mm/memcontrol,alloc_tag: handle slabobj_ext access under KASAN poison

In the near future, slabobj_ext may reside outside the allocated slab
object range within a slab, which could be reported as an out-of-bounds
access by KASAN.

As suggested by Andrey Konovalov [1], explicitly disable KASAN and KMSAN
checks when accessing slabobj_ext within slab allocator, memory profiling,
and memory cgroup code. While an alternative approach could be to unpoison
slabobj_ext, out-of-bounds accesses outside the slab allocator are
generally more common.

Move metadata_access_enable()/disable() helpers to mm/slab.h so that
it can be used outside mm/slub.c. However, as suggested by Suren
Baghdasaryan [2], instead of calling them directly from mm code (which is
more prone to errors), change users to access slabobj_ext via get/put
APIs:

  - Users should call get_slab_obj_exts() to access slabobj_metadata
    and call put_slab_obj_exts() when it's done.

  - From now on, accessing it outside the section covered by
    get_slab_obj_exts() ~ put_slab_obj_exts() is illegal.
    This ensures that accesses to slabobj_ext metadata won't be reported
    as access violations.

Call kasan_reset_tag() in slab_obj_ext() before returning the address to
prevent SW or HW tag-based KASAN from reporting false positives.

Suggested-by: Andrey Konovalov <andreyknvl@gmail.com>
Suggested-by: Suren Baghdasaryan <surenb@google.com>
Link: https://lore.kernel.org/linux-mm/CA+fCnZezoWn40BaS3cgmCeLwjT+5AndzcQLc=wH3BjMCu6_YCw@mail.gmail.com
Link: https://lore.kernel.org/linux-mm/CAJuCfpG=Lb4WhYuPkSpdNO4Ehtjm1YcEEK0OM=3g9i=LxmpHSQ@mail.gmail.com
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20260113061845.159790-7-harry.yoo@oracle.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 276e87090a75a4e26c2dcb5a9d104ef6af260321..2d6dfba540d46ab82d477ef5745b08b2a0421d97 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2604,10 +2604,16 @@ struct mem_cgroup *mem_cgroup_from_obj_slab(struct slab *slab, void *p)
        if (!obj_exts)
                return NULL;
 
+       get_slab_obj_exts(obj_exts);
        off = obj_to_index(slab->slab_cache, slab, p);
        obj_ext = slab_obj_ext(slab, obj_exts, off);
-       if (obj_ext->objcg)
-               return obj_cgroup_memcg(obj_ext->objcg);
+       if (obj_ext->objcg) {
+               struct obj_cgroup *objcg = obj_ext->objcg;
+
+               put_slab_obj_exts(obj_exts);
+               return obj_cgroup_memcg(objcg);
+       }
+       put_slab_obj_exts(obj_exts);
 
        return NULL;
 }
@@ -3219,10 +3225,12 @@ bool __memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
                        return false;
 
                obj_exts = slab_obj_exts(slab);
+               get_slab_obj_exts(obj_exts);
                off = obj_to_index(s, slab, p[i]);
                obj_ext = slab_obj_ext(slab, obj_exts, off);
                obj_cgroup_get(objcg);
                obj_ext->objcg = objcg;
+               put_slab_obj_exts(obj_exts);
        }
 
        return true;

diff --git a/mm/slab.h b/mm/slab.h
index be0e62c98da03fc03bbab309ccb863a74b665aaa..d5da2f69e2d5ca6d7c8204fe80fa345deb4710cc 100644 (file)
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -508,6 +508,24 @@ bool slab_in_kunit_test(void);
 static inline bool slab_in_kunit_test(void) { return false; }
 #endif
 
+/*
+ * slub is about to manipulate internal object metadata.  This memory lies
+ * outside the range of the allocated object, so accessing it would normally
+ * be reported by kasan as a bounds error.  metadata_access_enable() is used
+ * to tell kasan that these accesses are OK.
+ */
+static inline void metadata_access_enable(void)
+{
+       kasan_disable_current();
+       kmsan_disable_current();
+}
+
+static inline void metadata_access_disable(void)
+{
+       kmsan_enable_current();
+       kasan_enable_current();
+}
+
 #ifdef CONFIG_SLAB_OBJ_EXT
 
