linux-kernel - [55/80] KVM: Prevent internal slots from being COWed

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Message-Id: <20100924162619.660917470@clark.site>
Date:	Fri, 24 Sep 2010 09:24:43 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org, greg@...ah.com
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	avi@...hat.com, mtosatti@...hat.com
Subject: [55/80] KVM: Prevent internal slots from being COWed

2.6.35-stable review patch.  If anyone has any objections, please let us know.

------------------


From: Avi Kivity <avi@...hat.com>

commit 7ac77099ce88a0c31b75acd0ec5ef3da4415a6d8 upstream.

If a process with a memory slot is COWed, the page will change its address
(despite having an elevated reference count).  This breaks internal memory
slots which have their physical addresses loaded into vmcs registers (see
the APIC access memory slot).

Signed-off-by: Avi Kivity <avi@...hat.com>
Cc: Marcelo Tosatti <mtosatti@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>
---
 arch/x86/kvm/x86.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5438,6 +5438,11 @@ int kvm_arch_prepare_memory_region(struc
 				int user_alloc)
 {
 	int npages = memslot->npages;
+	int map_flags = MAP_PRIVATE | MAP_ANONYMOUS;
+
+	/* Prevent internal slot pages from being moved by fork()/COW. */
+	if (memslot->id >= KVM_MEMORY_SLOTS)
+		map_flags = MAP_SHARED | MAP_ANONYMOUS;
 
 	/*To keep backward compatibility with older userspace,
 	 *x86 needs to hanlde !user_alloc case.
@@ -5450,7 +5455,7 @@ int kvm_arch_prepare_memory_region(struc
 			userspace_addr = do_mmap(NULL, 0,
 						 npages * PAGE_SIZE,
 						 PROT_READ | PROT_WRITE,
-						 MAP_PRIVATE | MAP_ANONYMOUS,
+						 map_flags,
 						 0);
 			up_write(&current->mm->mmap_sem);
 


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