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Subject: Re: [kernel-hardening] [PATCH 09/38] usercopy: Mark kmalloc caches as
 usercopy caches
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Christopher Lameter <cl@linux.com>,
        Kees Cook <keescook@chromium.org>, Jiri Slaby <jslaby@suse.cz>,
        Julian Wiedmann <jwi@linux.ibm.com>,
        Ursula Braun <ubraun@linux.ibm.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-kernel@vger.kernel.org, David Windsor <dave@nullcore.net>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-xfs@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Laura Abbott <labbott@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Christoffer Dall <christoffer.dall@linaro.org>,
        Dave Kleikamp <dave.kleikamp@oracle.com>,
        Jan Kara <jack@suse.cz>,
        Luis de Bethencourt <luisbg@kernel.org>,
        Marc Zyngier <marc.zyngier@arm.com>,
        Rik van Riel <riel@redhat.com>,
        Matthew Garrett <mjg59@google.com>,
        linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org,
        netdev@vger.kernel.org, kernel-hardening@lists.openwall.com,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Kubecek <mkubecek@suse.cz>
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From:   Christian Borntraeger <borntraeger@de.ibm.com>
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Date:   Wed, 29 Jan 2020 18:19:56 +0100
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On 29.01.20 18:09, Christoph Hellwig wrote:
> On Wed, Jan 29, 2020 at 06:07:14PM +0100, Christian Borntraeger wrote:
>>> 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).
> 
> The normal way to allocate memory with addressing limits would be to
> use dma_alloc_coherent and friends.  Any chance to switch iucv over to
> that?  Or is there no device associated with it?

There is not necessarily a device for that. It is a hypervisor interface (an
instruction that is interpreted by z/VM). We do have the netiucv driver that
creates a virtual nic, but there is also AF_IUCV which works without a device.

But back to the original question: If we mark kmalloc caches as usercopy caches,
we should do the same for DMA kmalloc caches. As outlined by Christoph, this has
nothing to do with device DMA.