fw_cfg: avoid calculating invalid current entry pointer

When calculating a pointer to the currently selected fw_cfg item, the
following is used:

  FWCfgEntry *e = &s->entries[arch][s->cur_entry & FW_CFG_ENTRY_MASK];

When s->cur_entry is FW_CFG_INVALID, we are calculating the address of
a non-existent element in s->entries[arch][...], which is undefined.

This patch ensures the resulting entry pointer is set to NULL whenever
s->cur_entry is FW_CFG_INVALID.
Reported-by: NLaszlo Ersek <lersek@redhat.com>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NGabriel Somlo <somlo@cmu.edu>
Message-id: 1446733972-1602-5-git-send-email-somlo@cmu.edu
Cc: Marc Marí <markmb@redhat.com>
Signed-off-by: NGabriel Somlo <somlo@cmu.edu>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/nvram/fw_cfg.c

@@ -277,7 +277,8 @@ static int fw_cfg_select(FWCfgState *s, uint16_t key)
 static uint8_t fw_cfg_read(FWCfgState *s)
 {
     int arch = !!(s->cur_entry & FW_CFG_ARCH_LOCAL);
-    FWCfgEntry *e = &s->entries[arch][s->cur_entry & FW_CFG_ENTRY_MASK];
+    FWCfgEntry *e = (s->cur_entry == FW_CFG_INVALID) ? NULL :
+                    &s->entries[arch][s->cur_entry & FW_CFG_ENTRY_MASK];
     uint8_t ret;
 
     if (s->cur_entry == FW_CFG_INVALID || !e->data || s->cur_offset >= e->len)
@@ -342,7 +343,8 @@ static void fw_cfg_dma_transfer(FWCfgState *s)
     }
 
     arch = !!(s->cur_entry & FW_CFG_ARCH_LOCAL);
-    e = &s->entries[arch][s->cur_entry & FW_CFG_ENTRY_MASK];
+    e = (s->cur_entry == FW_CFG_INVALID) ? NULL :
+        &s->entries[arch][s->cur_entry & FW_CFG_ENTRY_MASK];
 
     if (dma.control & FW_CFG_DMA_CTL_READ) {
         read = 1;