The fixup_page_fault function in arch/x86/traps.c in Xen 4.0.1 and earlier on 64-bit platforms, when paravirtualization is enabled, does not verify that kernel mode is used to call the handle_gdt_ldt_mapping_fault function, which allows guest OS users to cause a denial of service (host OS BUG_ON) via a crafted memory access.
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