linux-kernel - Re: [PATCH 1/2] dma-direct: Validate DMA mask against canonical DMA addresses

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Message-ID: <20260120151127.GP961572@ziepe.ca>
Date: Tue, 20 Jan 2026 11:11:27 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Suzuki K Poulose <suzuki.poulose@....com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-coco@...ts.linux.dev,
	Catalin Marinas <catalin.marinas@....com>, will@...nel.org,
	robin.murphy@....com, steven.price@....com,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH 1/2] dma-direct: Validate DMA mask against canonical DMA
 addresses

On Tue, Jan 20, 2026 at 02:39:14PM +0000, Suzuki K Poulose wrote:
> > > > diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> > > > index 8e04f72baaa3..a5639e9415f5 100644
> > > > --- a/kernel/dma/direct.c
> > > > +++ b/kernel/dma/direct.c
> > > > @@ -580,12 +580,12 @@ int dma_direct_supported(struct device *dev, u64 mask)
> > > >    	/*
> > > >    	 * This check needs to be against the actual bit mask value, so use
> > > > -	 * phys_to_dma_unencrypted() here so that the SME encryption mask isn't
> > > > +	 * __phys_to_dma() here so that the arch specific encryption mask isn't
> > > >    	 * part of the check.
> > > >    	 */
> > > >    	if (IS_ENABLED(CONFIG_ZONE_DMA))
> > > >    		min_mask = min_t(u64, min_mask, zone_dma_limit);
> > > > -	return mask >= phys_to_dma_unencrypted(dev, min_mask);
> > > > +	return mask >= __phys_to_dma(dev, min_mask);
> > > 
> > > This is wrong, isn't it ? For e.g., for CCA, even though the "Flag" is
> > > added to the PA, it is really part of the actual "PA" and thus must be
> > > checked against the full PA ?
> > > 
> > 
> > That is true only when the device is operating in untrusted mode?. For a
> > trusted device that mask is valid mask right?
> 
> Irrespective of the mode in which the device is operating, the DMA
> address must include the fully qualified "{I}PA" address, right ?
> i.e., "the Unencrypted" bit is only a software construct and the full
> PA must be used, irrespective of the mode of the device.

But you could make an argument that a trusted device won't DMA to
shared memory, ie it would SWIOTLB to private memory if that is
required.

Otherwise these two limitations will exclude huge numbers of real
devices from working with ARM CCA at all.

Jason