kdc: validate KDC-REQ-BODY invariance in GSS preauth
Whilst channel bindings are used to bind the KDC-REQ-BODY to the GSS-API context, we need to also bind the KDC-REQ-BODY across multiple requests in a pre-authentication conversation. Do this by making a digest of the first KDC-REQ-BODY (with the nonce zeroed, as this may change), and verifying it in each subsequent request.
This commit is contained in:
Luke Howard
@ -50,6 +50,7 @@ struct gss_client_params {
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gss_buffer_desc output_token;
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OM_uint32 flags;
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OM_uint32 lifetime;
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krb5_checksum req_body_checksum;
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};
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static void
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@ -64,6 +65,135 @@ pa_gss_display_name(gss_name_t name,
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gss_buffer_t namebuf,
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gss_const_buffer_t *namebuf_p);
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/*
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* Create a checksum over KDC-REQ-BODY (without the nonce), used to
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* assert the request is invariant within the preauth conversation.
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*/
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static krb5_error_code
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pa_gss_create_req_body_checksum(astgs_request_t r,
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krb5_checksum *checksum)
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{
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krb5_error_code ret;
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KDC_REQ_BODY b = r->req.req_body;
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krb5_data data;
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size_t size;
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b.nonce = 0;
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ASN1_MALLOC_ENCODE(KDC_REQ_BODY, data.data, data.length, &b, &size, ret);
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heim_assert(ret || data.length,
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"internal asn1 encoder error");
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ret = krb5_create_checksum(r->context, NULL, 0, CKSUMTYPE_SHA256,
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data.data, data.length, checksum);
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krb5_data_free(&data);
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return ret;
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}
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/*
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* Verify a checksum over KDC-REQ-BODY (without the nonce), used to
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* assert the request is invariant within the preauth conversation.
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*/
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static krb5_error_code
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pa_gss_verify_req_body_checksum(astgs_request_t r,
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krb5_checksum *checksum)
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{
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krb5_error_code ret;
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KDC_REQ_BODY b = r->req.req_body;
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krb5_data data;
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size_t size;
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b.nonce = 0;
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ASN1_MALLOC_ENCODE(KDC_REQ_BODY, data.data, data.length, &b, &size, ret);
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heim_assert(ret || data.length,
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"internal asn1 encoder error");
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ret = krb5_verify_checksum(r->context, NULL, 0,
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data.data, data.length, checksum);
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krb5_data_free(&data);
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return ret;
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}
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/*
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* Decode the FX-COOKIE context state, consisting of the exported
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* GSS context token concatenated with the checksum of the initial
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* KDC-REQ-BODY.
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*/
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static krb5_error_code
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pa_gss_decode_context_state(astgs_request_t r,
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const krb5_data *state,
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gss_buffer_t sec_context_token,
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krb5_checksum *req_body_checksum)
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{
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krb5_error_code ret;
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krb5_storage *sp;
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size_t cksumsize;
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krb5_data data;
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memset(req_body_checksum, 0, sizeof(*req_body_checksum));
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sec_context_token->length = 0;
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sec_context_token->value = NULL;
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krb5_data_zero(&data);
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sp = krb5_storage_from_readonly_mem(state->data, state->length);
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if (sp == NULL) {
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ret = krb5_enomem(r->context);
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goto out;
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}
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krb5_storage_set_eof_code(sp, KRB5_BAD_MSIZE);
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krb5_storage_set_byteorder(sp, KRB5_STORAGE_BYTEORDER_PACKED);
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ret = krb5_ret_data(sp, &data);
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if (ret)
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goto out;
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ret = krb5_ret_int32(sp, &req_body_checksum->cksumtype);
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if (ret)
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goto out;
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if (req_body_checksum->cksumtype == CKSUMTYPE_NONE ||
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krb5_checksum_is_keyed(r->context, req_body_checksum->cksumtype)) {
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ret = KRB5KDC_ERR_SUMTYPE_NOSUPP;
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goto out;
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}
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ret = krb5_checksumsize(r->context, req_body_checksum->cksumtype,
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&cksumsize);
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if (ret)
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goto out;
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req_body_checksum->checksum.data = malloc(cksumsize);
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if (req_body_checksum->checksum.data == NULL) {
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ret = krb5_enomem(r->context);
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goto out;
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}
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if (krb5_storage_read(sp, req_body_checksum->checksum.data,
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cksumsize) != cksumsize) {
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ret = KRB5_BAD_MSIZE;
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goto out;
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}
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req_body_checksum->checksum.length = cksumsize;
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_krb5_gss_data_to_buffer(&data, sec_context_token);
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out:
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if (ret) {
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krb5_data_free(&data);
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free_Checksum(req_body_checksum);
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memset(req_body_checksum, 0, sizeof(*req_body_checksum));
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}
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krb5_storage_free(sp);
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return ret;
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}
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/*
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* Deserialize a GSS-API security context from the FAST cookie.
