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

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Date:   Wed, 29 Jan 2020 18:07:14 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Christopher Lameter <cl@...ux.com>,
        Kees Cook <keescook@...omium.org>
Cc:     Jiri Slaby <jslaby@...e.cz>, Julian Wiedmann <jwi@...ux.ibm.com>,
        Ursula Braun <ubraun@...ux.ibm.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org, David Windsor <dave@...lcore.net>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-xfs@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        "David S. Miller" <davem@...emloft.net>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Dave Kleikamp <dave.kleikamp@...cle.com>,
        Jan Kara <jack@...e.cz>,
        Luis de Bethencourt <luisbg@...nel.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Rik van Riel <riel@...hat.com>,
        Matthew Garrett <mjg59@...gle.com>,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        netdev@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Kubecek <mkubecek@...e.cz>
Subject: Re: [kernel-hardening] [PATCH 09/38] usercopy: Mark kmalloc caches as
 usercopy caches



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).