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Date: Tue, 23 Apr 2019 12:00:11 -0700
From: Kees Cook <keescook@...omium.org>
To: Alexander Potapenko <glider@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter <cl@...ux.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Laura Abbott <labbott@...hat.com>, Linux-MM <linux-mm@...ck.org>, 
	linux-security-module <linux-security-module@...r.kernel.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/3] mm: security: introduce the init_allocations=1 boot option

On Thu, Apr 18, 2019 at 8:42 AM Alexander Potapenko <glider@...gle.com> wrote:
> This option adds the possibility to initialize newly allocated pages and
> heap objects with zeroes. This is needed to prevent possible information
> leaks and make the control-flow bugs that depend on uninitialized values
> more deterministic.
>
> Initialization is done at allocation time at the places where checks for
> __GFP_ZERO are performed. We don't initialize slab caches with
> constructors to preserve their semantics. To reduce runtime costs of
> checking cachep->ctor we replace a call to memset with a call to
> cachep->poison_fn, which is only executed if the memory block needs to
> be initialized.
>
> For kernel testing purposes filling allocations with a nonzero pattern
> would be more suitable, but may require platform-specific code. To have
> a simple baseline we've decided to start with zero-initialization.

The memory tagging future may be worth mentioning here too. We'll
always need an allocation-time hook. What we do in that hook (tag,
zero, poison) can be manage from there.

> No performance optimizations are done at the moment to reduce double
> initialization of memory regions.

Isn't this addressed in later patches?

>
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: James Morris <jmorris@...ei.org>
> Cc: "Serge E. Hallyn" <serge@...lyn.com>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Kostya Serebryany <kcc@...gle.com>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Sandeep Patil <sspatil@...roid.com>
> Cc: Laura Abbott <labbott@...hat.com>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Cc: Jann Horn <jannh@...gle.com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Qian Cai <cai@....pw>
> Cc: Vlastimil Babka <vbabka@...e.cz>
> Cc: linux-mm@...ck.org
> Cc: linux-security-module@...r.kernel.org
> Cc: kernel-hardening@...ts.openwall.com
> ---
>  drivers/infiniband/core/uverbs_ioctl.c |  2 +-
>  include/linux/mm.h                     |  8 ++++++++
>  include/linux/slab_def.h               |  1 +
>  include/linux/slub_def.h               |  1 +
>  kernel/kexec_core.c                    |  2 +-
>  mm/dmapool.c                           |  2 +-
>  mm/page_alloc.c                        | 18 +++++++++++++++++-
>  mm/slab.c                              | 12 ++++++------
>  mm/slab.h                              |  1 +
>  mm/slab_common.c                       | 15 +++++++++++++++
>  mm/slob.c                              |  2 +-
>  mm/slub.c                              |  8 ++++----
>  net/core/sock.c                        |  2 +-
>  13 files changed, 58 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/infiniband/core/uverbs_ioctl.c b/drivers/infiniband/core/uverbs_ioctl.c
> index e1379949e663..f31234906be2 100644
> --- a/drivers/infiniband/core/uverbs_ioctl.c
> +++ b/drivers/infiniband/core/uverbs_ioctl.c
> @@ -127,7 +127,7 @@ __malloc void *_uverbs_alloc(struct uverbs_attr_bundle *bundle, size_t size,
>         res = (void *)pbundle->internal_buffer + pbundle->internal_used;
>         pbundle->internal_used =
>                 ALIGN(new_used, sizeof(*pbundle->internal_buffer));
> -       if (flags & __GFP_ZERO)
> +       if (want_init_memory(flags))
>                 memset(res, 0, size);
>         return res;
>  }
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 76769749b5a5..b38b71a5efaa 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2597,6 +2597,14 @@ static inline void kernel_poison_pages(struct page *page, int numpages,
>                                         int enable) { }
>  #endif
>
> +DECLARE_STATIC_KEY_FALSE(init_allocations);

I'd like a CONFIG to control this default. We can keep the boot-time
option to change it, but I think a CONFIG is warranted here.

