Re: [PATCH RFC v3 22/36] kmsan: mm: call KMSAN hooks from SLUB code

[Marco Elver <elver@google.com>]

On Fri, 22 Nov 2019 at 12:27, <glider@google.com> wrote:
>
> In order to report uninitialized memory coming from heap allocations
> KMSAN has to poison them unless they're created with __GFP_ZERO.
>
> It's handy that we need KMSAN hooks in the places where
> init_on_alloc/init_on_free initialization is performed.
>
> Signed-off-by: Alexander Potapenko <glider@google.com>
> To: Alexander Potapenko <glider@google.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Vegard Nossum <vegard.nossum@oracle.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: linux-mm@kvack.org
> ---
> v3:
>  - reverted unrelated whitespace changes
>
> Change-Id: I51103b7981d3aabed747d0c85cbdc85568665871
> ---
>  mm/slub.c | 34 +++++++++++++++++++++++++++++-----
>  1 file changed, 29 insertions(+), 5 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index b25c807a111f..b5d2be1ac755 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -21,6 +21,8 @@
>  #include <linux/proc_fs.h>
>  #include <linux/seq_file.h>
>  #include <linux/kasan.h>
> +#include <linux/kmsan.h>
> +#include <linux/kmsan-checks.h> /* KMSAN_INIT_VALUE */
>  #include <linux/cpu.h>
>  #include <linux/cpuset.h>
>  #include <linux/mempolicy.h>
> @@ -285,17 +287,27 @@ static void prefetch_freepointer(const struct kmem_cache *s, void *object)
>         prefetch(object + s->offset);
>  }
>
> +/*
> + * When running under KMSAN, get_freepointer_safe() may return an uninitialized
> + * pointer value in the case the current thread loses the race for the next
> + * memory chunk in the freelist. In that case this_cpu_cmpxchg_double() in
> + * slab_alloc_node() will fail, so the uninitialized value won't be used, but
> + * KMSAN will still check all arguments of cmpxchg because of imperfect
> + * handling of inline assembly.
> + * To work around this problem, use KMSAN_INIT_VALUE() to force initialize the
> + * return value of get_freepointer_safe().
> + */

Isn't this a general problem with cmpxchg? I.e. does other code using
it have the same problem?

Would it be better to just use KMSAN_INIT_VALUE in cmpxchg, rather
than having the one-off workaround here?

Thanks,
-- Marco