[PATCH 3.12 109/175] KVM: emulate: fix CMPXCHG8B on 32-bit hosts

From: Jiri Slaby
Date: Tue Mar 17 2015 - 05:05:29 EST

From: Paolo Bonzini <pbonzini@xxxxxxxxxx>

3.12-stable review patch. If anyone has any objections, please let me know.


commit 4ff6f8e61eb7f96d3ca535c6d240f863ccd6fb7d upstream.

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
Reported-by: Erik Rull <erik.rull@xxxxxxxxxxxxx>
Tested-by: Erik Rull <erik.rull@xxxxxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
arch/x86/kvm/emulate.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 8ab43ac68f06..c412bab82d1f 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4617,7 +4617,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
if (rc != X86EMUL_CONTINUE)
goto done;
- ctxt->dst.orig_val = ctxt->dst.val;
+ /* Copy full 64-bit value for CMPXCHG8B. */
+ ctxt->dst.orig_val64 = ctxt->dst.val64;



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