hw/misc/zynq_slcr: Avoid #DIV/0! error

Malicious user can set the feedback divisor for the PLLs
to zero, triggering a floating-point exception (SIGFPE).

As the datasheet [*] is not clear how hardware behaves
when these bits are zeroes, use the maximum divisor
possible (128) to avoid the software FPE.

[*] Zynq-7000 TRM, UG585 (v1.12.2)
    B.28 System Level Control Registers (slcr)
    -> "Register (slcr) ARM_PLL_CTRL"
    25.10.4 PLLs
    -> "Software-Controlled PLL Update"

Fixes: 38867cb7ec9 ("hw/misc/zynq_slcr: add clock generation for uarts")
Reported-by: Gaoning Pan <pgn@zju.edu.cn>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Damien Hedde <damien.hedde@greensocs.com>
Message-id: 20201210141610.884600-1-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/misc/zynq_slcr.c

@@ -217,6 +217,11 @@ static uint64_t zynq_slcr_compute_pll(uint64_t input, uint32_t ctrl_reg)
         return 0;
     }
 
+    /* Consider zero feedback as maximum divide ratio possible */
+    if (!mult) {
+        mult = 1 << R_xxx_PLL_CTRL_PLL_FPDIV_LENGTH;
+    }
+
     /* frequency multiplier -> period division */
     return input / mult;
 }