HVM guest creation crashing with 1 MB of guest memory

Hi,

When an HVM guest is started and assigned exactly one Megabyte of RAM, XEN will crash due to a overflow in building the E820 map in line 90 of tools/libxc/xc_hvm_build.c:
e820entry[nr_map].size = mem_size - 0x100000 - PAGE_SIZE * 3;
If memsize is one MB, the result will be negative (and the member is unsigned). As I don't think this is worth fixing here, I simply increased the minimal required memory for guest from zero to one Megabyte. Could someone check the effect on paravirtualized guests?

Regards,
Andre.

--
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany

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diff -r 3a5722420de7 tools/python/xen/xend/XendConfig.py
@@ -360,9 +360,9 @@ class XendConfig(dict):
         if not self["memory_dynamic_max"] <= self["memory_static_max"]:
             raise XendConfigError("memory_dynamic_max must be less " \
                                   "than or equal to memory_static_max")
-        if not self["memory_dynamic_max"] > 0:
+        if not self["memory_dynamic_max"] > 1048576:
             raise XendConfigError("memory_dynamic_max must be greater " \
-                                  "than zero")
+                                  "than one MB")
         if not self["memory_static_max"] > 0:
             raise XendConfigError("memory_static_max must be greater " \
                                   "than zero")

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Better to check and reject the memory size in xc_hvm_build() itself. I'll make a patch to do this.

-- Keir