[PATCH 1/1] mm: thp: kvm: fix memory corruption in KVM with THP enabled

[Andrea Arcangeli]

After the THP refcounting change, obtaining a compound pages from
get_user_pages() no longer allows us to assume the entire compound
page is immediately mappable from a secondary MMU.

A secondary MMU doesn't want to call get_user_pages() more than once
for each compound page, in order to know if it can map the whole
compound page. So a secondary MMU needs to know from a single
get_user_pages() invocation when it can map immediately the entire
compound page to avoid a flood of unnecessary secondary MMU faults and
spurious atomic_inc()/atomic_dec() (pages don't have to be pinned by
MMU notifier users).

Ideally instead of the page->_mapcount < 1 check, get_user_pages()
should return the granularity of the "page" mapping in the "mm" passed
to get_user_pages(). However it's non trivial change to pass the "pmd"
status belonging to the "mm" walked by get_user_pages up the stack (up
to the caller of get_user_pages). So the fix just checks if there is
not a single pte mapping on the page returned by get_user_pages, and
in turn if the caller can assume that the whole compound page is
mapped in the current "mm" (in a pmd_trans_huge()). In such case the
entire compound page is safe to map into the secondary MMU without
additional get_user_pages() calls on the surrounding tail/head
pages. In addition of being faster, not having to run other
get_user_pages() calls also reduces the memory footprint of the
secondary MMU fault in case the pmd split happened as result of memory
pressure.

Without this fix after a MADV_DONTNEED (like invoked by QEMU during
postcopy live migration or balloning) or after generic swapping (with
a failure in split_huge_page() that would only result in pmd splitting
and not a physical page split), KVM would map the whole compound page
into the shadow pagetables, despite regular faults or userfaults (like
UFFDIO_COPY) may map regular pages into the primary MMU as result of
the pte faults, leading to the guest mode and userland mode going out
of sync and not working on the same memory at all times.

Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
---
 arch/arm/kvm/mmu.c         |  2 +-
 arch/x86/kvm/mmu.c         |  4 ++--
 include/linux/page-flags.h | 22 ++++++++++++++++++++++
 3 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index 58dbd5c..d6d4191 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -1004,7 +1004,7 @@ static bool transparent_hugepage_adjust(kvm_pfn_t *pfnp, phys_addr_t *ipap)
 	kvm_pfn_t pfn = *pfnp;
 	gfn_t gfn = *ipap >> PAGE_SHIFT;
 
-	if (PageTransCompound(pfn_to_page(pfn))) {
+	if (PageTransCompoundMap(pfn_to_page(pfn))) {
 		unsigned long mask;
 		/*
 		 * The address we faulted on is backed by a transparent huge
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 1ff4dbb..b6f50e8 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -2823,7 +2823,7 @@ static void transparent_hugepage_adjust(struct kvm_vcpu *vcpu,
 	 */
 	if (!is_error_noslot_pfn(pfn) && !kvm_is_reserved_pfn(pfn) &&
 	    level == PT_PAGE_TABLE_LEVEL &&
-	    PageTransCompound(pfn_to_page(pfn)) &&
+	    PageTransCompoundMap(pfn_to_page(pfn)) &&
 	    !mmu_gfn_lpage_is_disallowed(vcpu, gfn, PT_DIRECTORY_LEVEL)) {
 		unsigned long mask;
 		/*
@@ -4785,7 +4785,7 @@ restart:
 		 */
 		if (sp->role.direct &&
 			!kvm_is_reserved_pfn(pfn) &&
-			PageTransCompound(pfn_to_page(pfn))) {
+			PageTransCompoundMap(pfn_to_page(pfn))) {
 			drop_spte(kvm, sptep);
 			need_tlb_flush = 1;
 			goto restart;
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index f4ed4f1b..6b052aa 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -517,6 +517,27 @@ static inline int PageTransCompound(struct page *page)
 }
 
 /*
+ * PageTransCompoundMap is the same as PageTransCompound, but it also
+ * guarantees the primary MMU has the entire compound page mapped
+ * through pmd_trans_huge, which in turn guarantees the secondary MMUs
+ * can also map the entire compound page. This allows the secondary
+ * MMUs to call get_user_pages() only once for each compound page and
+ * to immediately map the entire compound page with a single secondary
+ * MMU fault. If there will be a pmd split later, the secondary MMUs
+ * will get an update through the MMU notifier invalidation through
+ * split_huge_pmd().
+ *
+ * Unlike PageTransCompound, this is safe to be called only while
+ * split_huge_pmd() cannot run from under us, like if protected by the
+ * MMU notifier, otherwise it may result in page->_mapcount < 0 false
+ * positives.
+ */
+static inline int PageTransCompoundMap(struct page *page)
+{
+	return PageTransCompound(page) && atomic_read(&page->_mapcount) < 0;
+}
+
+/*
  * PageTransTail returns true for both transparent huge pages
  * and hugetlbfs pages, so it should only be called when it's known
  * that hugetlbfs pages aren't involved.
@@ -559,6 +580,7 @@ static inline int TestClearPageDoubleMap(struct page *page)
 #else
 TESTPAGEFLAG_FALSE(TransHuge)
 TESTPAGEFLAG_FALSE(TransCompound)
+TESTPAGEFLAG_FALSE(TransCompoundMap)
 TESTPAGEFLAG_FALSE(TransTail)
 TESTPAGEFLAG_FALSE(DoubleMap)
 	TESTSETFLAG_FALSE(DoubleMap)