Re: [PATCH] mm/slab: ensure all metadata in slab object are word-aligned

[Andrey Konovalov <andreyknvl@gmail.com>]

On Thu, Oct 23, 2025 at 3:16 PM Harry Yoo <harry.yoo@oracle.com> wrote:
>
> When the SLAB_STORE_USER debug flag is used, any metadata placed after
> the original kmalloc request size (orig_size) is not properly aligned
> on 64-bit architectures because its type is unsigned int. When both KASAN
> and SLAB_STORE_USER are enabled, kasan_alloc_meta is misaligned.
>
> Because not all architectures support unaligned memory accesses,
> ensure that all metadata (track, orig_size, kasan_{alloc,free}_meta)
> in a slab object are word-aligned. struct track, kasan_{alloc,free}_meta
> are aligned by adding __aligned(sizeof(unsigned long)).
>
> For orig_size, use ALIGN(sizeof(unsigned int), sizeof(unsigned long)) to
> make clear that its size remains unsigned int but it must be aligned to
> a word boundary. On 64-bit architectures, this reserves 8 bytes for
> orig_size, which is acceptable since kmalloc's original request size
> tracking is intended for debugging rather than production use.
>
> Cc: <stable@vger.kernel.org>
> Fixes: 6edf2576a6cc ("mm/slub: enable debugging memory wasting of kmalloc")
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  mm/kasan/kasan.h |  4 ++--
>  mm/slub.c        | 16 +++++++++++-----
>  2 files changed, 13 insertions(+), 7 deletions(-)
>
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 129178be5e64..d4ea7ecc20c3 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -265,7 +265,7 @@ struct kasan_alloc_meta {
>         struct kasan_track alloc_track;
>         /* Free track is stored in kasan_free_meta. */
>         depot_stack_handle_t aux_stack[2];
> -};
> +} __aligned(sizeof(unsigned long));
>
>  struct qlist_node {
>         struct qlist_node *next;
> @@ -289,7 +289,7 @@ struct qlist_node {
>  struct kasan_free_meta {
>         struct qlist_node quarantine_link;
>         struct kasan_track free_track;
> -};
> +} __aligned(sizeof(unsigned long));
>
>  #endif /* CONFIG_KASAN_GENERIC */
>
> diff --git a/mm/slub.c b/mm/slub.c
> index a585d0ac45d4..b921f91723c2 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -344,7 +344,7 @@ struct track {
>         int cpu;                /* Was running on cpu */
>         int pid;                /* Pid context */
>         unsigned long when;     /* When did the operation occur */
> -};
> +} __aligned(sizeof(unsigned long));
>
>  enum track_item { TRACK_ALLOC, TRACK_FREE };
>
> @@ -1196,7 +1196,7 @@ static void print_trailer(struct kmem_cache *s, struct slab *slab, u8 *p)
>                 off += 2 * sizeof(struct track);
>
>         if (slub_debug_orig_size(s))
> -               off += sizeof(unsigned int);
> +               off += ALIGN(sizeof(unsigned int), sizeof(unsigned long));
>
>         off += kasan_metadata_size(s, false);
>
> @@ -1392,7 +1392,8 @@ static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p)
>                 off += 2 * sizeof(struct track);
>
>                 if (s->flags & SLAB_KMALLOC)
> -                       off += sizeof(unsigned int);
> +                       off += ALIGN(sizeof(unsigned int),
> +                                    sizeof(unsigned long));
>         }
>
>         off += kasan_metadata_size(s, false);
> @@ -7820,9 +7821,14 @@ static int calculate_sizes(struct kmem_cache_args *args, struct kmem_cache *s)
>                  */
>                 size += 2 * sizeof(struct track);
>
> -               /* Save the original kmalloc request size */
> +               /*
> +                * Save the original kmalloc request size.
> +                * Although the request size is an unsigned int,
> +                * make sure that is aligned to word boundary.
> +                */
>                 if (flags & SLAB_KMALLOC)
> -                       size += sizeof(unsigned int);
> +                       size += ALIGN(sizeof(unsigned int),
> +                                     sizeof(unsigned long));
>         }
>  #endif
>
> --
> 2.43.0
>

Acked-by: Andrey Konovalov <andreyknvl@gmail.com>