mm: close PageTail race

commit 668f9abbd4334e6c29fa8acd71635c4f9101caa7 upstream.

Commit bf6bddf1924e ("mm: introduce compaction and migration for
ballooned pages") introduces page_count(page) into memory compaction
which dereferences page->first_page if PageTail(page).

This results in a very rare NULL pointer dereference on the
aforementioned page_count(page).  Indeed, anything that does
compound_head(), including page_count() is susceptible to racing with
prep_compound_page() and seeing a NULL or dangling page->first_page
pointer.

This patch uses Andrea's implementation of compound_trans_head() that
deals with such a race and makes it the default compound_head()
implementation.  This includes a read memory barrier that ensures that
if PageTail(head) is true that we return a head page that is neither
NULL nor dangling.  The patch then adds a store memory barrier to
prep_compound_page() to ensure page->first_page is set.

This is the safest way to ensure we see the head page that we are
expecting, PageTail(page) is already in the unlikely() path and the
memory barriers are unfortunately required.

Hugetlbfs is the exception, we don't enforce a store memory barrier
during init since no race is possible.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Holger Kiehl <Holger.Kiehl@dwd.de>
Cc: Christoph Lameter <cl@linux.com>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

diff --git a/drivers/block/aoe/aoecmd.c b/drivers/block/aoe/aoecmd.c
index d2515435e23f2f87215558ec703f5a625ab8ac80..8fb295350efbbda8ecc23fc49707bd2152747c90 100644 (file)
--- a/drivers/block/aoe/aoecmd.c
+++ b/drivers/block/aoe/aoecmd.c
@@ -905,7 +905,7 @@ bio_pageinc(struct bio *bio)
                /* Non-zero page count for non-head members of
                 * compound pages is no longer allowed by the kernel.
                 */
-               page = compound_trans_head(bv->bv_page);
+               page = compound_head(bv->bv_page);
                atomic_inc(&page->_count);
        }
 }
@@ -918,7 +918,7 @@ bio_pagedec(struct bio *bio)
        int i;
 
        bio_for_each_segment(bv, bio, i) {
-               page = compound_trans_head(bv->bv_page);
+               page = compound_head(bv->bv_page);
                atomic_dec(&page->_count);
        }
 }

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 4fb7a8f83c8a99ff8d3412a5328b2407c98409a8..54af4e93369583e5629a4bac6d1ecdbd74f885cf 100644 (file)
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -186,12 +186,12 @@ static bool is_invalid_reserved_pfn(unsigned long pfn)
        if (pfn_valid(pfn)) {
                bool reserved;
                struct page *tail = pfn_to_page(pfn);
-               struct page *head = compound_trans_head(tail);
+               struct page *head = compound_head(tail);
                reserved = !!(PageReserved(head));
                if (head != tail) {
                        /*
                         * "head" is not a dangling pointer
-                        * (compound_trans_head takes care of that)
+                        * (compound_head takes care of that)
                         * but the hugepage may have been split
                         * from under us (and we may not hold a
                         * reference count on the head page so it can

diff --git a/fs/proc/page.c b/fs/proc/page.c
index b8730d9ebaee651244d0d7e46e687792b46b0c9f..2a8cc94bb641b65196b75a29c077f09253a7b727 100644 (file)
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -121,7 +121,7 @@ u64 stable_page_flags(struct page *page)
         * just checks PG_head/PG_tail, so we need to check PageLRU to make
         * sure a given page is a thp, not a non-huge compound page.
         */
-       else if (PageTransCompound(page) && PageLRU(compound_trans_head(page)))
+       else if (PageTransCompound(page) && PageLRU(compound_head(page)))
                u |= 1 << KPF_THP;
 
