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[PATCH v4 12/12] mm: SLUB hardened usercopy support

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  [PATCH v4 12/12] mm: SLUB hardened usercopy support Kees Cook <keescook@chromium.org> - 2016-07-20 22:30 +0200
    Re: [PATCH v4 12/12] mm: SLUB hardened usercopy support Laura Abbott <labbott@redhat.com> - 2016-07-25 21:20 +0200
      Re: [PATCH v4 12/12] mm: SLUB hardened usercopy support Kees Cook <keescook@chromium.org> - 2016-07-25 22:50 +0200
        Re: [PATCH v4 12/12] mm: SLUB hardened usercopy support Laura Abbott <labbott@redhat.com> - 2016-07-26 03:00 +0200
      Re: [PATCH v4 12/12] mm: SLUB hardened usercopy support Rik van Riel <riel@redhat.com> - 2016-07-25 23:50 +0200
        Re: [PATCH v4 12/12] mm: SLUB hardened usercopy support Laura Abbott <labbott@redhat.com> - 2016-07-26 01:40 +0200
          Re: [PATCH v4 12/12] mm: SLUB hardened usercopy support Rik van Riel <riel@redhat.com> - 2016-07-26 02:30 +0200

#1447434 — [PATCH v4 12/12] mm: SLUB hardened usercopy support

Under CONFIG_HARDENED_USERCOPY, this adds object size checking to the
SLUB allocator to catch any copies that may span objects. Includes a
redzone handling fix discovered by Michael Ellerman.

Based on code from PaX and grsecurity.

Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
---
 init/Kconfig |  1 +
 mm/slub.c    | 36 ++++++++++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index 798c2020ee7c..1c4711819dfd 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1765,6 +1765,7 @@ config SLAB
 
 config SLUB
 	bool "SLUB (Unqueued Allocator)"
+	select HAVE_HARDENED_USERCOPY_ALLOCATOR
 	help
 	   SLUB is a slab allocator that minimizes cache line usage
 	   instead of managing queues of cached objects (SLAB approach).
diff --git a/mm/slub.c b/mm/slub.c
index 825ff4505336..7dee3d9a5843 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3614,6 +3614,42 @@ void *__kmalloc_node(size_t size, gfp_t flags, int node)
 EXPORT_SYMBOL(__kmalloc_node);
 #endif
 
+#ifdef CONFIG_HARDENED_USERCOPY
+/*
+ * Rejects objects that are incorrectly sized.
+ *
+ * Returns NULL if check passes, otherwise const char * to name of cache
+ * to indicate an error.
+ */
+const char *__check_heap_object(const void *ptr, unsigned long n,
+				struct page *page)
+{
+	struct kmem_cache *s;
+	unsigned long offset;
+	size_t object_size;
+
+	/* Find object and usable object size. */
+	s = page->slab_cache;
+	object_size = slab_ksize(s);
+
+	/* Find offset within object. */
+	offset = (ptr - page_address(page)) % s->size;
+
+	/* Adjust for redzone and reject if within the redzone. */
+	if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) {
+		if (offset < s->red_left_pad)
+			return s->name;
+		offset -= s->red_left_pad;
+	}
+
+	/* Allow address range falling entirely within object size. */
+	if (offset <= object_size && n <= object_size - offset)
+		return NULL;
+
+	return s->name;
+}
+#endif /* CONFIG_HARDENED_USERCOPY */
+
 static size_t __ksize(const void *object)
 {
 	struct page *page;
-- 
2.7.4

