hw/i386/intel_iommu: vtd_slpte_nonzero_rsvd(): assert no overflow

We support only 3- and 4-level page-tables, which is firstly checked in
vtd_decide_config(), then setup in vtd_init(). Than level fields are
checked by vtd_is_level_supported().

So here we can't have level out from 1..4 inclusive range. Let's assert
it. That also explains Coverity that we are not going to overflow the
array.

CID: 1487158, 1487186
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Maksim Davydov <davydov-max@yandex-team.ru>
Message-id: 20231017125941.810461-2-vsementsov@yandex-team.ru
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/i386/intel_iommu.c

@@ -1045,18 +1045,35 @@ static dma_addr_t vtd_get_iova_pgtbl_base(IntelIOMMUState *s,
  * Rsvd field masks for spte:
  *     vtd_spte_rsvd 4k pages
  *     vtd_spte_rsvd_large large pages
+ *
+ * We support only 3-level and 4-level page tables (see vtd_init() which
+ * sets only VTD_CAP_SAGAW_39bit and maybe VTD_CAP_SAGAW_48bit bits in s->cap).
  */
-static uint64_t vtd_spte_rsvd[5];
-static uint64_t vtd_spte_rsvd_large[5];
+#define VTD_SPTE_RSVD_LEN 5
+static uint64_t vtd_spte_rsvd[VTD_SPTE_RSVD_LEN];
+static uint64_t vtd_spte_rsvd_large[VTD_SPTE_RSVD_LEN];
 
 static bool vtd_slpte_nonzero_rsvd(uint64_t slpte, uint32_t level)
 {
-    uint64_t rsvd_mask = vtd_spte_rsvd[level];
+    uint64_t rsvd_mask;
+
+    /*
+     * We should have caught a guest-mis-programmed level earlier,
+     * via vtd_is_level_supported.
+     */
+    assert(level < VTD_SPTE_RSVD_LEN);
+    /*
+     * Zero level doesn't exist. The smallest level is VTD_SL_PT_LEVEL=1 and
+     * checked by vtd_is_last_slpte().
+     */
+    assert(level);
 
     if ((level == VTD_SL_PD_LEVEL || level == VTD_SL_PDP_LEVEL) &&
         (slpte & VTD_SL_PT_PAGE_SIZE_MASK)) {
         /* large page */
         rsvd_mask = vtd_spte_rsvd_large[level];
+    } else {
+        rsvd_mask = vtd_spte_rsvd[level];
     }
 
     return slpte & rsvd_mask;