evm: Check size of security.evm before using it

This patch checks the size for the EVM_IMA_XATTR_DIGSIG and
EVM_XATTR_PORTABLE_DIGSIG types to ensure that the algorithm is read from
the buffer returned by vfs_getxattr_alloc().

Cc: [e-mail] # 4.19.x
Fixes: 5feeb61183dde ("evm: Allow non-SHA1 digital signatures")
Signed-off-by: Roberto Sassu <[e-mail]>
Signed-off-by: Mimi Zohar <[e-mail]>

security/integrity/evm/evm_main.c

···
 		break;
 	case EVM_IMA_XATTR_DIGSIG:
 	case EVM_XATTR_PORTABLE_DIGSIG:
+		/* accept xattr with non-empty signature field */
+		if (xattr_len <= sizeof(struct signature_v2_hdr)) {
+			evm_status = INTEGRITY_FAIL;
+			goto out;
+		}
+
 		hdr = (struct signature_v2_hdr *)xattr_data;
 		digest.hdr.algo = hdr->hash_algo;
 		rc = evm_calc_hash(dentry, xattr_name, xattr_value,