 /*
@@ -517,8 +535,22 @@ static inline bool slab_in_kunit_test(void) { return false; }
  *
  * Returns the address of the object extension vector associated with the slab,
  * or zero if no such vector has been associated yet.
- * Do not dereference the return value directly; use slab_obj_ext() to access
- * its elements.
+ * Do not dereference the return value directly; use get/put_slab_obj_exts()
+ * pair and slab_obj_ext() to access individual elements.
+ *
+ * Example usage:
+ *
+ * obj_exts = slab_obj_exts(slab);
+ * if (obj_exts) {
+ *         get_slab_obj_exts(obj_exts);
+ *         obj_ext = slab_obj_ext(slab, obj_exts, obj_to_index(s, slab, obj));
+ *         // do something with obj_ext
+ *         put_slab_obj_exts(obj_exts);
+ * }
+ *
+ * Note that the get/put semantics does not involve reference counting.
+ * Instead, it updates kasan/kmsan depth so that accesses to slabobj_ext
+ * won't be reported as access violations.
  */
 static inline unsigned long slab_obj_exts(struct slab *slab)
 {
@@ -537,6 +569,17 @@ static inline unsigned long slab_obj_exts(struct slab *slab)
        return obj_exts & ~OBJEXTS_FLAGS_MASK;
 }
 
+static inline void get_slab_obj_exts(unsigned long obj_exts)
+{
+       VM_WARN_ON_ONCE(!obj_exts);
+       metadata_access_enable();
+}
+
+static inline void put_slab_obj_exts(unsigned long obj_exts)
+{
+       metadata_access_disable();
+}
+
 #ifdef CONFIG_64BIT
 static inline void slab_set_stride(struct slab *slab, unsigned short stride)
 {
@@ -565,14 +608,19 @@ static inline unsigned short slab_get_stride(struct slab *slab)
  * @index: an index of the object
  *
  * Returns a pointer to the object extension associated with the object.
+ * Must be called within a section covered by get/put_slab_obj_exts().
  */
 static inline struct slabobj_ext *slab_obj_ext(struct slab *slab,
                                               unsigned long obj_exts,
                                               unsigned int index)
 {
+       struct slabobj_ext *obj_ext;
+
        VM_WARN_ON_ONCE(obj_exts != slab_obj_exts(slab));
 
-       return (struct slabobj_ext *)(obj_exts + slab_get_stride(slab) * index);
+       obj_ext = (struct slabobj_ext *)(obj_exts +
+                                        slab_get_stride(slab) * index);
+       return kasan_reset_tag(obj_ext);
 }
 
 int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,

diff --git a/mm/slub.c b/mm/slub.c
index 020a2c4afb4599823528cae344027a9a4528f8c6..b0c50df49cf3550e4162fbe3121c71c73db16ef5 100644 (file)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -972,24 +972,6 @@ static slab_flags_t slub_debug;
 static const char *slub_debug_string __ro_after_init;
 static int disable_higher_order_debug;
 
-/*
- * slub is about to manipulate internal object metadata.  This memory lies
- * outside the range of the allocated object, so accessing it would normally
- * be reported by kasan as a bounds error.  metadata_access_enable() is used
- * to tell kasan that these accesses are OK.
- */
-static inline void metadata_access_enable(void)
-{
-       kasan_disable_current();
-       kmsan_disable_current();
-}
-
-static inline void metadata_access_disable(void)
-{
-       kmsan_enable_current();
-       kasan_enable_current();
-}
-
 /*
  * Object debugging
  */
@@ -2055,23 +2037,27 @@ static bool freelist_corrupted(struct kmem_cache *s, struct slab *slab,
 
 static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
 {
-       unsigned long slab_exts;
        struct slab *obj_exts_slab;
+       unsigned long slab_exts;
 
        obj_exts_slab = virt_to_slab(obj_exts);
        slab_exts = slab_obj_exts(obj_exts_slab);
        if (slab_exts) {
+               get_slab_obj_exts(slab_exts);
                unsigned int offs = obj_to_index(obj_exts_slab->slab_cache,
                                                 obj_exts_slab, obj_exts);
                struct slabobj_ext *ext = slab_obj_ext(obj_exts_slab,
                                                       slab_exts, offs);
 
-               if (unlikely(is_codetag_empty(&ext->ref)))
+               if (unlikely(is_codetag_empty(&ext->ref))) {
+                       put_slab_obj_exts(slab_exts);
                        return;
+               }
 
