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    Subject[PATCH 5.4 154/168] include/linux/sched/mm.h: use rcu_dereference in in_vfork()
    Date
    From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    From: Matthew Wilcox (Oracle) <willy@infradead.org>

    [ Upstream commit 149fc787353f65b7e72e05e7b75d34863266c3e2 ]

    Fix a sparse warning by using rcu_dereference(). Technically this is a
    bug and a sufficiently aggressive compiler could reload the `real_parent'
    pointer outside the protection of the rcu lock (and access freed memory),
    but I think it's pretty unlikely to happen.

    Link: https://lkml.kernel.org/r/20210221194207.1351703-1-willy@infradead.org
    Fixes: b18dc5f291c0 ("mm, oom: skip vforked tasks from being selected")
    Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
    Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
    Acked-by: Michal Hocko <mhocko@suse.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>
    ---
    include/linux/sched/mm.h | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
    index a132d875d351..3a1d899019af 100644
    --- a/include/linux/sched/mm.h
    +++ b/include/linux/sched/mm.h
    @@ -167,7 +167,8 @@ static inline bool in_vfork(struct task_struct *tsk)
    * another oom-unkillable task does this it should blame itself.
    */
    rcu_read_lock();
    - ret = tsk->vfork_done && tsk->real_parent->mm == tsk->mm;
    + ret = tsk->vfork_done &&
    + rcu_dereference(tsk->real_parent)->mm == tsk->mm;
    rcu_read_unlock();

    return ret;
    --
    2.30.1


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