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Subject[PATCH 23/55] KVM: Fix gfn_to_page() acquiring mmap_sem twice
Date
From: Anthony Liguori <aliguori@us.ibm.com>

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>
---
drivers/kvm/kvm_main.c | 22 ++++++++++++++++++----
1 files changed, 18 insertions(+), 4 deletions(-)
diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index c3dc24f..7186791 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;
--
1.5.3.7


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