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 <redacted>
---
 mm/slab.c | 6 +++++-
 mm/slub.c | 4 ++++
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/mm/slab.c b/mm/slab.c
index 6fdca9ec2ea4..3b4227059f2e 100644
--- a/mm/slab.c
+++ b/mm/slab.c

@@ -403,7 +403,11 @@ static inline struct kmem_cache *virt_to_cache(const void *obj)
 static inline void *index_to_obj(struct kmem_cache *cache, struct page *page,
                                 unsigned int idx)
 {
-       return page->s_mem + cache->size * idx;
+       void *obj;
+
+       obj = page->s_mem + cache->size * idx;
+       obj = khwasan_preset_slab_tag(cache, idx, obj);
+       return obj;
 }

 /*

diff --git a/mm/slub.c b/mm/slub.c
index 4206e1b616e7..086d6558a6b6 100644
--- a/mm/slub.c
+++ b/mm/slub.c

@@ -1531,12 +1531,14 @@ 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 = khwasan_preset_slub_tag(s, 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 = khwasan_preset_slub_tag(s, next);
                set_freepointer(s, cur, next);
                cur = next;
        }

@@ -1613,8 +1615,10 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
        shuffle = shuffle_freelist(s, page);

        if (!shuffle) {
+               start = khwasan_preset_slub_tag(s, start);
                for_each_object_idx(p, idx, s, start, page->objects) {
                        setup_object(s, page, p);
+                       p = khwasan_preset_slub_tag(s, p);