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#1449754

On 07/20/2016 01:27 PM, Kees Cook wrote:
> Under CONFIG_HARDENED_USERCOPY, this adds object size checking to the
> SLUB allocator to catch any copies that may span objects. Includes a
> redzone handling fix discovered by Michael Ellerman.
>
> Based on code from PaX and grsecurity.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Tested-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
>  init/Kconfig |  1 +
>  mm/slub.c    | 36 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 37 insertions(+)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index 798c2020ee7c..1c4711819dfd 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1765,6 +1765,7 @@ config SLAB
>
>  config SLUB
>  	bool "SLUB (Unqueued Allocator)"
> +	select HAVE_HARDENED_USERCOPY_ALLOCATOR
>  	help
>  	   SLUB is a slab allocator that minimizes cache line usage
>  	   instead of managing queues of cached objects (SLAB approach).
> diff --git a/mm/slub.c b/mm/slub.c
> index 825ff4505336..7dee3d9a5843 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3614,6 +3614,42 @@ void *__kmalloc_node(size_t size, gfp_t flags, int node)
>  EXPORT_SYMBOL(__kmalloc_node);
>  #endif
>
> +#ifdef CONFIG_HARDENED_USERCOPY
> +/*
> + * Rejects objects that are incorrectly sized.
> + *
> + * Returns NULL if check passes, otherwise const char * to name of cache
> + * to indicate an error.
> + */
> +const char *__check_heap_object(const void *ptr, unsigned long n,
> +				struct page *page)
> +{
> +	struct kmem_cache *s;
> +	unsigned long offset;
> +	size_t object_size;
> +
> +	/* Find object and usable object size. */
> +	s = page->slab_cache;
> +	object_size = slab_ksize(s);
> +
> +	/* Find offset within object. */
> +	offset = (ptr - page_address(page)) % s->size;
> +
> +	/* Adjust for redzone and reject if within the redzone. */
> +	if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) {
> +		if (offset < s->red_left_pad)
> +			return s->name;
> +		offset -= s->red_left_pad;
> +	}
> +
> +	/* Allow address range falling entirely within object size. */
> +	if (offset <= object_size && n <= object_size - offset)
> +		return NULL;
> +
> +	return s->name;
> +}
> +#endif /* CONFIG_HARDENED_USERCOPY */
> +

I compared this against what check_valid_pointer does for SLUB_DEBUG
checking. I was hoping we could utilize that function to avoid
duplication but a) __check_heap_object needs to allow accesses anywhere
in the object, not just the beginning b) accessing page->objects
is racy without the addition of locking in SLUB_DEBUG.

Still, the ptr < page_address(page) check from __check_heap_object would
be good to add to avoid generating garbage large offsets and trying to
infer C math.

diff --git a/mm/slub.c b/mm/slub.c
index 7dee3d9..5370e4f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3632,6 +3632,9 @@ const char *__check_heap_object(const void *ptr, unsigned long n,
         s = page->slab_cache;
         object_size = slab_ksize(s);
  
+       if (ptr < page_address(page))
+               return s->name;
+
         /* Find offset within object. */
         offset = (ptr - page_address(page)) % s->size;
  

With that, you can add

Reviwed-by: Laura Abbott <labbott@redhat.com>

>  static size_t __ksize(const void *object)
>  {
>  	struct page *page;
>

Thanks,
Laura

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#1449795

On Mon, Jul 25, 2016 at 12:16 PM, Laura Abbott <labbott@redhat.com> wrote:
