Re: [PATCH v6 08/12] PCI: liveupdate: Inherit ACS flags in incoming preserved devices

[David Matlack <dmatlack@google.com>]

On 2026-06-07 08:37 PM, Pranjal Shrivastava wrote:
> On Fri, May 22, 2026 at 08:24:06PM +0000, David Matlack wrote:
> > Inherit Access Control Services (ACS) flags on all incoming preserved
> > devices (endpoints and upstream bridges) during a Live Update.
> > 
> > Inheriting ACS flags avoids changing routing rules while memory
> > transactions are in flight from preserved devices. This is also strictly
> > necessary to ensure that IOMMU group assignments do not change across
> > a Live Update for preserved devices, as changing ACS configurations can
> > split or merge IOMMU groups.
> > 
> > Cache the inherited ACS controls established by the previous kernel in
> > struct pci_dev so that ACS controls do not change after a reset
> > (pci_restore_state() calls pci_enable_acs()).
> > 
> > To simplify ACS inheritance, reject preserving any devices that require
> > quirks to enable ACS as those quirks would also have to take Live Update
> > into account.
> > 
> > Signed-off-by: David Matlack <dmatlack@google.com>
> > ---
> >  drivers/pci/liveupdate.c       | 68 ++++++++++++++++++++++++++++++++++
> >  drivers/pci/liveupdate.h       | 11 ++++++
> >  drivers/pci/pci.c              |  5 +++
> >  drivers/pci/pci.h              |  5 +++
> >  drivers/pci/quirks.c           |  7 ++++
> >  include/linux/pci_liveupdate.h |  6 +++
> >  6 files changed, 102 insertions(+)
> > 
> 
> [...]
> 
> >  
> > +void pci_liveupdate_init_acs(struct pci_dev *dev)
> > +{
> > +	guard(rwsem_read)(&pci_liveupdate.rwsem);
> > +
> > +	if (!dev->acs_cap || !dev->liveupdate.incoming)
> > +		return;
> > +
> > +	pci_read_config_word(dev, dev->acs_cap + PCI_ACS_CTRL, &dev->liveupdate.acs_ctrl);
> 
> I might be thinking out loud here, but as an attacker, this motivates me
> to somehow hack the EP FW to mis-report the PCI_ACS_CTRL register across
> a liveupdate to fool the incoming kernel. If the FW feeds a 0, it silently
> strips ACS protections.
> 
> Should we also serialize ACS state in ser somehow to ensure we aren't 
> fooled by something like this?

What does "EP FW" mean?

Does such an attacker even need Live Update to attack the system? It
seems like such an attacker could route TLPs in whatever malicious way
they want regardless of Live Update.

> 
> > +}
> > +
> > +int pci_liveupdate_enable_acs(struct pci_dev *dev)
> > +{
> > +	u16 acs_ctrl = dev->liveupdate.acs_ctrl;
> > +	u16 acs_cap = dev->acs_cap;
> > +
> > +	/*
> > +	 * Use liveupdate.was_preserved instead of liveupdate.incoming since the
> > +	 * device's ACS controls should not change even after the device is
> > +	 * finished participating in the Live Update.
> > +	 */
> > +	if (!dev->liveupdate.was_preserved)
> > +		return -EINVAL;
> > +
> > +	/*
> > +	 * The previous kernel should not have preserved any devices that
> > +	 * require device-specific quirks to enable ACS, but if such a device is
> > +	 * detected, log a big warning and fall back to the normal enable ACS
> > +	 * path.
> > +	 */
> 
> Nit: It might be worth adding a note here that this can also happen if a
> new device-specific ACS quirk is introduced in the incoming kernel for a
> device that was preserved by the old kernel (which didn't have the quirk).
> In such cases, the two kernels are essentially non-LUO-compatible..

Yes will do.

> 
> > +	if (pci_need_dev_specific_enable_acs(dev)) {
> > +		pci_warn(dev, "Device-specific quirk required to enable ACS!\n");
> > +		WARN_ON_ONCE(true);
> > +		return -EINVAL;
> > +	}
> > +
> > +	if (acs_cap)
> > +		pci_write_config_word(dev, acs_cap + PCI_ACS_CTRL, acs_ctrl);
> > +
> > +	return 0;
> > +}
> > +
> >  /**
> >   * pci_liveupdate_is_incoming() - Check if a device is incoming-preserved
> >   * @dev: The PCI device to check
> 
> [...]
> 
> Thanks,
> Praan