                /* codetag should be NULL here */
                WARN_ON(ext->ref.ct);
                set_codetag_empty(&ext->ref);
+               put_slab_obj_exts(slab_exts);
        }
 }
 
@@ -2287,30 +2273,28 @@ static inline void free_slab_obj_exts(struct slab *slab)
 
 #ifdef CONFIG_MEM_ALLOC_PROFILING
 
-static inline struct slabobj_ext *
-prepare_slab_obj_ext_hook(struct kmem_cache *s, gfp_t flags, void *p)
+static inline unsigned long
+prepare_slab_obj_exts_hook(struct kmem_cache *s, struct slab *slab,
+                          gfp_t flags, void *p)
 {
-       struct slab *slab;
-       unsigned long obj_exts;
-
-       slab = virt_to_slab(p);
-       obj_exts = slab_obj_exts(slab);
-       if (!obj_exts &&
+       if (!slab_obj_exts(slab) &&
            alloc_slab_obj_exts(slab, s, flags, false)) {
                pr_warn_once("%s, %s: Failed to create slab extension vector!\n",
                             __func__, s->name);
-               return NULL;
+               return 0;
        }
 
-       obj_exts = slab_obj_exts(slab);
-       return slab_obj_ext(slab, obj_exts, obj_to_index(s, slab, p));
+       return slab_obj_exts(slab);
 }
 
+
 /* Should be called only if mem_alloc_profiling_enabled() */
 static noinline void
 __alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags)
 {
+       unsigned long obj_exts;
        struct slabobj_ext *obj_ext;
+       struct slab *slab;
 
        if (!object)
                return;
@@ -2321,16 +2305,23 @@ __alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags)
        if (flags & __GFP_NO_OBJ_EXT)
                return;
 
-       obj_ext = prepare_slab_obj_ext_hook(s, flags, object);
+       slab = virt_to_slab(object);
+       obj_exts = prepare_slab_obj_exts_hook(s, slab, flags, object);
        /*
         * Currently obj_exts is used only for allocation profiling.
         * If other users appear then mem_alloc_profiling_enabled()
         * check should be added before alloc_tag_add().
         */
-       if (likely(obj_ext))
+       if (obj_exts) {
+               unsigned int obj_idx = obj_to_index(s, slab, object);
+
+               get_slab_obj_exts(obj_exts);
+               obj_ext = slab_obj_ext(slab, obj_exts, obj_idx);
                alloc_tag_add(&obj_ext->ref, current->alloc_tag, s->size);
-       else
+               put_slab_obj_exts(obj_exts);
+       } else {
                alloc_tag_set_inaccurate(current->alloc_tag);
+       }
 }
 
 static inline void
@@ -2356,11 +2347,13 @@ __alloc_tagging_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p
        if (!obj_exts)
                return;
 
+       get_slab_obj_exts(obj_exts);
        for (i = 0; i < objects; i++) {
                unsigned int off = obj_to_index(s, slab, p[i]);
 
                alloc_tag_sub(&slab_obj_ext(slab, obj_exts, off)->ref, s->size);
        }
+       put_slab_obj_exts(obj_exts);
 }
 
 static inline void
@@ -2427,7 +2420,9 @@ void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
        if (likely(!obj_exts))
                return;
 
+       get_slab_obj_exts(obj_exts);
        __memcg_slab_free_hook(s, slab, p, objects, obj_exts);
+       put_slab_obj_exts(obj_exts);
 }
 
 static __fastpath_inline
@@ -2477,10 +2472,14 @@ bool memcg_slab_post_charge(void *p, gfp_t flags)
        /* Ignore already charged objects. */
        obj_exts = slab_obj_exts(slab);
        if (obj_exts) {
+               get_slab_obj_exts(obj_exts);
                off = obj_to_index(s, slab, p);
                obj_ext = slab_obj_ext(slab, obj_exts, off);
-               if (unlikely(obj_ext->objcg))
+               if (unlikely(obj_ext->objcg)) {
+                       put_slab_obj_exts(obj_exts);
                        return true;
+               }
+               put_slab_obj_exts(obj_exts);
        }
 
        return __memcg_slab_post_alloc_hook(s, NULL, flags, 1, &p);