KVM: Fix gfn_to_page() acquiring mmap_sem twice

KVM's nopage handler calls gfn_to_page() which acquires the mmap_sem when
calling out to get_user_pages().  nopage handlers are already invoked with the
mmap_sem held though.  Introduce a __gfn_to_page() for use by the nopage
handler which requires the lock to already be held.

This was noticed by tglx.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

1 files changed, 18 insertions, 4 deletions

diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index ac5ed00e9065..f439e45233f2 100644
--- a/drivers/kvm/kvm_main.c
+++ b/drivers/kvm/kvm_main.c
@@ -633,7 +633,10 @@ int kvm_is_visible_gfn(struct kvm *kvm, gfn_t gfn)
 }
 EXPORT_SYMBOL_GPL(kvm_is_visible_gfn);
 
-struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
+/*
+ * Requires current->mm->mmap_sem to be held
+ */
+static struct page *__gfn_to_page(struct kvm *kvm, gfn_t gfn)
 {
         struct kvm_memory_slot *slot;
         struct page *page[1];
@@ -648,12 +651,10 @@ struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
                 return bad_page;
         }
 
-        down_read(&current->mm->mmap_sem);
         npages = get_user_pages(current, current->mm,
                                 slot->userspace_addr
                                 + (gfn - slot->base_gfn) * PAGE_SIZE, 1,
                                 1, 1, page, NULL);
-        up_read(&current->mm->mmap_sem);
         if (npages != 1) {
                 get_page(bad_page);
                 return bad_page;
@@ -661,6 +662,18 @@ struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
 
         return page[0];
 }
+
+struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
+{
+        struct page *page;
+
+        down_read(&current->mm->mmap_sem);
+        page = __gfn_to_page(kvm, gfn);
+        up_read(&current->mm->mmap_sem);
+
+        return page;
+}
+
 EXPORT_SYMBOL_GPL(gfn_to_page);
 
 void kvm_release_page(struct page *page)
@@ -2621,7 +2634,8 @@ static struct page *kvm_vm_nopage(struct vm_area_struct *vma,
         pgoff = ((address - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
         if (!kvm_is_visible_gfn(kvm, pgoff))
                 return NOPAGE_SIGBUS;
-        page = gfn_to_page(kvm, pgoff);
+        /* current->mm->mmap_sem is already held so call lockless version */
+        page = __gfn_to_page(kvm, pgoff);
         if (is_error_page(page)) {
                 kvm_release_page(page);
                 return NOPAGE_SIGBUS;