Re: [PATCH 2/2] pci: Add parameter to disable attaching untrusted devices

[Rajat Jain]

Hello,

Thanks for taking a look.

On Fri, Jun 26, 2020 at 7:18 AM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Jun 25, 2020 at 05:27:10PM -0700, Rajat Jain wrote:
> > Introduce a PCI parameter that disables the automatic attachment of
> > untrusted devices to their drivers.
> >
> > Signed-off-by: Rajat Jain <rajatja@xxxxxxxxxx>
> > ---
> > Context:
> >
> >   I set out to implement the approach outlined in
> >     https://lkml.org/lkml/2020/6/9/1331
> >     https://lkml.org/lkml/2020/6/15/1453
> >
> >   But to my surprise, I found that the new hotplugged PCI devices
> >   were getting automatically attached to drivers even though
> >   /sys/bus/pci/drivers_autoprobe was set to 0.
> >
> >   I realized that the device core's "drivers_autoprobe":
> >
> >   * only disables the *initial* probe of the device (i.e. from
> >     device_add()). If a subsystem calls device_attach() explicitly
> >     for its devices like PCI subsystem does, the drivers_autoprobe
> >     setting does not matter. The core will attach device to the driver.
> >     This looks like correct semantic behavior to me because PCI is
> >     explicitly calling device_attach(), which is a way to explicitly
> >     ask the core to find and attach a driver for a device.
> >
> >   * "drivers_autoprobe" cannot be controlled at boot time (to restrict
> >     any drivers before userspace comes up).
> >
> >   The options I considered were:
> >
> >   1) Change device_attach() so that it takes into consideration the
> >      drivers_autoprobe property. Not sure if this is semantically correct
> >      thing to do though. If I do this, then the only way a driver can
> >      be attached to the drivers would be via userspace
> >      (/sys/bus/pci/drivers/bind) (Good for our use case though!).
>
> This is the correct thing to do here, haven't I been asking you do move
> this logic into the driver core so that all busses can use it?

(please see below)

>
> >   2) Make the drivers_autoprobe property available to PCI to use
> >      (currently it is private to device core). The PCI could use this
> >      to determine whether or not to call device_attach(). This still
> >      leaves the other problem (of not being able to set
> >      drivers_autoprobe via command line open).
>
> Ick, command lines are horrible, don't do that if at all possible.  On
> some systems they are not able to be changed which can be good or bad...

(please see below)

>
> >   3) I found the pci_dev->match_driver, which seemed similar to what I
> >      am trying to do, but can't be controlled from userspace. I considered
> >      populating that field based on drivers_autoprobe (still need (2)).
> >      But the problem is that there is the AMD IOMMU driver which is setting
> >      this independently, so setting the match_driver based on
> >      drivers_autoprobe may not be a good idea. May be we can populate it
> >      for untrusted devicesi, based on the parameter that I'm introducing?
> >
> >   4) This patch was my option 4 that helps fix both the problems for me.
>
> I suggest putting some of the above text in the changelog, as it has a
> lot of good context, while your existing changelog is pretty sparse and
> does not explain anything...

Will do.

>
>
> >
> >  drivers/pci/bus.c | 11 ++++++++---
> >  drivers/pci/pci.c |  9 +++++++++
> >  drivers/pci/pci.h |  1 +
> >  3 files changed, 18 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
> > index 3cef835b375fd..336aeeb4c4ebf 100644
> > --- a/drivers/pci/bus.c
> > +++ b/drivers/pci/bus.c
> > @@ -321,9 +321,14 @@ void pci_bus_add_device(struct pci_dev *dev)
> >       pci_bridge_d3_update(dev);
> >
> >       dev->match_driver = true;
> > -     retval = device_attach(&dev->dev);
> > -     if (retval < 0 && retval != -EPROBE_DEFER)
> > -             pci_warn(dev, "device attach failed (%d)\n", retval);
> > +
> > +     if (dev->untrusted && pci_dont_attach_untrusted_devs) {
> > +             pci_info(dev, "not attaching untrusted device\n");
> > +     } else {
> > +             retval = device_attach(&dev->dev);
> > +             if (retval < 0 && retval != -EPROBE_DEFER)
> > +                     pci_warn(dev, "device attach failed (%d)\n", retval);
> > +     }
> >
> >       pci_dev_assign_added(dev, true);
> >  }
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index ce096272f52b1..dec1f9ef27d71 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -127,6 +127,13 @@ static bool pcie_ats_disabled;
> >  /* If set, the PCI config space of each device is printed during boot. */
> >  bool pci_early_dump;
> >
> > +/*
> > + * If set, the devices with "untrusted" flag shall not be attached automatically
> > + * Userspace will need to attach them manually:
> > + * echo <pci device>  > /sys/bus/pci/drivers/<driver>/bind
> > + */
> > +bool pci_dont_attach_untrusted_devs;
> > +
> >  bool pci_ats_disabled(void)
> >  {
> >       return pcie_ats_disabled;
> > @@ -6522,6 +6529,8 @@ static int __init pci_setup(char *str)
> >                               pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
> >                       } else if (!strncmp(str, "disable_acs_redir=", 18)) {
> >                               disable_acs_redir_param = str + 18;
> > +                     } else if (!strcmp(str, "dont_attach_untrusted_devs")) {
> > +                             pci_dont_attach_untrusted_devs = true;
> >                       } else {
> >                               pr_err("PCI: Unknown option `%s'\n", str);
> >                       }
> > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > index 6d3f758671064..30ffad047d926 100644
> > --- a/drivers/pci/pci.h
> > +++ b/drivers/pci/pci.h
> > @@ -13,6 +13,7 @@
> >
> >  extern const unsigned char pcie_link_speed[];
> >  extern bool pci_early_dump;
> > +extern bool pci_dont_attach_untrusted_devs;
> >
> >  bool pcie_cap_has_lnkctl(const struct pci_dev *dev);
> >  bool pcie_cap_has_rtctl(const struct pci_dev *dev);
> > --
> > 2.27.0.212.ge8ba1cc988-goog
> >
>
> What happened to the split of "trust" and "internal/external" logic that
> we discussed before?

a) I think what was decided was introducing a device core "location"
property that can be exposed to userspace to help it to decide whether
or not to attach a driver to a device. Yes, that is still the plan.
(Mild sidenote: userspace may not need to distinguish between internal
and external devices if it can assume that no internal PCI devices
will show up after "echo 0 > /sys/bus/pci/drivers_autoprobe". But
nevertheless...)

b) Note that even with (a) in place, we still need a parameter that
can ensure that drivers are not bound to external devices at boot,
*before* userspace gets a chance to disable "drivers_autoprobe".
https://lkml.org/lkml/2020/6/15/1453
Is it OK to add such a parameter in device core?

Thanks,

Rajat





> This seems to ignore all of that and go straight
> to some form of "we know what we trust, so all is fine!".
>
> It's not obvious what this is really doing here at all, sorry...
>
> greg k-h