+ mm-page_mapped-dont-assume-compound-page-is-huge-or-thp.patch added to -mm tree - Linux-stable-mirror

5 Jan 2019

The patch titled
     Subject: mm: page_mapped: don't assume compound page is huge or THP
has been added to the -mm tree.  Its filename is
     mm-page_mapped-dont-assume-compound-page-is-huge-or-thp.patch
This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/mm-page_mapped-dont-assume-compound...
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/mm-page_mapped-dont-assume-compound...
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------------------------------------------------------
From: Jan Stancek jstancek@redhat.com
Subject: mm: page_mapped: don't assume compound page is huge or THP
LTP proc01 testcase has been observed to rarely trigger crashes
on arm64:
    page_mapped+0x78/0xb4
    stable_page_flags+0x27c/0x338
    kpageflags_read+0xfc/0x164
    proc_reg_read+0x7c/0xb8
    __vfs_read+0x58/0x178
    vfs_read+0x90/0x14c
    SyS_read+0x60/0xc0
Issue is that page_mapped() assumes that if compound page is not huge,
then it must be THP.  But if this is 'normal' compound page
(COMPOUND_PAGE_DTOR), then following loop can keep running (for
HPAGE_PMD_NR iterations) until it tries to read from memory that isn't
mapped and triggers a panic:
for (i = 0; i < hpage_nr_pages(page); i++) {
                if (atomic_read(&page[i]._mapcount) >= 0)
                        return true;
    }
I could replicate this on x86 (v4.20-rc4-98-g60b548237fed) only
with a custom kernel module [1] which:
- allocates compound page (PAGEC) of order 1
- allocates 2 normal pages (COPY), which are initialized to 0xff
  (to satisfy _mapcount >= 0)
- 2 PAGEC page structs are copied to address of first COPY page
- second page of COPY is marked as not present
- call to page_mapped(COPY) now triggers fault on access to 2nd
  COPY page at offset 0x30 (_mapcount)
[1] https://github.com/jstancek/reproducers/blob/master/kernel/page_mapped_crash...
Fix the loop to iterate for "1 << compound_order" pages.
Kirrill said "IIRC, sound subsystem can producuce custom mapped compound
pages".
Link: http://lkml.kernel.org/r/c440d69879e34209feba21e12d236d06bc0a25db.1543577156...
Fixes: e1534ae95004 ("mm: differentiate page_mapped() from page_mapcount() for compound pages")
Signed-off-by: Jan Stancek jstancek@redhat.com
Debugged-by: Laszlo Ersek lersek@redhat.com
Suggested-by: "Kirill A. Shutemov" kirill@shutemov.name
Acked-by: Michal Hocko mhocko@suse.com
Acked-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com
Reviewed-by: David Hildenbrand david@redhat.com
Reviewed-by: Andrea Arcangeli aarcange@redhat.com
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Morton akpm@linux-foundation.org
---

--- a/mm/util.c~mm-page_mapped-dont-assume-compound-page-is-huge-or-thp
+++ a/mm/util.c
@@ -478,7 +478,7 @@ bool page_mapped(struct page *page)
    	return true;
    if (PageHuge(page))
    	return false;
-	for (i = 0; i < hpage_nr_pages(page); i++) {
+	for (i = 0; i < (1 << compound_order(page)); i++) {
    	if (atomic_read(&page[i]._mapcount) >= 0)
    		return true;
    }
_
Patches currently in -mm which might be from jstancek@redhat.com are
mm-page_mapped-dont-assume-compound-page-is-huge-or-thp.patch

    