[kernel-hardening] Re: [PATCH 9/9] mm: SLUB hardened usercopy support

[Kees Cook]

On Thu, Jul 7, 2016 at 12:35 AM, Michael Ellerman <mpe at ellerman.id.au> wrote:
> Kees Cook <keescook at chromium.org> writes:
>
>> Under CONFIG_HARDENED_USERCOPY, this adds object size checking to the
>> SLUB allocator to catch any copies that may span objects.
>>
>> Based on code from PaX and grsecurity.
>>
>> Signed-off-by: Kees Cook <keescook at chromium.org>
>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 825ff4505336..0c8ace04f075 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -3614,6 +3614,33 @@ 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;
>> +
>> +     /* Find object. */
>> +     s = page->slab_cache;
>> +
>> +     /* Find offset within object. */
>> +     offset = (ptr - page_address(page)) % s->size;
>> +
>> +     /* Allow address range falling entirely within object size. */
>> +     if (offset <= s->object_size && n <= s->object_size - offset)
>> +             return NULL;
>> +
>> +     return s->name;
>> +}
>
> I gave this a quick spin on powerpc, it blew up immediately :)

Wheee :) This series is rather easy to test: blows up REALLY quickly
if it's wrong. ;)

FWIW, -next also has a bunch of additional lkdtm tests for the various
protections and directions.

>
>   Brought up 16 CPUs
>   usercopy: kernel memory overwrite attempt detected to c0000001fe023868 (kmalloc-16) (9 bytes)
>   CPU: 8 PID: 103 Comm: kdevtmpfs Not tainted 4.7.0-rc3-00098-g09d9556ae5d1 #55
>   Call Trace:
>   [c0000001fa0cfb40] [c0000000009bdbe8] dump_stack+0xb0/0xf0 (unreliable)
>   [c0000001fa0cfb80] [c00000000029cf44] __check_object_size+0x74/0x320
>   [c0000001fa0cfc00] [c00000000005d4d0] copy_from_user+0x60/0xd4
>   [c0000001fa0cfc40] [c00000000022b6cc] memdup_user+0x5c/0xf0
>   [c0000001fa0cfc80] [c00000000022b90c] strndup_user+0x7c/0x110
>   [c0000001fa0cfcc0] [c0000000002d6c28] SyS_mount+0x58/0x180
>   [c0000001fa0cfd10] [c0000000005ee908] devtmpfsd+0x98/0x210
>   [c0000001fa0cfd80] [c0000000000df810] kthread+0x110/0x130
>   [c0000001fa0cfe30] [c0000000000095e8] ret_from_kernel_thread+0x5c/0x74
>
> SLUB tracing says:
>
>   TRACE kmalloc-16 alloc 0xc0000001fe023868 inuse=186 fp=0x          (null)
>
> Which is not 16-byte aligned, which seems to be caused by the red zone?
> The following patch fixes it for me, but I don't know SLUB enough to say
> if it's always correct.
>
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 0c8ace04f075..66191ea4545a 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3630,6 +3630,9 @@ const char *__check_heap_object(const void *ptr, unsigned long n,
>         /* Find object. */
>         s = page->slab_cache;
>
> +       /* Subtract red zone if enabled */
> +       ptr = restore_red_left(s, ptr);
> +

Ah, interesting. Just to make sure: you've built with
CONFIG_SLUB_DEBUG and either CONFIG_SLUB_DEBUG_ON or booted with
either slub_debug or slub_debug=z ?

Thanks for the slub fix!

I wonder if this code should be using size_from_object() instead of s->size?

(It looks like slab is already handling this via the obj_offset() call.)

-Kees

>         /* Find offset within object. */
>         offset = (ptr - page_address(page)) % s->size;
>
> cheers



-- 
Kees Cook
Chrome OS & Brillo Security