> On 07/20/2016 01:27 PM, Kees Cook wrote:
>>
>> Under CONFIG_HARDENED_USERCOPY, this adds object size checking to the
>> SLUB allocator to catch any copies that may span objects. Includes a
>> redzone handling fix discovered by Michael Ellerman.
>>
>> Based on code from PaX and grsecurity.
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> Tested-by: Michael Ellerman <mpe@ellerman.id.au>
>> ---
>>  init/Kconfig |  1 +
>>  mm/slub.c    | 36 ++++++++++++++++++++++++++++++++++++
>>  2 files changed, 37 insertions(+)
>>
>> diff --git a/init/Kconfig b/init/Kconfig
>> index 798c2020ee7c..1c4711819dfd 100644
>> --- a/init/Kconfig
>> +++ b/init/Kconfig
>> @@ -1765,6 +1765,7 @@ config SLAB
>>
>>  config SLUB
>>         bool "SLUB (Unqueued Allocator)"
>> +       select HAVE_HARDENED_USERCOPY_ALLOCATOR
>>         help
>>            SLUB is a slab allocator that minimizes cache line usage
>>            instead of managing queues of cached objects (SLAB approach).
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 825ff4505336..7dee3d9a5843 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -3614,6 +3614,42 @@ void *__kmalloc_node(size_t size, gfp_t flags, int
>> node)
>>  EXPORT_SYMBOL(__kmalloc_node);
>>  #endif
>>
>> +#ifdef CONFIG_HARDENED_USERCOPY
>> +/*
>> + * Rejects objects that are incorrectly sized.
>> + *
>> + * Returns NULL if check passes, otherwise const char * to name of cache
>> + * to indicate an error.
>> + */
>> +const char *__check_heap_object(const void *ptr, unsigned long n,
>> +                               struct page *page)
>> +{
>> +       struct kmem_cache *s;
>> +       unsigned long offset;
>> +       size_t object_size;
>> +
>> +       /* Find object and usable object size. */
>> +       s = page->slab_cache;
>> +       object_size = slab_ksize(s);
>> +
>> +       /* Find offset within object. */
>> +       offset = (ptr - page_address(page)) % s->size;
>> +
>> +       /* Adjust for redzone and reject if within the redzone. */
>> +       if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) {
>> +               if (offset < s->red_left_pad)
>> +                       return s->name;
>> +               offset -= s->red_left_pad;
>> +       }
>> +
>> +       /* Allow address range falling entirely within object size. */
>> +       if (offset <= object_size && n <= object_size - offset)
>> +               return NULL;
>> +
>> +       return s->name;
>> +}
>> +#endif /* CONFIG_HARDENED_USERCOPY */
>> +
>
>
> I compared this against what check_valid_pointer does for SLUB_DEBUG
> checking. I was hoping we could utilize that function to avoid
> duplication but a) __check_heap_object needs to allow accesses anywhere
> in the object, not just the beginning b) accessing page->objects
> is racy without the addition of locking in SLUB_DEBUG.
>
> Still, the ptr < page_address(page) check from __check_heap_object would
> be good to add to avoid generating garbage large offsets and trying to
> infer C math.
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 7dee3d9..5370e4f 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3632,6 +3632,9 @@ const char *__check_heap_object(const void *ptr,
> unsigned long n,
>         s = page->slab_cache;
>         object_size = slab_ksize(s);
>  +       if (ptr < page_address(page))
> +               return s->name;
> +
>         /* Find offset within object. */
>         offset = (ptr - page_address(page)) % s->size;
>
> With that, you can add
>
> Reviwed-by: Laura Abbott <labbott@redhat.com>

