hw/display/qxl: Avoid buffer overrun in qxl_phys2virt (CVE-2022-4144)

Have qxl_get_check_slot_offset() return false if the requested
buffer size does not fit within the slot memory region.

Similarly qxl_phys2virt() now returns NULL in such case, and
qxl_dirty_one_surface() aborts.

This avoids buffer overrun in the host pointer returned by
memory_region_get_ram_ptr().

Fixes: CVE-2022-4144 (out-of-bounds read)
Reported-by: Wenxu Yin (@awxylitol)
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1336
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221128202741.4945-5-philmd@linaro.org>

hw/display/qxl.c

@@ -1424,11 +1424,13 @@ static void qxl_reset_surfaces(PCIQXLDevice *d)
 
 /* can be also called from spice server thread context */
 static bool qxl_get_check_slot_offset(PCIQXLDevice *qxl, QXLPHYSICAL pqxl,
-                                      uint32_t *s, uint64_t *o)
+                                      uint32_t *s, uint64_t *o,
+                                      size_t size_requested)
 {
     uint64_t phys   = le64_to_cpu(pqxl);
     uint32_t slot   = (phys >> (64 -  8)) & 0xff;
     uint64_t offset = phys & 0xffffffffffff;
+    uint64_t size_available;
 
     if (slot >= NUM_MEMSLOTS) {
         qxl_set_guest_bug(qxl, "slot too large %d >= %d", slot,
@@ -1452,6 +1454,23 @@ static bool qxl_get_check_slot_offset(PCIQXLDevice *qxl, QXLPHYSICAL pqxl,
                           slot, offset, qxl->guest_slots[slot].size);
         return false;
     }
+    size_available = memory_region_size(qxl->guest_slots[slot].mr);
+    if (qxl->guest_slots[slot].offset + offset >= size_available) {
+        qxl_set_guest_bug(qxl,
+                          "slot %d offset %"PRIu64" > region size %"PRIu64"\n",
+                          slot, qxl->guest_slots[slot].offset + offset,
+                          size_available);
+        return false;
+    }
+    size_available -= qxl->guest_slots[slot].offset + offset;
+    if (size_requested > size_available) {
+        qxl_set_guest_bug(qxl,
+                          "slot %d offset %"PRIu64" size %zu: "
+                          "overrun by %"PRIu64" bytes\n",
+                          slot, offset, size_requested,
+                          size_requested - size_available);
+        return false;
+    }
 
     *s = slot;
     *o = offset;
@@ -1471,7 +1490,7 @@ void *qxl_phys2virt(PCIQXLDevice *qxl, QXLPHYSICAL pqxl, int group_id,
         offset = le64_to_cpu(pqxl) & 0xffffffffffff;
         return (void *)(intptr_t)offset;
     case MEMSLOT_GROUP_GUEST:
-        if (!qxl_get_check_slot_offset(qxl, pqxl, &slot, &offset)) {
+        if (!qxl_get_check_slot_offset(qxl, pqxl, &slot, &offset, size)) {
             return NULL;
         }
         ptr = memory_region_get_ram_ptr(qxl->guest_slots[slot].mr);
@@ -1937,9 +1956,9 @@ static void qxl_dirty_one_surface(PCIQXLDevice *qxl, QXLPHYSICAL pqxl,
     uint32_t slot;
     bool rc;
 
-    rc = qxl_get_check_slot_offset(qxl, pqxl, &slot, &offset);
-    assert(rc == true);
     size = (uint64_t)height * abs(stride);
+    rc = qxl_get_check_slot_offset(qxl, pqxl, &slot, &offset, size);
+    assert(rc == true);
     trace_qxl_surfaces_dirty(qxl->id, offset, size);
     qxl_set_dirty(qxl->guest_slots[slot].mr,
                   qxl->guest_slots[slot].offset + offset,

hw/display/qxl.h

@@ -157,7 +157,7 @@ OBJECT_DECLARE_SIMPLE_TYPE(PCIQXLDevice, PCI_QXL)
  *
  * Returns a host pointer to a buffer placed at offset @phys within the
  * active slot @group_id of the PCI VGA RAM memory region associated with
- * the @qxl device. If the slot is inactive, or the offset is out
+ * the @qxl device. If the slot is inactive, or the offset + size are out
  * of the memory region, returns NULL.
  *
  * Use with care; by the time this function returns, the returned pointer is