Silicon/Synopsys/DesignWare: DwEmacSnpDxe: Fix bug in EmacGetDmaStatus

The EFI spec (see UEFI 2.10, 24.1.12) requires EFI_SIMPLE_NETWORK's
GetStatus() method to handle NULL InterruptStatus pointers by not
reading nor clearing the interrupt status from the device.

However, EmacGetDmaStatus (part of the DwEmacSnpDxe GetStatus()
implementation) did not correctly handle NULL IrqStat, despite already
being tagged as an OPTIONAL argument. This made calling GetStatus() with
a NULL pointer (for example, the call in MnpRecycleTxBuf) either corrupt
memory or straight-up crash.

Make it EFI spec compliant, by adding proper NULL pointer checks around
RI_SET_MSK and TI_SET_MSK retrieval/clearing.

Signed-off-by: Yang Wang <wangyzhaoz@163.com>
[ardb: update commit log according to Pedro's suggestion]
Acked-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

Silicon/Synopsys/DesignWare/Drivers/DwEmacSnpDxe/EmacDxeUtil.c

@@ -500,24 +500,30 @@ EmacGetDmaStatus (
   UINT32  ErrorBit;
   UINT32  Mask = 0;
 
+  if (IrqStat != NULL) {
+    *IrqStat = 0;
+  }
+
   DmaStatus = MmioRead32 (MacBaseAddress +
                            DW_EMAC_DMAGRP_STATUS_OFST);
   if (DmaStatus & DW_EMAC_DMAGRP_STATUS_NIS_SET_MSK) {
     Mask |= DW_EMAC_DMAGRP_STATUS_NIS_SET_MSK;
     // Rx interrupt
     if (DmaStatus & DW_EMAC_DMAGRP_STATUS_RI_SET_MSK) {
-      *IrqStat |= EFI_SIMPLE_NETWORK_RECEIVE_INTERRUPT;
-      Mask |= DW_EMAC_DMAGRP_STATUS_RI_SET_MSK;
-    } else {
-      *IrqStat &= ~EFI_SIMPLE_NETWORK_RECEIVE_INTERRUPT;
+      if (IrqStat != NULL) {
+        *IrqStat |= EFI_SIMPLE_NETWORK_RECEIVE_INTERRUPT;
+        Mask |= DW_EMAC_DMAGRP_STATUS_RI_SET_MSK;
+      }
     }
+
     // Tx interrupt
     if (DmaStatus & DW_EMAC_DMAGRP_STATUS_TI_SET_MSK) {
-      *IrqStat |= EFI_SIMPLE_NETWORK_TRANSMIT_INTERRUPT;
-      Mask |= DW_EMAC_DMAGRP_STATUS_TI_SET_MSK;
-    } else {
-      *IrqStat &= ~EFI_SIMPLE_NETWORK_TRANSMIT_INTERRUPT;
+      if (IrqStat != NULL) {
+        *IrqStat |= EFI_SIMPLE_NETWORK_TRANSMIT_INTERRUPT;
+        Mask |= DW_EMAC_DMAGRP_STATUS_TI_SET_MSK;
+      }
     }
+
     // Tx Buffer
     if (DmaStatus & DW_EMAC_DMAGRP_STATUS_TU_SET_MSK){
       Mask |= DW_EMAC_DMAGRP_STATUS_TU_SET_MSK;