linux-kernel - Re: DMA Engine: Transfer From Userspace

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Message-ID: <20200626200815.GC2454695@iweiny-DESK2.sc.intel.com>
Date:   Fri, 26 Jun 2020 13:08:15 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Thomas Ruf <freelancer@...usul.de>
Cc:     Dave Jiang <dave.jiang@...el.com>, Vinod Koul <vkoul@...nel.org>,
        Federico Vaga <federico.vaga@...n.ch>,
        Dan Williams <dan.j.williams@...el.com>,
        dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>
Subject: Re: DMA Engine: Transfer From Userspace

On Thu, Jun 25, 2020 at 10:11:28AM +0200, Thomas Ruf wrote:
> 
> > On 25 June 2020 at 02:42 Dave Jiang <dave.jiang@...el.com> wrote:
> > 
> > 
> > 
> > 
> > On 6/21/2020 12:24 AM, Vinod Koul wrote:
> > > On 19-06-20, 16:31, Dave Jiang wrote:
> > >>
> > >>
> > >> On 6/19/2020 3:47 PM, Federico Vaga wrote:
> > >>> Hello,
> > >>>
> > >>> is there the possibility of using a DMA engine channel from userspace?
> > >>>
> > >>> Something like:
> > >>> - configure DMA using ioctl() (or whatever configuration mechanism)
> > >>> - read() or write() to trigger the transfer
> > >>>
> > >>
> > >> I may have supposedly promised Vinod to look into possibly providing
> > >> something like this in the future. But I have not gotten around to do that
> > >> yet. Currently, no such support.
> > > 
> > > And I do still have serious reservations about this topic :) Opening up
> > > userspace access to DMA does not sound very great from security point of
> > > view.
> > 
> > What about doing it with DMA engine that supports PASID? That way the user can 
> > really only trash its own address space and kernel is protected.
> 
> Sounds interesting! Not sure if this is really needed in that case...
> I have already implemented checks of vm_area_struct for contiguous memory or even do a get_user_pages_fast for user memory to pin it (hope that is the correct term here). Of course i have to do that for every involved page.

FWIW there is a new pin_user_pages_fast()/unpin_user_page() interface now.

Ira

> But i will do some checks if my code is really suitable to avoid misusage.
> 
> Best regards,
> Thomas