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

[Kees Cook]

On Wed, Aug 17, 2016 at 2:30 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Aug 17, 2016 at 2:25 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>
>> I had forwarded this bug Rik's way since the page-cross checking was
>> suggested by him. I'm happy to drop it; it was a suggested improvement
>> that was suspected to be safe (none of the folks testing this ran into
>> it and we saw no report during its time in -next). I can prepare a
>> patch if there isn't a better way to detect this kind of allocation.
>> (FWIW, slab is handled separately.)
>
> I can't think of any sane way to notice it normally.
>
> Yes, with __GFP_COMPOUND you get the compound flag bits set, but as
> mentioned, that's a special case for the large page VM handling, and
> not applicable in general.
>
> Very few things do higher order allocations outside of slab and the
> task struct. But it does happen. Even fewer of those then have
> contents that might get copied to user space, but it clearly happens
> at least for _one_ case, and I can't convince myself that there might
> not be other cases too..

Yup, totally, I'll send a patch to remove this and Rik and I can
investigate re-adding it later.

-Kees

-- 
Kees Cook
Nexus Security