commit 7ac77099ce88a0c31b75acd0ec5ef3da4415a6d8
author Avi Kivity <avi@redhat.com> Mon Jun 21 10:57:45 2010 +0300
committer Avi Kivity <avi@redhat.com> Sun Aug 01 10:47:08 2010 +0300
tree 20fcf29491d762cb0d378c6bbda0aadb81748418
parent e36d96f7cfaa71870c407131eb4fbd38ea285c01

KVM: Prevent internal slots from being COWed

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@redhat.com>

arch/x86/kvm/x86.c

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7ef4410..68be38e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5491,6 +5491,11 @@
 				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.
@@ -5503,7 +5508,7 @@
 			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);