Cool, I'll add that.

Should I add your reviewed-by for this patch only or for the whole series?

Thanks!

-Kees

>
>>  static size_t __ksize(const void *object)
>>  {
>>         struct page *page;
>>
>
> Thanks,
> Laura



-- 
Kees Cook
Chrome OS & Brillo Security

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#1450307

On 07/25/2016 01:45 PM, Kees Cook wrote:
> On Mon, Jul 25, 2016 at 12:16 PM, Laura Abbott <labbott@redhat.com> wrote:
>> On 07/20/2016 01:27 PM, Kees Cook wrote:
>>>
>>> Under CONFIG_HARDENED_USERCOPY, this adds object size checking to the
>>> SLUB allocator to catch any copies that may span objects. Includes a
>>> redzone handling fix discovered by Michael Ellerman.
>>>
>>> Based on code from PaX and grsecurity.
>>>
>>> Signed-off-by: Kees Cook <keescook@chromium.org>
>>> Tested-by: Michael Ellerman <mpe@ellerman.id.au>
>>> ---
>>>  init/Kconfig |  1 +
>>>  mm/slub.c    | 36 ++++++++++++++++++++++++++++++++++++
>>>  2 files changed, 37 insertions(+)
>>>
>>> diff --git a/init/Kconfig b/init/Kconfig
>>> index 798c2020ee7c..1c4711819dfd 100644
>>> --- a/init/Kconfig
>>> +++ b/init/Kconfig
>>> @@ -1765,6 +1765,7 @@ config SLAB
>>>
>>>  config SLUB
>>>         bool "SLUB (Unqueued Allocator)"
>>> +       select HAVE_HARDENED_USERCOPY_ALLOCATOR
>>>         help
>>>            SLUB is a slab allocator that minimizes cache line usage
>>>            instead of managing queues of cached objects (SLAB approach).
>>> diff --git a/mm/slub.c b/mm/slub.c
>>> index 825ff4505336..7dee3d9a5843 100644
>>> --- a/mm/slub.c
>>> +++ b/mm/slub.c
>>> @@ -3614,6 +3614,42 @@ void *__kmalloc_node(size_t size, gfp_t flags, int
>>> node)
>>>  EXPORT_SYMBOL(__kmalloc_node);
>>>  #endif
>>>
>>> +#ifdef CONFIG_HARDENED_USERCOPY
>>> +/*
>>> + * Rejects objects that are incorrectly sized.
>>> + *
>>> + * Returns NULL if check passes, otherwise const char * to name of cache
>>> + * to indicate an error.
>>> + */
>>> +const char *__check_heap_object(const void *ptr, unsigned long n,
>>> +                               struct page *page)
>>> +{
>>> +       struct kmem_cache *s;
>>> +       unsigned long offset;
>>> +       size_t object_size;
>>> +
>>> +       /* Find object and usable object size. */
>>> +       s = page->slab_cache;
>>> +       object_size = slab_ksize(s);
>>> +
>>> +       /* Find offset within object. */
>>> +       offset = (ptr - page_address(page)) % s->size;
>>> +
>>> +       /* Adjust for redzone and reject if within the redzone. */
>>> +       if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) {
>>> +               if (offset < s->red_left_pad)
>>> +                       return s->name;
>>> +               offset -= s->red_left_pad;
>>> +       }
>>> +
>>> +       /* Allow address range falling entirely within object size. */
>>> +       if (offset <= object_size && n <= object_size - offset)
>>> +               return NULL;
>>> +
>>> +       return s->name;
>>> +}
>>> +#endif /* CONFIG_HARDENED_USERCOPY */
>>> +
>>
>>
>> I compared this against what check_valid_pointer does for SLUB_DEBUG
>> checking. I was hoping we could utilize that function to avoid
>> duplication but a) __check_heap_object needs to allow accesses anywhere
>> in the object, not just the beginning b) accessing page->objects
>> is racy without the addition of locking in SLUB_DEBUG.
>>
>> Still, the ptr < page_address(page) check from __check_heap_object would
>> be good to add to avoid generating garbage large offsets and trying to
>> infer C math.
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 7dee3d9..5370e4f 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -3632,6 +3632,9 @@ const char *__check_heap_object(const void *ptr,
>> unsigned long n,
>>         s = page->slab_cache;
>>         object_size = slab_ksize(s);
>>  +       if (ptr < page_address(page))
>> +               return s->name;
>> +
>>         /* Find offset within object. */
>>         offset = (ptr - page_address(page)) % s->size;
>>
>> With that, you can add
>>
>> Reviwed-by: Laura Abbott <labbott@redhat.com>
>
> Cool, I'll add that.
>
> Should I add your reviewed-by for this patch only or for the whole series?
>
> Thanks!
>
> -Kees
>

