linux-kernel - Re: [v3 PATCH] iommu/arm-smmu-v3: Fix L1 stream table index calculation for 32-bit sid size

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Message-ID: <20241008151506.GC762027@ziepe.ca>
Date: Tue, 8 Oct 2024 12:15:06 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Will Deacon <will@...nel.org>
Cc: Yang Shi <yang@...amperecomputing.com>, nicolinc@...dia.com,
	james.morse@....com, robin.murphy@....com,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [v3 PATCH] iommu/arm-smmu-v3: Fix L1 stream table index
 calculation for 32-bit sid size

On Tue, Oct 08, 2024 at 02:34:58PM +0100, Will Deacon wrote:

> This all looks a bit messy to me. The architecture guarantees that
> 2-level stream tables are supported once we hit 7-bit SIDs and, although
> the driver relaxes this to > 8-bit SIDs, we'll never run into overflow
> problems in the linear table code above.

My original point was about the confidential compute position (sigh)
that the untrusted hypverisor should not corrupt the driver.

So your statement is architecturally true, but we never check that
IDR0_ST_LVL_2LVL is set if IDR1_SIDSIZE > 2**7, and so we can get into
this situation where the hypervisor could trigger some kind of bad
behavior.

> So I'm inclined to take Daniel's one-liner [1] which just chucks the
> 'ULL' suffix into the 2-level case. Otherwise, we're in a weird

I think you should take it and let better be for the CC crowd.

Jason