linux-kernel - Re: [PATCH] PCI/PM: Move ASUS EHCI workaround out of generic code

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Message-ID: <CAJZ5v0iNdLp_=GPg2AQ+r06tSdpZ1HQJMu=bBTNOyyic1no1EQ@mail.gmail.com>
Date: Fri, 12 Sep 2025 11:34:39 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Lukas Wunner <lukas@...ner.de>
Cc: Mario Limonciello <superm1@...nel.org>, Bjorn Helgaas <helgaas@...nel.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Alan Stern <stern@...land.harvard.edu>, linux-pci@...r.kernel.org, 
	linux-pm@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org, 
	Oleksij Rempel <o.rempel@...gutronix.de>, Timo Jyrinki <timo.jyrinki@...il.com>, 
	Ernst Persson <ernstp@...il.com>, Steven Harms <sjharms@...il.com>, James Ettle <james@...le.org.uk>, 
	Nick Coghlan <ncoghlan@...il.com>, Weng Xuetian <wengxt@...il.com>, 
	Andrey Rahmatullin <wrar@...r.name>, Boris Barbour <boris.barbour@....fr>, 
	Vlastimil Zima <vlastimil.zima@...il.com>, David Banks <amoebae@...il.com>, 
	Chris Moeller <kode54@...il.com>, Daniel Fraga <fragabr@...il.com>, Javier Marcet <jmarcet@...il.com>, 
	Pavel Pisa <pisa@....felk.cvut.cz>
Subject: Re: [PATCH] PCI/PM: Move ASUS EHCI workaround out of generic code

On Fri, Sep 12, 2025 at 9:26 AM Lukas Wunner <lukas@...ner.de> wrote:
>
> On Thu, Sep 11, 2025 at 08:34:56AM -0500, Mario Limonciello wrote:
> > On 9/11/25 8:11 AM, Lukas Wunner wrote:
> > > pci_disable_device() does not clear I/O and Memory Space Enable, although
> > > its name suggests otherwise.  The kernel has never disabled these bits
> > > once they're enabled.  Doing so would avoid the need for the quirk, but it
> > > is unclear what will break if this fundamental behavior is changed.
> >
> > It's too late for this cycle to do so, but how would you feel about making
> > this change at the start of the next cycle so it had a whole cycle to bake
> > in linux-next and see if there is a problem in doing so?
>
> I can look into it.
>
> The change could be justified as a security enhancement to prevent
> unauthorized traffic between devices through peer-to-peer transactions.
>
> pci_disable_device() was introduced with v2.4.3.5 in 2002:
> https://git.kernel.org/tglx/history/c/9102e0eb3e9e
>
> I suspect back in the day, clearing Bus Master Enable seemed sufficient
> because the only concern was to prevent DMA (and by extension MSIs)
> from broken devices.  Attacks *between* devices were probably not
> considered realistic.
>
> ACS is meant to prevent such attacks, but is an optional capability
> and might be configured incorrectly.  A zero trust, defense in depth
> approach as is common today requires not leaving doors open without need.
>
> If the kernel would clear Memory Space Enable, a malicious device could
> not re-enable it on its own because "propagation of Configuration Requests
> from Downstream to Upstream as well as peer-to-peer are not supported"
> (PCIe r7.0 sec 7.3.3).
>
> It seemed too risky to make such a sweeping change only to get rid of
> the EHCI quirk.  The present patch is meant as a low-risk refactoring,
> but we can consider clearing IO + Memory Space Enable as a long-term
> solution.

Yes, we can.

Obviously, the additional reason for doing it, which appears to be
more significant than avoiding the EHCI quirk alone, can be regarded
as sufficient justification IMV.