KVM: emulate: fix CMPXCHG8B on 32-bit hosts

This has been broken for a long time: it broke first in 2.6.35, then was
almost fixed in 2.6.36 but this one-liner slipped through the cracks.
The bug shows up as an infinite loop in Windows 7 (and newer) boot on
32-bit hosts without EPT.

Windows uses CMPXCHG8B to write to page tables, which causes a
page fault if running without EPT; the emulator is then called from
kvm_mmu_page_fault.  The loop then happens if the higher 4 bytes are
not 0; the common case for this is that the NX bit (bit 63) is 1.

Fixes: 6550e1f165f384f3a46b60a1be9aba4bc3c2adad
Fixes: 16518d5ada690643453eb0aef3cc7841d3623c2d
Cc: stable@vger.kernel.org   # 2.6.35+
Reported-by: Erik Rull <erik.rull@rdsoftware.de>
Tested-by: Erik Rull <erik.rull@rdsoftware.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index e0b794a..106c015 100644 (file)
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4950,7 +4950,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
                        goto done;
                }
        }
-       ctxt->dst.orig_val = ctxt->dst.val;
+       /* Copy full 64-bit value for CMPXCHG8B.  */
+       ctxt->dst.orig_val64 = ctxt->dst.val64;
 
 special_insn: