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    Subject[PATCH v8 09/20] kasan: preassign tags to objects with ctors or SLAB_TYPESAFE_BY_RCU
    Date
    An object constructor can initialize pointers within this objects based on
    the address of the object. Since the object address might be tagged, we
    need to assign a tag before calling constructor.

    The implemented approach is to assign tags to objects with constructors
    when a slab is allocated and call constructors once as usual. The
    downside is that such object would always have the same tag when it is
    reallocated, so we won't catch use-after-frees on it.

    Also pressign tags for objects from SLAB_TYPESAFE_BY_RCU caches, since
    they can be validy accessed after having been freed.

    Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
    ---
    mm/slab.c | 2 +-
    mm/slub.c | 24 ++++++++++++++----------
    2 files changed, 15 insertions(+), 11 deletions(-)

    diff --git a/mm/slab.c b/mm/slab.c
    index 6fdca9ec2ea4..fe0ddf08aa2c 100644
    --- a/mm/slab.c
    +++ b/mm/slab.c
    @@ -2574,7 +2574,7 @@ static void cache_init_objs(struct kmem_cache *cachep,

    for (i = 0; i < cachep->num; i++) {
    objp = index_to_obj(cachep, page, i);
    - kasan_init_slab_obj(cachep, objp);
    + objp = kasan_init_slab_obj(cachep, objp);

    /* constructor could break poison info */
    if (DEBUG == 0 && cachep->ctor) {
    diff --git a/mm/slub.c b/mm/slub.c
    index c4d5f4442ff1..75fc76e42a1e 100644
    --- a/mm/slub.c
    +++ b/mm/slub.c
    @@ -1413,16 +1413,17 @@ static inline bool slab_free_freelist_hook(struct kmem_cache *s,
    #endif
    }

    -static void setup_object(struct kmem_cache *s, struct page *page,
    +static void *setup_object(struct kmem_cache *s, struct page *page,
    void *object)
    {
    setup_object_debug(s, page, object);
    - kasan_init_slab_obj(s, object);
    + object = kasan_init_slab_obj(s, object);
    if (unlikely(s->ctor)) {
    kasan_unpoison_object_data(s, object);
    s->ctor(object);
    kasan_poison_object_data(s, object);
    }
    + return object;
    }

    /*
    @@ -1530,16 +1531,16 @@ static bool shuffle_freelist(struct kmem_cache *s, struct page *page)
    /* First entry is used as the base of the freelist */
    cur = next_freelist_entry(s, page, &pos, start, page_limit,
    freelist_count);
    + cur = setup_object(s, page, cur);
    page->freelist = cur;

    for (idx = 1; idx < page->objects; idx++) {
    - setup_object(s, page, cur);
    next = next_freelist_entry(s, page, &pos, start, page_limit,
    freelist_count);
    + next = setup_object(s, page, next);
    set_freepointer(s, cur, next);
    cur = next;
    }
    - setup_object(s, page, cur);
    set_freepointer(s, cur, NULL);

    return true;
    @@ -1561,7 +1562,7 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
    struct page *page;
    struct kmem_cache_order_objects oo = s->oo;
    gfp_t alloc_gfp;
    - void *start, *p;
    + void *start, *p, *next;
    int idx, order;
    bool shuffle;

    @@ -1613,13 +1614,16 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)

    if (!shuffle) {
    for_each_object_idx(p, idx, s, start, page->objects) {
    - setup_object(s, page, p);
    - if (likely(idx < page->objects))
    - set_freepointer(s, p, p + s->size);
    - else
    + if (likely(idx < page->objects)) {
    + next = p + s->size;
    + next = setup_object(s, page, next);
    + set_freepointer(s, p, next);
    + } else
    set_freepointer(s, p, NULL);
    }
    - page->freelist = fixup_red_left(s, start);
    + start = fixup_red_left(s, start);
    + start = setup_object(s, page, start);
    + page->freelist = start;
    }

    page->inuse = page->objects;
    --
    2.19.0.397.gdd90340f6a-goog
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