pass", "pass_immutable", "fail", "no_label", "no_xattrs", "unknown"
> +	"pass", "pass_immutable", "fail", "fail_immutable", "no_label",
> +	"no_xattrs", "unknown"
>  };
>  int evm_hmac_attrs;
>  
> @@ -134,7 +135,7 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
>  	enum integrity_status evm_status = INTEGRITY_PASS;
>  	struct evm_digest digest;
>  	struct inode *inode;
> -	int rc, xattr_len;
> +	int rc, xattr_len, evm_immutable = 0;
>  
>  	if (iint && (iint->evm_status == INTEGRITY_PASS ||
>  		     iint->evm_status == INTEGRITY_PASS_IMMUTABLE))
> @@ -179,8 +180,10 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
>  		if (rc)
>  			rc = -EINVAL;
>  		break;
> -	case EVM_IMA_XATTR_DIGSIG:
>  	case EVM_XATTR_PORTABLE_DIGSIG:
> +		evm_immutable = 1;
> +		fallthrough;
> +	case EVM_IMA_XATTR_DIGSIG:
>  		/* accept xattr with non-empty signature field */
>  		if (xattr_len <= sizeof(struct signature_v2_hdr)) {
>  			evm_status = INTEGRITY_FAIL;
> @@ -219,7 +222,8 @@ static enum integrity_status evm_verify_hmac(struct dentry *dentry,
>  
>  	if (rc)
>  		evm_status = (rc == -ENODATA) ?
> -				INTEGRITY_NOXATTRS : INTEGRITY_FAIL;
> +				INTEGRITY_NOXATTRS : evm_immutable ?
> +				INTEGRITY_FAIL_IMMUTABLE : INTEGRITY_FAIL;

Embedded ternary operator should be replaced with normal C syntax.

>  out:
>  	if (iint)
>  		iint->evm_status = evm_status;
> @@ -351,6 +355,12 @@ static int evm_protect_xattr(struct dentry *dentry, const char *xattr_name,
>  				    -EPERM, 0);
>  	}
>  out:
> +	/* It is safe to allow fail_immutable, portable signatures can never be
> +	 * updated
> +	 */

Replace "It" with "Writing other xattrs".   Writing other xattrs is
safe for portable signatures, as portable signatures are immutable and
...."

> +	if (evm_status == INTEGRITY_FAIL_IMMUTABLE)
> +		return 0;
> +
>  	if (evm_status != INTEGRITY_PASS)
>  		integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
>  				    dentry->d_name.name, "appraise_metadata",
> @@ -488,9 +498,14 @@ int evm_inode_setattr(struct dentry *dentry, struct iattr *attr)
>  	if (!(ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID)))
>  		return 0;
>  	evm_status = evm_verify_current_integrity(dentry);
> +	/* It is safe to allow fail_immutable, portable signatures can never
> +	 * be updated
> +	 */

Replace "It" with what is safe.

Mimi

>  	if ((evm_status == INTEGRITY_PASS) ||
> -	    (evm_status == INTEGRITY_NOXATTRS))
> +	    (evm_status == INTEGRITY_NOXATTRS) ||
> +	    (evm_status == INTEGRITY_FAIL_IMMUTABLE))
>  		return 0;
> +
>  	integrity_audit_msg(AUDIT_INTEGRITY_METADATA, d_backing_inode(dentry),
>  			    dentry->d_name.name, "appraise_metadata",
>  			    integrity_status_msg[evm_status], -EPERM, 0);
> diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
> index a9649b04b9f1..21bda264fc30 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -393,6 +393,7 @@ int ima_appraise_measurement(enum ima_hooks func,
>  	case INTEGRITY_NOLABEL:		/* No security.evm xattr. */
>  		cause = "missing-HMAC";
>  		goto out;
> +	case INTEGRITY_FAIL_IMMUTABLE:
>  	case INTEGRITY_FAIL:		/* Invalid HMAC/signature. */
>  		cause = "invalid-HMAC";
>  		goto out;