Re: [x86/uaccess] 5b710f34e1: kernel BUG at mm/usercopy.c:75!

[Kees Cook]

On Wed, Aug 17, 2016 at 2:45 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Aug 17, 2016 at 2:37 PM, Rik van Riel <riel@xxxxxxxxxx> wrote:
>>
>> This particular allocation is through kmalloc, but the
>> kernel in question has CONFIG_SLOB=y, and usercopy has
>> no code in mm/slob.c
>
> Oh, I didn't notice that.
>
> Maybe we can just say that HARDENING depends on !SLOB for now, and see
> if anything else shows up.

This logic (for avoiding uninstrumented allocators, which is only
SLOB) already exists (via CONFIG_HAVE_HARDENED_USERCOPY_ALLOCATOR).
And PageSlab(page) should be catching this, so that the logic of "this
is from the allocator, so we must use its checker" is supposed to get
invoked.

> Maybe we don't have any code that copies data from (non-kmalloc)
> multi-order allocations to user space.
>
> Networking does, but seems to use __GFP_COMP, at least in the one case
> I checked (skbuff).

Was this allocation really through kmalloc?

-Kees

-- 
Kees Cook
Nexus Security