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    Subject[PATCH 11/16] uprobes: convert uprobe.ref to refcount_t
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    atomic_t variables are currently used to implement reference
    counters with the following properties:
    - counter is initialized to 1 using atomic_set()
    - a resource is freed upon counter reaching zero
    - once counter reaches zero, its further
    increments aren't allowed
    - counter schema uses basic atomic operations
    (set, inc, inc_not_zero, dec_and_test, etc.)

    Such atomic variables should be converted to a newly provided
    refcount_t type and API that prevents accidental counter overflows
    and underflows. This is important since overflows and underflows
    can lead to use-after-free situation and be exploitable.

    The variable uprobe.ref is used as pure reference counter.
    Convert it to refcount_t and fix up the operations.

    **Important note for maintainers:

    Some functions from refcount_t API defined in lib/refcount.c
    have different memory ordering guarantees than their atomic
    counterparts.
    The full comparison can be seen in
    https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
    in state to be merged to the documentation tree.
    Normally the differences should not matter since refcount_t provides
    enough guarantees to satisfy the refcounting use cases, but in
    some rare cases it might matter.
    Please double check that you don't have some undocumented
    memory guarantees for this variable usage.

    For the uprobe.ref it might make a difference
    in following places:
    - put_uprobe(): decrement in refcount_dec_and_test() only
    provides RELEASE ordering and control dependency on success
    vs. fully ordered atomic counterpart

    Suggested-by: Kees Cook <keescook@chromium.org>
    Reviewed-by: David Windsor <dwindsor@gmail.com>
    Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
    Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
    ---
    kernel/events/uprobes.c | 8 ++++----
    1 file changed, 4 insertions(+), 4 deletions(-)

    diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
    index 8d42d8f..3514b42 100644
    --- a/kernel/events/uprobes.c
    +++ b/kernel/events/uprobes.c
    @@ -66,7 +66,7 @@ static struct percpu_rw_semaphore dup_mmap_sem;

    struct uprobe {
    struct rb_node rb_node; /* node in the rb tree */
    - atomic_t ref;
    + refcount_t ref;
    struct rw_semaphore register_rwsem;
    struct rw_semaphore consumer_rwsem;
    struct list_head pending_list;
    @@ -371,13 +371,13 @@ set_orig_insn(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long v

    static struct uprobe *get_uprobe(struct uprobe *uprobe)
    {
    - atomic_inc(&uprobe->ref);
    + refcount_inc(&uprobe->ref);
    return uprobe;
    }

    static void put_uprobe(struct uprobe *uprobe)
    {
    - if (atomic_dec_and_test(&uprobe->ref))
    + if (refcount_dec_and_test(&uprobe->ref))
    kfree(uprobe);
    }

    @@ -459,7 +459,7 @@ static struct uprobe *__insert_uprobe(struct uprobe *uprobe)
    rb_link_node(&uprobe->rb_node, parent, p);
    rb_insert_color(&uprobe->rb_node, &uprobes_tree);
    /* get access + creation ref */
    - atomic_set(&uprobe->ref, 2);
    + refcount_set(&uprobe->ref, 2);

    return u;
    }
    --
    2.7.4
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