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Date: Mon, 25 Jul 2016 16:29:51 -0700
From: Laura Abbott <labbott@...hat.com>
To: Rik van Riel <riel@...hat.com>, Kees Cook <keescook@...omium.org>,
kernel-hardening@...ts.openwall.com
Cc: Laura Abbott <labbott@...oraproject.org>,
Balbir Singh <bsingharora@...il.com>, Daniel Micay <danielmicay@...il.com>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Casey Schaufler <casey@...aufler-ca.com>, PaX Team <pageexec@...email.hu>,
Brad Spengler <spender@...ecurity.net>, Russell King
<linux@...linux.org.uk>, Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>, Ard Biesheuvel
<ard.biesheuvel@...aro.org>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Michael Ellerman <mpe@...erman.id.au>, Tony Luck <tony.luck@...el.com>,
Fenghua Yu <fenghua.yu@...el.com>, "David S. Miller" <davem@...emloft.net>,
x86@...nel.org, Christoph Lameter <cl@...ux.com>,
Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski
<luto@...nel.org>, Borislav Petkov <bp@...e.de>,
Mathias Krause <minipli@...glemail.com>, Jan Kara <jack@...e.cz>,
Vitaly Wool <vitalywool@...il.com>, Andrea Arcangeli <aarcange@...hat.com>,
Dmitry Vyukov <dvyukov@...gle.com>, linux-arm-kernel@...ts.infradead.org,
linux-ia64@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
sparclinux@...r.kernel.org, linux-arch@...r.kernel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 12/12] mm: SLUB hardened usercopy support
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@...omium.org>
>>> Tested-by: Michael Ellerman <mpe@...erman.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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