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    Subject[PATCH 4.14] mm: page_alloc: validate buddy page before using
    Date
    Commit 787af64d05cd ("mm: page_alloc: validate buddy before check its migratetype.")
    fixes a bug in 1dd214b8f21c and there is a similar bug in d9dddbf55667 that
    can be fixed in a similar way too.

    In addition, for RISC-V arch the first 2MB RAM could be reserved for opensbi,
    so it would have pfn_base=512 and mem_map began with 512th PFN when
    CONFIG_FLATMEM=y.
    But __find_buddy_pfn algorithm thinks the start pfn 0, it could get 0 pfn or
    less than the pfn_base value. We need page_is_buddy() to verify the buddy to
    prevent accessing an invalid buddy.

    Fixes: d9dddbf55667 ("mm/page_alloc: prevent merging between isolated and other pageblocks")
    Cc: stable@vger.kernel.org
    Reported-by: zjb194813@alibaba-inc.com
    Reported-by: tianhu.hh@alibaba-inc.com
    Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
    ---
    mm/page_alloc.c | 3 +++
    1 file changed, 3 insertions(+)

    diff --git a/mm/page_alloc.c b/mm/page_alloc.c
    index bfbccc739332..6e994a2013aa 100644
    --- a/mm/page_alloc.c
    +++ b/mm/page_alloc.c
    @@ -866,6 +866,9 @@ static inline void __free_one_page(struct page *page,

    buddy_pfn = __find_buddy_pfn(pfn, order);
    buddy = page + (buddy_pfn - pfn);
    +
    + if (!page_is_buddy(page, buddy, order))
    + goto done_merging;
    buddy_mt = get_pageblock_migratetype(buddy);

    if (migratetype != buddy_mt
    --
    2.17.1
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