Re: [kernel-hardening] [PATCH 09/38] usercopy: Mark kmalloc caches as usercopy caches

Christian Borntraeger
2020-01-29T17:07:49Z

On 29.01.20 17:43, Christopher Lameter wrote:
> On Tue, 28 Jan 2020, Kees Cook wrote:
>
>>> On the other hand not marking the DMA caches still seems questionable.
>>
>> My understanding is that exposing DMA memory to userspace copies can
>> lead to unexpected results, especially for misbehaving hardware, so I'm
>> not convinced this is a generically bad hardening choice.
>
> "DMA" memory (and thus DMA caches) have nothing to do with DMA. Its a
> legacy term. "DMA Memory" is memory limited to a certain
> physical address boundary (old restrictions on certain devices only
> supporting a limited number of address bits).
>
> DMA can be done to NORMAL memory as well.

Exactly.
I think iucv uses GFP_DMA because z/VM needs those buffers to reside below 2GB (which is ZONA_DMA for s390).