Just this patch for now, I'm working through a couple of others

>>
>>>  static size_t __ksize(const void *object)
>>>  {
>>>         struct page *page;
>>>
>>
>> Thanks,
>> Laura
>
>
>

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#1450023

[Multipart message — attachments visible in raw view] — view raw

On Mon, 2016-07-25 at 12:16 -0700, Laura Abbott wrote:
> On 07/20/2016 01:27 PM, Kees Cook wrote:
> > Under CONFIG_HARDENED_USERCOPY, this adds object size checking to
> > the
> > SLUB allocator to catch any copies that may span objects. Includes
> > a
> > redzone handling fix discovered by Michael Ellerman.
> > 
> > Based on code from PaX and grsecurity.
> > 
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > Tested-by: Michael Ellerman <mpe@ellerman.id.au>
> > ---
> >  init/Kconfig |  1 +
> >  mm/slub.c    | 36 ++++++++++++++++++++++++++++++++++++
> >  2 files changed, 37 insertions(+)
> > 
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 798c2020ee7c..1c4711819dfd 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1765,6 +1765,7 @@ config SLAB
> > 
> >  config SLUB
> >  	bool "SLUB (Unqueued Allocator)"
> > +	select HAVE_HARDENED_USERCOPY_ALLOCATOR
> >  	help
> >  	   SLUB is a slab allocator that minimizes cache line
> > usage
> >  	   instead of managing queues of cached objects (SLAB
> > approach).
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 825ff4505336..7dee3d9a5843 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3614,6 +3614,42 @@ void *__kmalloc_node(size_t size, gfp_t
> > flags, int node)
> >  EXPORT_SYMBOL(__kmalloc_node);
> >  #endif
> > 
> > +#ifdef CONFIG_HARDENED_USERCOPY
> > +/*
> > + * Rejects objects that are incorrectly sized.
> > + *
> > + * Returns NULL if check passes, otherwise const char * to name of
> > cache
> > + * to indicate an error.
> > + */
> > +const char *__check_heap_object(const void *ptr, unsigned long n,
> > +				struct page *page)
> > +{
> > +	struct kmem_cache *s;
> > +	unsigned long offset;
> > +	size_t object_size;
> > +
> > +	/* Find object and usable object size. */
> > +	s = page->slab_cache;
> > +	object_size = slab_ksize(s);
> > +
> > +	/* Find offset within object. */
> > +	offset = (ptr - page_address(page)) % s->size;
> > +
> > +	/* Adjust for redzone and reject if within the redzone. */
> > +	if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) {
> > +		if (offset < s->red_left_pad)
> > +			return s->name;
> > +		offset -= s->red_left_pad;
> > +	}
> > +
> > +	/* Allow address range falling entirely within object
> > size. */
> > +	if (offset <= object_size && n <= object_size - offset)
> > +		return NULL;
> > +
> > +	return s->name;
> > +}
> > +#endif /* CONFIG_HARDENED_USERCOPY */
> > +
> 
> I compared this against what check_valid_pointer does for SLUB_DEBUG
> checking. I was hoping we could utilize that function to avoid
> duplication but a) __check_heap_object needs to allow accesses
> anywhere
> in the object, not just the beginning b) accessing page->objects
> is racy without the addition of locking in SLUB_DEBUG.
> 
> Still, the ptr < page_address(page) check from __check_heap_object
> would
> be good to add to avoid generating garbage large offsets and trying
> to
> infer C math.
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 7dee3d9..5370e4f 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3632,6 +3632,9 @@ const char *__check_heap_object(const void
> *ptr, unsigned long n,
>          s = page->slab_cache;
>          object_size = slab_ksize(s);
>   
> +       if (ptr < page_address(page))
> +               return s->name;
> +
>          /* Find offset within object. */
>          offset = (ptr - page_address(page)) % s->size;
> 