> +static inline bool want_init_memory(gfp_t flags)
> +{
> +       if (static_branch_unlikely(&init_allocations))
> +               return true;
> +       return flags & __GFP_ZERO;
> +}
> +
>  #ifdef CONFIG_DEBUG_PAGEALLOC
>  extern bool _debug_pagealloc_enabled;
>  extern void __kernel_map_pages(struct page *page, int numpages, int enable);
> diff --git a/include/linux/slab_def.h b/include/linux/slab_def.h
> index 9a5eafb7145b..9dfe9eb639d7 100644
> --- a/include/linux/slab_def.h
> +++ b/include/linux/slab_def.h
> @@ -37,6 +37,7 @@ struct kmem_cache {
>
>         /* constructor func */
>         void (*ctor)(void *obj);
> +       void (*poison_fn)(struct kmem_cache *c, void *object);
>
>  /* 4) cache creation/removal */
>         const char *name;
> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
> index d2153789bd9f..afb928cb7c20 100644
> --- a/include/linux/slub_def.h
> +++ b/include/linux/slub_def.h
> @@ -99,6 +99,7 @@ struct kmem_cache {
>         gfp_t allocflags;       /* gfp flags to use on each alloc */
>         int refcount;           /* Refcount for slab cache destroy */
>         void (*ctor)(void *);
> +       void (*poison_fn)(struct kmem_cache *c, void *object);
>         unsigned int inuse;             /* Offset to metadata */
>         unsigned int align;             /* Alignment */
>         unsigned int red_left_pad;      /* Left redzone padding size */
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index d7140447be75..be84f5f95c97 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -315,7 +315,7 @@ static struct page *kimage_alloc_pages(gfp_t gfp_mask, unsigned int order)
>                 arch_kexec_post_alloc_pages(page_address(pages), count,
>                                             gfp_mask);
>
> -               if (gfp_mask & __GFP_ZERO)
> +               if (want_init_memory(gfp_mask))
>                         for (i = 0; i < count; i++)
>                                 clear_highpage(pages + i);
>         }
> diff --git a/mm/dmapool.c b/mm/dmapool.c
> index 76a160083506..796e38160d39 100644
> --- a/mm/dmapool.c
> +++ b/mm/dmapool.c
> @@ -381,7 +381,7 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
>  #endif
>         spin_unlock_irqrestore(&pool->lock, flags);
>
> -       if (mem_flags & __GFP_ZERO)
> +       if (want_init_memory(mem_flags))
>                 memset(retval, 0, pool->size);
>
>         return retval;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d96ca5bc555b..e2a21d866ac9 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -133,6 +133,22 @@ unsigned long totalcma_pages __read_mostly;
>
>  int percpu_pagelist_fraction;
>  gfp_t gfp_allowed_mask __read_mostly = GFP_BOOT_MASK;
> +bool want_init_allocations __read_mostly;

This can be a stack variable in early_init_allocations() -- it's never
used again.

> +EXPORT_SYMBOL(want_init_allocations);
> +DEFINE_STATIC_KEY_FALSE(init_allocations);
> +
> +static int __init early_init_allocations(char *buf)
> +{
> +       int ret;
> +
> +       if (!buf)
> +               return -EINVAL;
> +       ret = kstrtobool(buf, &want_init_allocations);
> +       if (want_init_allocations)
> +               static_branch_enable(&init_allocations);

With the CONFIG, this should have a _disable on an "else" here...

> +       return ret;
> +}
> +early_param("init_allocations", early_init_allocations);

Does early_init_allocations() get called before things like
prep_new_page() are up and running to call want_init_memory()?

>
>  /*
>   * A cached value of the page's pageblock's migratetype, used when the page is
> @@ -2014,7 +2030,7 @@ static void prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags
>
>         post_alloc_hook(page, order, gfp_flags);
>
> -       if (!free_pages_prezeroed() && (gfp_flags & __GFP_ZERO))
> +       if (!free_pages_prezeroed() && want_init_memory(gfp_flags))
>                 for (i = 0; i < (1 << order); i++)
>                         clear_highpage(page + i);
>
> diff --git a/mm/slab.c b/mm/slab.c
> index 47a380a486ee..dcc5b73cf767 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3331,8 +3331,8 @@ slab_alloc_node(struct kmem_cache *cachep, gfp_t flags, int nodeid,
>         local_irq_restore(save_flags);
>         ptr = cache_alloc_debugcheck_after(cachep, flags, ptr, caller);
>
> -       if (unlikely(flags & __GFP_ZERO) && ptr)
> -               memset(ptr, 0, cachep->object_size);
> +       if (unlikely(want_init_memory(flags)) && ptr)
> +               cachep->poison_fn(cachep, ptr);

So... this _must_ zero when __GFP_ZERO is present, so I'm not sure
"poison_fn" is the right name, and it likely needs to take the "flags"
argument.

>
>         slab_post_alloc_hook(cachep, flags, 1, &ptr);
>         return ptr;
> @@ -3388,8 +3388,8 @@ slab_alloc(struct kmem_cache *cachep, gfp_t flags, unsigned long caller)
>         objp = cache_alloc_debugcheck_after(cachep, flags, objp, caller);
>         prefetchw(objp);
>
> -       if (unlikely(flags & __GFP_ZERO) && objp)
> -               memset(objp, 0, cachep->object_size);
> +       if (unlikely(want_init_memory(flags)) && objp)
> +               cachep->poison_fn(cachep, objp);

Same.

