lib: sbi: Respect scounteren when emulating the time CSR

This optimization creates a correctness issue, as it prevents supervisor
software from restricting VS-mode or U-mode access to the time CSR for
its own purposes.

Closes: https://github.com/riscv-software-src/opensbi/issues/370
Fixes: ebc8ebc0f8 ("lib: sbi: Improve HPM CSR read/write emulation")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

lib/sbi/sbi_emulate_csr.c

@@ -66,12 +66,11 @@ int sbi_emulate_csr_read(int csr_num, struct sbi_trap_regs *regs,
 		*csr_val = csr_read(CSR_MCYCLE);
 		break;
 	case CSR_TIME:
+		if (!hpm_allowed(csr_num - CSR_CYCLE, prev_mode, virt))
+			return SBI_ENOTSUPP;
 		/*
 		 * We emulate TIME CSR for both Host (HS/U-mode) and
 		 * Guest (VS/VU-mode).
-		 *
-		 * Faster TIME CSR reads are critical for good performance
-		 * in S-mode software so we don't check CSR permissions.
 		 */
 		*csr_val = (virt) ? sbi_timer_virt_value():
 				    sbi_timer_value();