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*/
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@ -73,25 +203,89 @@ pa_gss_get_context_state(astgs_request_t r,
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{
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int idx = 0;
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PA_DATA *fast_pa;
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krb5_error_code ret;
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OM_uint32 major, minor;
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gss_buffer_desc sec_context_token;
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fast_pa = krb5_find_padata(r->fast.fast_state.val,
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r->fast.fast_state.len,
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KRB5_PADATA_GSS, &idx);
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if (fast_pa) {
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gss_buffer_desc sec_context_token;
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OM_uint32 major, minor;
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if (fast_pa == NULL)
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return 0;
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_krb5_gss_data_to_buffer(&fast_pa->padata_value, &sec_context_token);
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major = gss_import_sec_context(&minor, &sec_context_token,
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&gcp->context_handle);
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if (GSS_ERROR(major))
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pa_gss_display_status(r, major, minor, gcp,
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"Failed to import GSS pre-authentication context");
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ret = pa_gss_decode_context_state(r, &fast_pa->padata_value,
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&sec_context_token,
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&gcp->req_body_checksum);
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if (ret)
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return ret;
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return _krb5_gss_map_error(major, minor);
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ret = pa_gss_verify_req_body_checksum(r, &gcp->req_body_checksum);
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if (ret)
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return ret;
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major = gss_import_sec_context(&minor, &sec_context_token,
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&gcp->context_handle);
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if (GSS_ERROR(major)) {
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pa_gss_display_status(r, major, minor, gcp,
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"Failed to import GSS pre-authentication context");
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ret = _krb5_gss_map_error(major, minor);
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} else
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ret = 0;
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gss_release_buffer(&minor, &sec_context_token);
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return ret;
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}
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/*
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* Encode the FX-COOKIE context state, consisting of the exported
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* GSS context token concatenated with the checksum of the initial
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* KDC-REQ-BODY.
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*/
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static krb5_error_code
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pa_gss_encode_context_state(astgs_request_t r,
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gss_const_buffer_t sec_context_token,
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const krb5_checksum *req_body_checksum,
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krb5_data *state)
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{
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krb5_error_code ret;
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krb5_storage *sp;
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krb5_data data;
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krb5_data_zero(state);
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sp = krb5_storage_emem();
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if (sp == NULL) {
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ret = krb5_enomem(r->context);
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goto out;
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}
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return 0;
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krb5_storage_set_byteorder(sp, KRB5_STORAGE_BYTEORDER_PACKED);
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_krb5_gss_buffer_to_data(sec_context_token, &data);
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ret = krb5_store_data(sp, data);
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if (ret)
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goto out;
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ret = krb5_store_int32(sp, req_body_checksum->cksumtype);
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if (ret)
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goto out;
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ret = krb5_store_bytes(sp, req_body_checksum->checksum.data,
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req_body_checksum->checksum.length);
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if (ret)
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goto out;
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ret = krb5_storage_to_data(sp, state);
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if (ret)
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goto out;
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out:
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krb5_storage_free(sp);
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return ret;
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}
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/*
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@ -104,10 +298,22 @@ pa_gss_set_context_state(astgs_request_t r,
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krb5_error_code ret;
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PA_DATA *fast_pa;
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int idx = 0;
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krb5_data state;
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OM_uint32 major, minor;
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gss_buffer_desc sec_context_token = GSS_C_EMPTY_BUFFER;
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/*
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* On second and subsequent responses, we can recycle the checksum
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* from the request as it is validated and invariant. This saves
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* re-encoding the request body again.
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*/
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if (gcp->req_body_checksum.cksumtype == CKSUMTYPE_NONE) {
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ret = pa_gss_create_req_body_checksum(r, &gcp->req_body_checksum);
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if (ret)
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return ret;
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}
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major = gss_export_sec_context(&minor, &gcp->context_handle,
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&sec_context_token);
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if (GSS_ERROR(major)) {
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@ -116,25 +322,27 @@ pa_gss_set_context_state(astgs_request_t r,
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return _krb5_gss_map_error(major, minor);
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}
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ret = pa_gss_encode_context_state(r, &sec_context_token,
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&gcp->req_body_checksum, &state);
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gss_release_buffer(&minor, &sec_context_token);
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if (ret)
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return ret;
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fast_pa = krb5_find_padata(r->fast.fast_state.val,
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r->fast.fast_state.len,
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KRB5_PADATA_GSS, &idx);
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if (fast_pa) {
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krb5_data_free(&fast_pa->padata_value);
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_krb5_gss_buffer_to_data(&sec_context_token, &fast_pa->padata_value);
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fast_pa->padata_value = state;
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} else {
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ret = krb5_padata_add(r->context,
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&r->fast.fast_state,
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ret = krb5_padata_add(r->context, &r->fast.fast_state,
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KRB5_PADATA_GSS,
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sec_context_token.value,
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sec_context_token.length);
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if (ret) {
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gss_release_buffer(&minor, &sec_context_token);
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return ret;
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}
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state.data, state.length);
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if (ret)
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krb5_data_free(&state);
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}
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return 0;
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return ret;
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}
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static krb5_error_code
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@ -656,6 +864,7 @@ _kdc_gss_free_client_param(astgs_request_t r,
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gss_delete_sec_context(&minor, &gcp->context_handle, GSS_C_NO_BUFFER);
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gss_release_name(&minor, &gcp->initiator_name);
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gss_release_buffer(&minor, &gcp->output_token);
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free_Checksum(&gcp->req_body_checksum);
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memset(gcp, 0, sizeof(*gcp));
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free(gcp);
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}
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