[3.16.y-ckt stable] Patch "KVM: emulate: fix CMPXCHG8B on 32-bit hosts" has been added to staging queue

Luis Henriques luis.henriques at canonical.com
Thu Mar 19 10:12:29 UTC 2015

This is a note to let you know that I have just added a patch titled

    KVM: emulate: fix CMPXCHG8B on 32-bit hosts

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:


This patch is scheduled to be released in version 3.16.7-ckt9.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see



>From 91a77f5eabc664edfc161ba0da6e26a0b25ec425 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini at redhat.com>
Date: Thu, 12 Feb 2015 17:04:47 +0100
Subject: KVM: emulate: fix CMPXCHG8B on 32-bit hosts

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 at rdsoftware.de>
Tested-by: Erik Rull <erik.rull at rdsoftware.de>
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
 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 8c9f719453da..57604c0e5a53 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -4720,7 +4720,8 @@ 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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