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Date: Wed, 22 Nov 2023 14:44:53 +0000
From: Greg KH <gregkh@linuxfoundation.org>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Robin Murphy <robin.murphy@arm.com>, joro@8bytes.org, will@kernel.org,
	iommu@lists.linux.dev, linux-kernel@vger.kernel.org,
	linux-acpi@vger.kernel.org, rafael@kernel.org, lenb@kernel.org,
	lpieralisi@kernel.org, andre.draszik@linaro.org,
	quic_zhenhuah@quicinc.com
Subject: Re: [PATCH] iommu: Avoid more races around device probe
Message-ID: <2023112238-sierra-chewable-86fc@gregkh>
References: <16f433658661d7cadfea51e7c65da95826112a2b.1700071477.git.robin.murphy@arm.com>
 <ZVZL9kZuVsb9VPnM@nvidia.com>
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In-Reply-To: <ZVZL9kZuVsb9VPnM@nvidia.com>

On Thu, Nov 16, 2023 at 01:05:58PM -0400, Jason Gunthorpe wrote:
> On Wed, Nov 15, 2023 at 06:25:44PM +0000, Robin Murphy wrote:
> > It turns out there are more subtle races beyond just the main part of
> > __iommu_probe_device() itself running in parallel - the dev_iommu_free()
> > on the way out of an unsuccessful probe can still manage to trip up
> > concurrent accesses to a device's fwspec. Thus, extend the scope of
> > iommu_probe_device_lock() to also serialise fwspec creation and initial
> > retrieval.
> > 
> > Reported-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
> > Link: https://lore.kernel.org/linux-iommu/e2e20e1c-6450-4ac5-9804-b0000acdf7de@quicinc.com/
> > Fixes: 01657bc14a39 ("iommu: Avoid races around device probe")
> > Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> > ---
> > 
> > This is my idea of a viable fix, since it does not need a 700-line
> > diffstat to make the code do what it was already *trying* to do anyway.
> > This stuff should fundamentally not be hanging off driver probe in the
> > first place, so I'd rather get on with removing the underlying
> > brokenness than waste time and effort polishing it any further.
> 
> I'm fine with this as some hacky backport, but I don't want to see
> this cross-layer leakage left in the next merge window.
> 
> ie we should still do my other series on top of and reverting this.
> 
> I've poked at moving parts of it under probe and I think we can do
> substantial amounts in about two more series and a tidy a bunch of
> other stuff too.

I agree, it's messy and acpi should not need this, BUT at the moment, I
can't see any other way to resolve this simply.

So here's a begrudged ack:

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

and hopefully the larger series should resolve this correctly?  Can that
be rebased on top of this?

Also, cc: stable on this for whomever applies it?

thanks,

greg k-h