linux-kernel - Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

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Date:   Thu, 7 Oct 2021 16:10:53 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     Barry Song <21cnbao@...il.com>, iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Raj Ashok <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Tony Luck <tony.luck@...el.com>, mike.campin@...el.com,
        Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [RFC 0/7] Support in-kernel DMA with PASID and SVA

On Thu, Oct 07, 2021 at 12:11:27PM -0700, Jacob Pan wrote:
> Hi Barry,
> 
> On Thu, 7 Oct 2021 18:43:33 +1300, Barry Song <21cnbao@...il.com> wrote:
> 
> > > > Security-wise, KVA respects kernel mapping. So permissions are better
> > > > enforced than pass-through and identity mapping.  
> > >
> > > Is this meaningful? Isn't the entire physical map still in the KVA and
> > > isn't it entirely RW ?  
> > 
> > Some areas are RX, for example, ARCH64 supports KERNEL_TEXT_RDONLY.
> > But the difference is really minor.
> That brought up a good point if we were to use DMA API to give out KVA as
> dma_addr for trusted devices. We cannot satisfy DMA direction requirements
> since we can't change kernel mapping. It will be similar to DMA direct
> where dir is ignored AFAICT.

Right.

Using the DMA API to DMA to read only kernel memory is a bug in the
first place.

> Or we are saying if the device is trusted, using pass-through is allowed.
> i.e. physical address.

I don't see trusted being relavent here beyond the usual decision to
use the trusted map or not.

Jason