1. 27 Mar, 2007 1 commit
    • Avi Kivity's avatar
      KVM: Prevent system selectors leaking into guest on real->protected mode transition on vmx · 6af11b9e
      Avi Kivity authored
      Intel virtualization extensions do not support virtualizing real mode.  So
      kvm uses virtualized vm86 mode to run real mode code.  Unfortunately, this
      virtualized vm86 mode does not support the so called "big real" mode, where
      the segment selector and base do not agree with each other according to the
      real mode rules (base == selector << 4).
      
      To work around this, kvm checks whether a selector/base pair violates the
      virtualized vm86 rules, and if so, forces it into conformance.  On a
      transition back to protected mode, if we see that the guest did not touch
      a forced segment, we restore it back to the original protected mode value.
      
      This pile of hacks breaks down if the gdt has changed in real mode, as it
      can cause a segment selector to point to a system descriptor instead of a
      normal data segment.  In fact, this happens with the Windows bootloader
      and the qemu acpi bios, where a protected mode memcpy routine issues an
      innocent 'pop %es' and traps on an attempt to load a system descriptor.
      
      "Fix" by checking if the to-be-restored selector points at a system segment,
      and if so, coercing it into a normal data segment.  The long term solution,
      of course, is to abandon vm86 mode and use emulation for big real mode.
      Signed-off-by: default avatarAvi Kivity <avi@qumranet.com>
      6af11b9e
  2. 26 Mar, 2007 39 commits