hw/net/fsl_etsec/rings.c: Avoid variable length array

In fill_rx_bd() we create a variable length array of size
etsec->rx_padding. In fact we know that this will never be
larger than 64 bytes, because rx_padding is set in rx_init_frame()
in a way that ensures it is only that large. Use a fixed sized
array and assert that it is big enough.

Since padd[] is now potentially rather larger than the actual
padding required, adjust the memset() we do on it to match the
size that we write with cpu_physical_memory_write(), rather than
clearing the entire array.

The codebase has very few VLAs, and if we can get rid of them all we
can make the compiler error on new additions.  This is a defensive
measure against security bugs where an on-stack dynamic allocation
isn't correctly size-checked (e.g.  CVE-2021-3527).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>

hw/net/fsl_etsec/rings.c

@@ -372,6 +372,12 @@ void etsec_walk_tx_ring(eTSEC *etsec, int ring_nbr)
     etsec->regs[TSTAT].value |= 1 << (31 - ring_nbr);
 }
 
+/*
+ * rx_init_frame() ensures we never do more padding than this
+ * (checksum plus minimum data packet size)
+ */
+#define MAX_RX_PADDING 64
+
 static void fill_rx_bd(eTSEC          *etsec,
                        eTSEC_rxtx_bd  *bd,
                        const uint8_t **buf,
@@ -380,9 +386,11 @@ static void fill_rx_bd(eTSEC          *etsec,
     uint16_t to_write;
     hwaddr   bufptr = bd->bufptr +
         ((hwaddr)(etsec->regs[TBDBPH].value & 0xF) << 32);
-    uint8_t  padd[etsec->rx_padding];
+    uint8_t  padd[MAX_RX_PADDING];
     uint8_t  rem;
 
+    assert(etsec->rx_padding <= MAX_RX_PADDING);
+
     RING_DEBUG("eTSEC fill Rx buffer @ 0x%016" HWADDR_PRIx
                " size:%zu(padding + crc:%u) + fcb:%u\n",
                bufptr, *size, etsec->rx_padding, etsec->rx_fcb_size);
@@ -426,7 +434,7 @@ static void fill_rx_bd(eTSEC          *etsec,
         rem = MIN(etsec->regs[MRBLR].value - bd->length, etsec->rx_padding);
 
         if (rem > 0) {
-            memset(padd, 0x0, sizeof(padd));
+            memset(padd, 0x0, rem);
             etsec->rx_padding -= rem;
             *size             -= rem;
             bd->length        += rem;