        /*

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 3935428c57cff80d01236428fdea979d7a9f911b..a291552ab76778b54d14149e089fb4fd55867a5a 100644 (file)
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -156,23 +156,6 @@ static inline int hpage_nr_pages(struct page *page)
                return HPAGE_PMD_NR;
        return 1;
 }
-static inline struct page *compound_trans_head(struct page *page)
-{
-       if (PageTail(page)) {
-               struct page *head;
-               head = page->first_page;
-               smp_rmb();
-               /*
-                * head may be a dangling pointer.
-                * __split_huge_page_refcount clears PageTail before
-                * overwriting first_page, so if PageTail is still
-                * there it means the head pointer isn't dangling.
-                */
-               if (PageTail(page))
-                       return head;
-       }
-       return page;
-}
 
 extern int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
                                unsigned long addr, pmd_t pmd, pmd_t *pmdp);
@@ -202,7 +185,6 @@ static inline int split_huge_page(struct page *page)
        do { } while (0)
 #define split_huge_page_pmd_mm(__mm, __address, __pmd) \
        do { } while (0)
-#define compound_trans_head(page) compound_head(page)
 static inline int hugepage_madvise(struct vm_area_struct *vma,
                                   unsigned long *vm_flags, int advice)
 {

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 648bcb007ebac076344b1e09cc5459ca75936e40..073734339583e2f67f63a6327ea3a8aee15009e4 100644 (file)
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -371,8 +371,18 @@ static inline void compound_unlock_irqrestore(struct page *page,
 
 static inline struct page *compound_head(struct page *page)
 {
-       if (unlikely(PageTail(page)))
-               return page->first_page;
+       if (unlikely(PageTail(page))) {
+               struct page *head = page->first_page;
+
+               /*
+                * page->first_page may be a dangling pointer to an old
+                * compound page, so recheck that it is still a tail
+                * page before returning.
+                */
+               smp_rmb();
+               if (likely(PageTail(page)))
+                       return head;
+       }
        return page;
 }
 

diff --git a/mm/ksm.c b/mm/ksm.c
index 0bea2b262a47837f1c0bd81492e9cfdd30daa041..c78fff1e9eaeea700fd83ef283989c897949956e 100644 (file)
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -444,7 +444,7 @@ static void break_cow(struct rmap_item *rmap_item)
 static struct page *page_trans_compound_anon(struct page *page)
 {
        if (PageTransCompound(page)) {
-               struct page *head = compound_trans_head(page);
+               struct page *head = compound_head(page);
                /*
                 * head may actually be splitted and freed from under
                 * us but it's ok here.

diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index de476c2f88278e78eb3c32e4dc9834fff4be141f..5ea3cf73413814aad4d6554da9434602861cb08a 100644 (file)
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1659,7 +1659,7 @@ int soft_offline_page(struct page *page, int flags)
 {
        int ret;
        unsigned long pfn = page_to_pfn(page);
-       struct page *hpage = compound_trans_head(page);
+       struct page *hpage = compound_head(page);
 
        if (PageHWPoison(page)) {
                pr_info("soft offline: %#lx page already poisoned\n", pfn);

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 06f847933eeb7dd288b263fd8a52565f3316ea5a..6fca39097766d38ac36d2f52d424ae56a3deb717 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -369,9 +369,11 @@ void prep_compound_page(struct page *page, unsigned long order)
        __SetPageHead(page);
        for (i = 1; i < nr_pages; i++) {
                struct page *p = page + i;
-               __SetPageTail(p);
                set_page_count(p, 0);
                p->first_page = page;
+               /* Make sure p->first_page is always valid for PageTail() */
+               smp_wmb();
+               __SetPageTail(p);
        }
 }
 

diff --git a/mm/swap.c b/mm/swap.c
index 0c8f7a471925ebfd2ad7d41cb8963b64029c171b..aa4da5d9401d0937c007a4e2ccf95c2604847304 100644 (file)
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -84,7 +84,7 @@ static void put_compound_page(struct page *page)
 {
        if (unlikely(PageTail(page))) {
                /* __split_huge_page_refcount can run under us */
-               struct page *page_head = compound_trans_head(page);
+               struct page *page_head = compound_head(page);
 
                if (likely(page != page_head &&
                           get_page_unless_zero(page_head))) {
@@ -222,7 +222,7 @@ bool __get_page_tail(struct page *page)
         */
        unsigned long flags;
        bool got = false;
-       struct page *page_head = compound_trans_head(page);
+       struct page *page_head = compound_head(page);
 
        if (likely(page != page_head && get_page_unless_zero(page_head))) {
                /* Ref to put_compound_page() comment. */