I don't get it, isn't that already guaranteed because we
look for the page that ptr is in, before __check_heap_object
is called?

Specifically, in patch 3/12:

+       page = virt_to_head_page(ptr);
+
+       /* Check slab allocator for flags and size. */
+       if (PageSlab(page))
+               return __check_heap_object(ptr, n, page);

How can that generate a ptr that is not inside the page?

What am I overlooking?  And, should it be in the changelog or
a comment? :)

-- 

All Rights Reversed.

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#1450255

On 07/25/2016 02:42 PM, Rik van Riel wrote:
> On Mon, 2016-07-25 at 12:16 -0700, Laura Abbott wrote:
>> On 07/20/2016 01:27 PM, Kees Cook wrote:
>>> Under CONFIG_HARDENED_USERCOPY, this adds object size checking to
>>> the
>>> SLUB allocator to catch any copies that may span objects. Includes
>>> a
>>> redzone handling fix discovered by Michael Ellerman.
>>>
>>> Based on code from PaX and grsecurity.
>>>
>>> Signed-off-by: Kees Cook <keescook@chromium.org>
>>> Tested-by: Michael Ellerman <mpe@ellerman.id.au>
>>> ---
>>>  init/Kconfig |  1 +
>>>  mm/slub.c    | 36 ++++++++++++++++++++++++++++++++++++
>>>  2 files changed, 37 insertions(+)
>>>
>>> diff --git a/init/Kconfig b/init/Kconfig
>>> index 798c2020ee7c..1c4711819dfd 100644
>>> --- a/init/Kconfig
>>> +++ b/init/Kconfig
>>> @@ -1765,6 +1765,7 @@ config SLAB
>>>
>>>  config SLUB
>>>  	bool "SLUB (Unqueued Allocator)"
>>> +	select HAVE_HARDENED_USERCOPY_ALLOCATOR
>>>  	help
>>>  	   SLUB is a slab allocator that minimizes cache line
>>> usage
>>>  	   instead of managing queues of cached objects (SLAB
>>> approach).
>>> diff --git a/mm/slub.c b/mm/slub.c
>>> index 825ff4505336..7dee3d9a5843 100644
>>> --- a/mm/slub.c
>>> +++ b/mm/slub.c
>>> @@ -3614,6 +3614,42 @@ void *__kmalloc_node(size_t size, gfp_t
>>> flags, int node)
>>>  EXPORT_SYMBOL(__kmalloc_node);
>>>  #endif
>>>
>>> +#ifdef CONFIG_HARDENED_USERCOPY
>>> +/*
>>> + * Rejects objects that are incorrectly sized.
>>> + *
>>> + * Returns NULL if check passes, otherwise const char * to name of
>>> cache
>>> + * to indicate an error.
>>> + */
>>> +const char *__check_heap_object(const void *ptr, unsigned long n,
>>> +				struct page *page)
>>> +{
>>> +	struct kmem_cache *s;
>>> +	unsigned long offset;
>>> +	size_t object_size;
>>> +
>>> +	/* Find object and usable object size. */
>>> +	s = page->slab_cache;
>>> +	object_size = slab_ksize(s);
>>> +
>>> +	/* Find offset within object. */
>>> +	offset = (ptr - page_address(page)) % s->size;
>>> +
>>> +	/* Adjust for redzone and reject if within the redzone. */
>>> +	if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) {
>>> +		if (offset < s->red_left_pad)
>>> +			return s->name;
>>> +		offset -= s->red_left_pad;
>>> +	}
>>> +
>>> +	/* Allow address range falling entirely within object
>>> size. */
>>> +	if (offset <= object_size && n <= object_size - offset)
>>> +		return NULL;
>>> +
>>> +	return s->name;
>>> +}
>>> +#endif /* CONFIG_HARDENED_USERCOPY */
>>> +
>>
>> I compared this against what check_valid_pointer does for SLUB_DEBUG
>> checking. I was hoping we could utilize that function to avoid
>> duplication but a) __check_heap_object needs to allow accesses
>> anywhere
>> in the object, not just the beginning b) accessing page->objects
>> is racy without the addition of locking in SLUB_DEBUG.
>>
>> Still, the ptr < page_address(page) check from __check_heap_object
>> would
>> be good to add to avoid generating garbage large offsets and trying
>> to
>> infer C math.
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 7dee3d9..5370e4f 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -3632,6 +3632,9 @@ const char *__check_heap_object(const void
>> *ptr, unsigned long n,
>>          s = page->slab_cache;
>>          object_size = slab_ksize(s);
>>
>> +       if (ptr < page_address(page))
>> +               return s->name;
>> +
>>          /* Find offset within object. */
>>          offset = (ptr - page_address(page)) % s->size;
>>
>
> I don't get it, isn't that already guaranteed because we
> look for the page that ptr is in, before __check_heap_object
> is called?
>
> Specifically, in patch 3/12:
>
> +       page = virt_to_head_page(ptr);
> +
> +       /* Check slab allocator for flags and size. */
> +       if (PageSlab(page))
> +               return __check_heap_object(ptr, n, page);
>
> How can that generate a ptr that is not inside the page?
>
> What am I overlooking?  And, should it be in the changelog or
> a comment? :)
>


I ran into the subtraction issue when the vmalloc detection wasn't
working on ARM64, somehow virt_to_head_page turned into a page
that happened to have PageSlab set. I agree if everything is working
properly this is redundant but given the type of feature this is, a
little bit of redundancy against a system running off into the weeds
or bad patches might be warranted.

I'm not super attached to the check if other maintainers think it
is redundant. Updating the __check_heap_object header comment
with a note of what we are assuming could work

