Re: [PATCH] mm: Remove HARDENED_USERCOPY_FALLBACK

[Joel Stanley <joel@jms.id.au>]

On Tue, 21 Sept 2021 at 09:50, Stephen Kitt <steve@sk2.org> wrote:
>
> This has served its purpose and is no longer used. All usercopy
> violations appear to have been handled by now, any remaining
> instances (or new bugs) will cause copies to be rejected.
>
> This isn't a direct revert of commit 2d891fbc3bb6 ("usercopy: Allow
> strict enforcement of whitelists"); since usercopy_fallback is
> effectively 0, the fallback handling is removed too.
>
> This also removes the usercopy_fallback module parameter on
> slab_common.
>
> Link: https://github.com/KSPP/linux/issues/153
> Signed-off-by: Stephen Kitt <steve@sk2.org>
> Suggested-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/powerpc/configs/skiroot_defconfig |  1 -

For the defconfig change:

Reviewed-by: Joel Stanley <joel@jms.id.au>

Cheers,

Joel

>  include/linux/slab.h                   |  2 --
>  mm/slab.c                              | 13 -------------
>  mm/slab_common.c                       |  8 --------
>  mm/slub.c                              | 14 --------------
>  security/Kconfig                       | 14 --------------
>  6 files changed, 52 deletions(-)
>
> diff --git a/arch/powerpc/configs/skiroot_defconfig b/arch/powerpc/configs/skiroot_defconfig
> index b806a5d3a695..c3ba614c973d 100644
> --- a/arch/powerpc/configs/skiroot_defconfig
> +++ b/arch/powerpc/configs/skiroot_defconfig
> @@ -275,7 +275,6 @@ CONFIG_NLS_UTF8=y
>  CONFIG_ENCRYPTED_KEYS=y
>  CONFIG_SECURITY=y
>  CONFIG_HARDENED_USERCOPY=y
> -# CONFIG_HARDENED_USERCOPY_FALLBACK is not set
>  CONFIG_HARDENED_USERCOPY_PAGESPAN=y
>  CONFIG_FORTIFY_SOURCE=y
>  CONFIG_SECURITY_LOCKDOWN_LSM=y
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 0c97d788762c..5b21515afae0 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -142,8 +142,6 @@ struct mem_cgroup;
>  void __init kmem_cache_init(void);
>  bool slab_is_available(void);
>
> -extern bool usercopy_fallback;
> -
>  struct kmem_cache *kmem_cache_create(const char *name, unsigned int size,
>                         unsigned int align, slab_flags_t flags,
>                         void (*ctor)(void *));
> diff --git a/mm/slab.c b/mm/slab.c
> index d0f725637663..4d826394ffcb 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -4207,19 +4207,6 @@ void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
>             n <= cachep->useroffset - offset + cachep->usersize)
>                 return;
>
> -       /*
> -        * If the copy is still within the allocated object, produce
> -        * a warning instead of rejecting the copy. This is intended
> -        * to be a temporary method to find any missing usercopy
> -        * whitelists.
> -        */
> -       if (usercopy_fallback &&
> -           offset <= cachep->object_size &&
> -           n <= cachep->object_size - offset) {
> -               usercopy_warn("SLAB object", cachep->name, to_user, offset, n);
> -               return;
> -       }
> -
>         usercopy_abort("SLAB object", cachep->name, to_user, offset, n);
>  }
>  #endif /* CONFIG_HARDENED_USERCOPY */
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index a4a571428c51..925b00c1d4e8 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -37,14 +37,6 @@ LIST_HEAD(slab_caches);
>  DEFINE_MUTEX(slab_mutex);
>  struct kmem_cache *kmem_cache;
>
> -#ifdef CONFIG_HARDENED_USERCOPY
> -bool usercopy_fallback __ro_after_init =
> -               IS_ENABLED(CONFIG_HARDENED_USERCOPY_FALLBACK);
> -module_param(usercopy_fallback, bool, 0400);
> -MODULE_PARM_DESC(usercopy_fallback,
> -               "WARN instead of reject usercopy whitelist violations");
> -#endif
> -
>  static LIST_HEAD(slab_caches_to_rcu_destroy);
>  static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work);
>  static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
> diff --git a/mm/slub.c b/mm/slub.c
> index 3f96e099817a..77f53e76a3c3 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4125,7 +4125,6 @@ void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
>  {
>         struct kmem_cache *s;
>         unsigned int offset;
> -       size_t object_size;
>         bool is_kfence = is_kfence_address(ptr);
>
>         ptr = kasan_reset_tag(ptr);
> @@ -4158,19 +4157,6 @@ void __check_heap_object(const void *ptr, unsigned long n, struct page *page,
>             n <= s->useroffset - offset + s->usersize)
>                 return;
>
> -       /*
> -        * If the copy is still within the allocated object, produce
> -        * a warning instead of rejecting the copy. This is intended
> -        * to be a temporary method to find any missing usercopy
> -        * whitelists.
> -        */
> -       object_size = slab_ksize(s);
> -       if (usercopy_fallback &&
> -           offset <= object_size && n <= object_size - offset) {
> -               usercopy_warn("SLUB object", s->name, to_user, offset, n);
> -               return;
> -       }
> -
>         usercopy_abort("SLUB object", s->name, to_user, offset, n);
>  }
>  #endif /* CONFIG_HARDENED_USERCOPY */
> diff --git a/security/Kconfig b/security/Kconfig
> index 0ced7fd33e4d..d9698900c9b7 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -163,20 +163,6 @@ config HARDENED_USERCOPY
>           or are part of the kernel text. This kills entire classes
>           of heap overflow exploits and similar kernel memory exposures.
>
> -config HARDENED_USERCOPY_FALLBACK
> -       bool "Allow usercopy whitelist violations to fallback to object size"
> -       depends on HARDENED_USERCOPY
> -       default y
> -       help
> -         This is a temporary option that allows missing usercopy whitelists
> -         to be discovered via a WARN() to the kernel log, instead of
> -         rejecting the copy, falling back to non-whitelisted hardened
> -         usercopy that checks the slab allocation size instead of the
> -         whitelist size. This option will be removed once it seems like
> -         all missing usercopy whitelists have been identified and fixed.
> -         Booting with "slab_common.usercopy_fallback=Y/N" can change
> -         this setting.
> -
>  config HARDENED_USERCOPY_PAGESPAN
>         bool "Refuse to copy allocations that span multiple pages"
>         depends on HARDENED_USERCOPY
>
> base-commit: 368094df48e680fa51cedb68537408cfa64b788e
> --
> 2.30.2
>