[csrng] Return error status response for GEN commands with glen = 0

[vogelpi]

Description

When taking care of #18350, I found out that CSRNG treats Generate commands with a glen = 0 similarly to commands with a glen = 1, i.e., it generates one single 128-bit block of random data instead of nothing.

I don't think this is a bug really because the CSRNG documentation specifies:

Generate Length: Only defined for the generate command, this field is the total number of cryptographic entropy blocks requested. Each unit represents 128 bits of entropy returned. This field allows values between 1 and 4095. A value of 1 returns 1 * 128 bits of entropy. A value of 4095 returns 4095 * 128 bits of entropy, which is less than the 219 bits allowed by NIST (referenced to as max_number_of_bit_per_request).

and further:

The glen field defines how many 128-bit words are to be returned to the application interface. The glen field needs to be a minimum value of one.

In terms of the NIST spec, we are a bit in a gray area here. NIST SP 800-90Ar1 says

What the returned_bits actually is if requested_number_of_bits = 0 isn't clear.

Right now, CSRNG produces one 128-bit block and also the following CTR_DRBG_Update is done and the reseed counter is incremented. I think this behavior is not intended from an RTL perspective. When receiving the Generate command in the command stage, there is just a check for glen = 1:

opentitan/hw/ip/csrng/rtl/csrng_cmd_stage.sv

Lines 378 to 391 in 9cfd621

	SendSOP: begin
	cmd_gen_1st_req = 1'b1;
	cmd_arb_sop_o = 1'b1;
	cmd_fifo_pop = 1'b1;
	if (sfifo_cmd_rdata[12+:GenBitsCntrWidth] == GenBitsCntrWidth'(1)) begin
	cmd_gen_cnt_last = 1'b1;
	end
	if (cmd_len == '0) begin
	cmd_arb_eop_o = 1'b1;
	state_d = GenCmdChk;
	end else begin
	state_d = SendMOP;
	end
	end

And then also the u_prim_count_cmd_gen_cntr output is compared against 1 again. But there is no check against 0. I think the current behavior is a coincidence.

Now that we have the capability to signal error status responses, it would probably be better to change the design to return an error response when receiving a Generate command with glen = 0.