Thanks,
Laura

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On Mon, 2016-07-25 at 16:29 -0700, Laura Abbott wrote:
> On 07/25/2016 02:42 PM, Rik van Riel wrote:
> > On Mon, 2016-07-25 at 12:16 -0700, Laura Abbott wrote:
> > > On 07/20/2016 01:27 PM, Kees Cook wrote:
> > > > Under CONFIG_HARDENED_USERCOPY, this adds object size checking
> > > > to
> > > > the
> > > > SLUB allocator to catch any copies that may span objects.
> > > > Includes
> > > > a
> > > > redzone handling fix discovered by Michael Ellerman.
> > > > 
> > > > Based on code from PaX and grsecurity.
> > > > 
> > > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > > > Tested-by: Michael Ellerman <mpe@ellerman.id.au>
> > > > ---
> > > >  init/Kconfig |  1 +
> > > >  mm/slub.c    | 36 ++++++++++++++++++++++++++++++++++++
> > > >  2 files changed, 37 insertions(+)
> > > > 
> > > > diff --git a/init/Kconfig b/init/Kconfig
> > > > index 798c2020ee7c..1c4711819dfd 100644
> > > > --- a/init/Kconfig
> > > > +++ b/init/Kconfig
> > > > @@ -1765,6 +1765,7 @@ config SLAB
> > > > 
> > > >  config SLUB
> > > >  	bool "SLUB (Unqueued Allocator)"
> > > > +	select HAVE_HARDENED_USERCOPY_ALLOCATOR
> > > >  	help
> > > >  	   SLUB is a slab allocator that minimizes cache line
> > > > usage
> > > >  	   instead of managing queues of cached objects (SLAB
> > > > approach).
> > > > diff --git a/mm/slub.c b/mm/slub.c
> > > > index 825ff4505336..7dee3d9a5843 100644
> > > > --- a/mm/slub.c
> > > > +++ b/mm/slub.c
> > > > @@ -3614,6 +3614,42 @@ void *__kmalloc_node(size_t size, gfp_t
> > > > flags, int node)
> > > >  EXPORT_SYMBOL(__kmalloc_node);
> > > >  #endif
> > > > 
> > > > +#ifdef CONFIG_HARDENED_USERCOPY
> > > > +/*
> > > > + * Rejects objects that are incorrectly sized.
> > > > + *
> > > > + * Returns NULL if check passes, otherwise const char * to
> > > > name of
> > > > cache
> > > > + * to indicate an error.
> > > > + */
> > > > +const char *__check_heap_object(const void *ptr, unsigned long
> > > > n,
> > > > +				struct page *page)
> > > > +{
> > > > +	struct kmem_cache *s;
> > > > +	unsigned long offset;
> > > > +	size_t object_size;
> > > > +
> > > > +	/* Find object and usable object size. */
> > > > +	s = page->slab_cache;
> > > > +	object_size = slab_ksize(s);
> > > > +
> > > > +	/* Find offset within object. */
> > > > +	offset = (ptr - page_address(page)) % s->size;
> > > > +
> > > > +	/* Adjust for redzone and reject if within the
> > > > redzone. */
> > > > +	if (kmem_cache_debug(s) && s->flags & SLAB_RED_ZONE) {
> > > > +		if (offset < s->red_left_pad)
> > > > +			return s->name;
> > > > +		offset -= s->red_left_pad;
> > > > +	}
> > > > +
> > > > +	/* Allow address range falling entirely within object
> > > > size. */
> > > > +	if (offset <= object_size && n <= object_size -
> > > > offset)
> > > > +		return NULL;
> > > > +
> > > > +	return s->name;
> > > > +}
> > > > +#endif /* CONFIG_HARDENED_USERCOPY */
> > > > +
> > > 
> > > I compared this against what check_valid_pointer does for
> > > SLUB_DEBUG
> > > checking. I was hoping we could utilize that function to avoid
> > > duplication but a) __check_heap_object needs to allow accesses
> > > anywhere
> > > in the object, not just the beginning b) accessing page->objects
> > > is racy without the addition of locking in SLUB_DEBUG.
> > > 
> > > Still, the ptr < page_address(page) check from
> > > __check_heap_object
> > > would
> > > be good to add to avoid generating garbage large offsets and
> > > trying
> > > to
> > > infer C math.
> > > 
> > > diff --git a/mm/slub.c b/mm/slub.c
> > > index 7dee3d9..5370e4f 100644
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -3632,6 +3632,9 @@ const char *__check_heap_object(const void
> > > *ptr, unsigned long n,
> > >          s = page->slab_cache;
> > >          object_size = slab_ksize(s);
> > > 
> > > +       if (ptr < page_address(page))
> > > +               return s->name;
> > > +
> > >          /* Find offset within object. */
> > >          offset = (ptr - page_address(page)) % s->size;
> > > 
> > 
> > I don't get it, isn't that already guaranteed because we
> > look for the page that ptr is in, before __check_heap_object
> > is called?
> > 
> > Specifically, in patch 3/12:
> > 
> > +       page = virt_to_head_page(ptr);
> > +
> > +       /* Check slab allocator for flags and size. */
> > +       if (PageSlab(page))
> > +               return __check_heap_object(ptr, n, page);
> > 
> > How can that generate a ptr that is not inside the page?
> > 
> > What am I overlooking?  And, should it be in the changelog or
> > a comment? :)
> > 
> 
> 
> I ran into the subtraction issue when the vmalloc detection wasn't
> working on ARM64, somehow virt_to_head_page turned into a page
> that happened to have PageSlab set. I agree if everything is working
> properly this is redundant but given the type of feature this is, a
> little bit of redundancy against a system running off into the weeds
> or bad patches might be warranted.
> 
That's fair.  I have no objection to the check, but would
like to see it documented, since it does look a little out
of place.

-- 

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