>
>         slab_post_alloc_hook(cachep, flags, 1, &objp);
>         return objp;
> @@ -3596,9 +3596,9 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>         cache_alloc_debugcheck_after_bulk(s, flags, size, p, _RET_IP_);
>
>         /* Clear memory outside IRQ disabled section */
> -       if (unlikely(flags & __GFP_ZERO))
> +       if (unlikely(want_init_memory(flags)))
>                 for (i = 0; i < size; i++)
> -                       memset(p[i], 0, s->object_size);
> +                       s->poison_fn(s, p[i]);

Same.

>
>         slab_post_alloc_hook(s, flags, size, p);
>         /* FIXME: Trace call missing. Christoph would like a bulk variant */
> diff --git a/mm/slab.h b/mm/slab.h
> index 43ac818b8592..3b541e8970ee 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -27,6 +27,7 @@ struct kmem_cache {
>         const char *name;       /* Slab name for sysfs */
>         int refcount;           /* Use counter */
>         void (*ctor)(void *);   /* Called on object slot creation */
> +       void (*poison_fn)(struct kmem_cache *c, void *object);

How about naming it just "initialize"?

>         struct list_head list;  /* List of all slab caches on the system */
>  };
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 58251ba63e4a..37810114b2ea 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -360,6 +360,16 @@ struct kmem_cache *find_mergeable(unsigned int size, unsigned int align,
>         return NULL;
>  }
>
> +static void poison_zero(struct kmem_cache *c, void *object)
> +{
> +       memset(object, 0, c->object_size);
> +}
> +
> +static void poison_dont(struct kmem_cache *c, void *object)
> +{
> +       /* Do nothing. Use for caches with constructors. */
> +}
> +
>  static struct kmem_cache *create_cache(const char *name,
>                 unsigned int object_size, unsigned int align,
>                 slab_flags_t flags, unsigned int useroffset,
> @@ -381,6 +391,10 @@ static struct kmem_cache *create_cache(const char *name,
>         s->size = s->object_size = object_size;
>         s->align = align;
>         s->ctor = ctor;
> +       if (ctor)
> +               s->poison_fn = poison_dont;
> +       else
> +               s->poison_fn = poison_zero;

As mentioned, we must still always zero when __GFP_ZERO is present.

>         s->useroffset = useroffset;
>         s->usersize = usersize;
>
> @@ -974,6 +988,7 @@ void __init create_boot_cache(struct kmem_cache *s, const char *name,
>         s->align = calculate_alignment(flags, ARCH_KMALLOC_MINALIGN, size);
>         s->useroffset = useroffset;
>         s->usersize = usersize;
> +       s->poison_fn = poison_zero;
>
>         slab_init_memcg_params(s);
>
> diff --git a/mm/slob.c b/mm/slob.c
> index 307c2c9feb44..18981a71e962 100644
> --- a/mm/slob.c
> +++ b/mm/slob.c
> @@ -330,7 +330,7 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
>                 BUG_ON(!b);
>                 spin_unlock_irqrestore(&slob_lock, flags);
>         }
> -       if (unlikely(gfp & __GFP_ZERO))
> +       if (unlikely(want_init_memory(gfp)))
>                 memset(b, 0, size);
>         return b;
>  }
> diff --git a/mm/slub.c b/mm/slub.c
> index d30ede89f4a6..e4efb6575510 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2750,8 +2750,8 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
>                 stat(s, ALLOC_FASTPATH);
>         }
>
> -       if (unlikely(gfpflags & __GFP_ZERO) && object)
> -               memset(object, 0, s->object_size);
> +       if (unlikely(want_init_memory(gfpflags)) && object)
> +               s->poison_fn(s, object);
>
>         slab_post_alloc_hook(s, gfpflags, 1, &object);
>
> @@ -3172,11 +3172,11 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>         local_irq_enable();
>
>         /* Clear memory outside IRQ disabled fastpath loop */
> -       if (unlikely(flags & __GFP_ZERO)) {
> +       if (unlikely(want_init_memory(flags))) {
>                 int j;
>
>                 for (j = 0; j < i; j++)
> -                       memset(p[j], 0, s->object_size);
> +                       s->poison_fn(s, p[j]);
>         }
>
>         /* memcg and kmem_cache debug support */
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 782343bb925b..99b288a19b39 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1601,7 +1601,7 @@ static struct sock *sk_prot_alloc(struct proto *prot, gfp_t priority,
>                 sk = kmem_cache_alloc(slab, priority & ~__GFP_ZERO);
>                 if (!sk)
>                         return sk;
> -               if (priority & __GFP_ZERO)
> +               if (want_init_memory(priority))
>                         sk_prot_clear_nulls(sk, prot->obj_size);
>         } else
>                 sk = kmalloc(prot->obj_size, priority);
> --
> 2.21.0.392.gf8f6787159e-goog
>

Looking good. :) Thanks for working on this!

-- 
Kees Cook

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