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    Subject[PATCH AUTOSEL 5.1 128/141] mm/mincore.c: fix race between swapoff and mincore
    Date
    From: Huang Ying <ying.huang@intel.com>

    [ Upstream commit aeb309b81c6bada783c3695528a3e10748e97285 ]

    Via commit 4b3ef9daa4fc ("mm/swap: split swap cache into 64MB trunks"),
    after swapoff, the address_space associated with the swap device will be
    freed. So swap_address_space() users which touch the address_space need
    some kind of mechanism to prevent the address_space from being freed
    during accessing.

    When mincore processes an unmapped range for swapped shmem pages, it
    doesn't hold the lock to prevent swap device from being swapped off. So
    the following race is possible:

    CPU1 CPU2
    do_mincore() swapoff()
    walk_page_range()
    mincore_unmapped_range()
    __mincore_unmapped_range
    mincore_page
    as = swap_address_space()
    ... exit_swap_address_space()
    ... kvfree(spaces)
    find_get_page(as)

    The address space may be accessed after being freed.

    To fix the race, get_swap_device()/put_swap_device() is used to enclose
    find_get_page() to check whether the swap entry is valid and prevent the
    swap device from being swapoff during accessing.

    Link: http://lkml.kernel.org/r/20190611020510.28251-1-ying.huang@intel.com
    Fixes: 4b3ef9daa4fc ("mm/swap: split swap cache into 64MB trunks")
    Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
    Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
    Acked-by: Michal Hocko <mhocko@suse.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Tim Chen <tim.c.chen@linux.intel.com>
    Cc: Mel Gorman <mgorman@techsingularity.net>
    Cc: Jérôme Glisse <jglisse@redhat.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Yang Shi <yang.shi@linux.alibaba.com>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Jan Kara <jack@suse.cz>
    Cc: Dave Jiang <dave.jiang@intel.com>
    Cc: Daniel Jordan <daniel.m.jordan@oracle.com>
    Cc: Andrea Parri <andrea.parri@amarulasolutions.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    mm/mincore.c | 12 ++++++++++--
    1 file changed, 10 insertions(+), 2 deletions(-)

    diff --git a/mm/mincore.c b/mm/mincore.c
    index c3f058bd0faf..4fe91d497436 100644
    --- a/mm/mincore.c
    +++ b/mm/mincore.c
    @@ -68,8 +68,16 @@ static unsigned char mincore_page(struct address_space *mapping, pgoff_t pgoff)
    */
    if (xa_is_value(page)) {
    swp_entry_t swp = radix_to_swp_entry(page);
    - page = find_get_page(swap_address_space(swp),
    - swp_offset(swp));
    + struct swap_info_struct *si;
    +
    + /* Prevent swap device to being swapoff under us */
    + si = get_swap_device(swp);
    + if (si) {
    + page = find_get_page(swap_address_space(swp),
    + swp_offset(swp));
    + put_swap_device(si);
    + } else
    + page = NULL;
    }
    } else
    page = find_get_page(mapping, pgoff);
    --
    2.20.1
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