linux-kernel - [PATCH RFC] kvm: write protect memory after slot swap

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Message-ID: <20101025012124.GA31262@redhat.com>
Date:	Mon, 25 Oct 2010 03:21:24 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	unlisted-recipients:; (no To-header on input)
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Gleb Natapov <gleb@...hat.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Sheng Yang <sheng@...ux.intel.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH RFC] kvm: write protect memory after slot swap

I have observed the following bug trigger:

1. userspace calls GET_DIRTY_LOG
2. kvm_mmu_slot_remove_write_access is called and makes a page ro
3. page fault happens and makes the page writeable
   fault is logged in the bitmap appropriately
4. kvm_vm_ioctl_get_dirty_log swaps slot pointers

a lot of time passes

5. guest writes into the page
6. userspace calls GET_DIRTY_LOG

At point (5), bitmap is clean and page is writeable,
thus, guest modification of memory is not logged
and GET_DIRTY_LOG returns an empty bitmap.

The rule is that all pages are either dirty in the current bitmap,
or write-protected, which is violated here.

It seems that just moving kvm_mmu_slot_remove_write_access down
to after the slot pointer swap should fix this bug.

Warning: completely untested.
Please comment.
Note: fix will be needed for -stable etc.

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
---
 arch/x86/kvm/x86.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3a09c62..4ca1d7f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2912,10 +2912,6 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 		struct kvm_memslots *slots, *old_slots;
 		unsigned long *dirty_bitmap;

-		spin_lock(&kvm->mmu_lock);
-		kvm_mmu_slot_remove_write_access(kvm, log->slot);
-		spin_unlock(&kvm->mmu_lock);
-
 		r = -ENOMEM;
 		dirty_bitmap = vmalloc(n);
 		if (!dirty_bitmap)
@@ -2937,6 +2933,10 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 		dirty_bitmap = old_slots->memslots[log->slot].dirty_bitmap;
 		kfree(old_slots);

+		spin_lock(&kvm->mmu_lock);
+		kvm_mmu_slot_remove_write_access(kvm, log->slot);
+		spin_unlock(&kvm->mmu_lock);
+
 		r = -EFAULT;
 		if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n)) {
 			vfree(dirty_bitmap);
-- 
1.7.3-rc1
--
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