linux-kernel - Re: [PATCH V3] PCI: pciehp: Disable ACS Source Validation during hot-remove

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Date: Thu, 18 Jan 2024 11:33:35 +0100
From: Lukas Wunner <lukas@...ner.de>
To: Vidya Sagar <vidyas@...dia.com>
Cc: bhelgaas@...gle.com, alex.williamson@...hat.com, treding@...dia.com,
	jonathanh@...dia.com, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, vsethi@...dia.com, kthota@...dia.com,
	mmaddireddy@...dia.com, sagar.tv@...il.com
Subject: Re: [PATCH V3] PCI: pciehp: Disable ACS Source Validation during
 hot-remove

On Thu, Jan 11, 2024 at 07:14:54PM +0530, Vidya Sagar wrote:
> On 1/8/2024 7:49 PM, Lukas Wunner wrote:
> > On Thu, Jan 04, 2024 at 08:01:06PM +0530, Vidya Sagar wrote:
> > > On 8/1/2023 1:29 AM, Lukas Wunner wrote:
> > > > As an alternative to disabling ACS, have you explored masking ACS
> > > > Violations (PCI_ERR_UNC_ACSV) upon de-enumeration of a device and
> > > > unmasking them after assignment of a bus number?
> > > 
> > > I explored this option and it seemed to work as expected. But, the issue
> > > is that this works only if the AER registers are owned by the OS. If the
> > > AER registers are owned by the firmware (i.e. Firmware-First approach of
> > > handling the errors), OS is not supposed to access the AER registers and
> > > there is no indication from the OS to the firmware as to when the
> > > enumeration is completed and time is apt to unmask the ACSViolation
> > > errors in the AER's Uncorrectable Error Mask register.
> > > Any thoughts on accommodating the Firmware-First approach also?

I'm sorry, I don't have any good ideas.

I just would like to avoid disabling ACS Source Validation because
it would diminish our security posture.

I guess setting the secondary bus number in the hotplug port to 0
isn't a good solution either because it would allow hotplugged devices
to temporarily spoof TLPs from devices on the root bus, right?

One option might be to have separate code paths:  If AER is owned by
the OS, mask PCI_ERR_UNC_ACSV on hot-removal, unmask on hot-add.
If AER is *not* owned by the OS, disable ACS Source Validation on
hot-removal, enable on hot-add, and warn loudly about the security
implications.

Another option might be to change error handling, i.e. ignore
ACS Source Validation errors if they occur before assignment of
a bus number.  And temporarily disable DPC.

None of these options look pretty.  I'm generally not a fan of
having the firmware own certain features.  The user experience
is better if everything is owned by the OS.  This is just one
more case in point. :(

Thanks,

Lukas