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authorJan Stancek <jstancek@redhat.com>2019-01-08 15:23:28 -0800
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2019-01-16 22:03:26 +0100
commit2ed2db6a69d8e29b734c6db0037accceb7b69a2f (patch)
parent096909af76066819a8e4a9880799b4969e519007 (diff)
mm: page_mapped: don't assume compound page is huge or THP
commit 8ab88c7169b7fba98812ead6524b9d05bc76cf00 upstream. 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 The 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/repro.c 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.git.jstancek@redhat.com 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> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
1 files changed, 1 insertions, 1 deletions
diff --git a/mm/util.c b/mm/util.c
index 8bf08b5b5760..5c9c7359ee8a 100644
--- a/mm/util.c
+++ b/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;