Re: [PATCH v4 0/3] driver core: add TAINT_FORCED_BIND for when userspace manually messes with devices and drivers

[Michal Pecio]

On Mon, 26 Oct 2026 21:34:57 +0100, Greg Kroah-Hartman wrote:
> > Even if we could come up with a set of device IDs, it's a policy
> > decision whether a given device is intended by the admin to be used
> > by the default in-kernel driver or a userspace driver.  
> 
> Great, then have the admin never allow the in-kernel driver to be on
> the system, or be loaded (modprobe has deny lists) and all is fine.

As some maintainers like to say, it's not the '90s anymore :)

Users don't reboot systems to kick a kernel driver off a PCI device to
try something in a disposable VM on a whim.

Even if they did, it's completely normal for desktop users to have two
GPUs from the same vendor or two xHCI controllers and pass only one of
them to Windows/macOS while using the other on the host.

bind/unbind feature prominently on every distribution wiki and forum,
this cat is absolutely out of the bag at this point.

> > That means that at one point we want the default driver bound and at
> > another we want a vfio driver bound.  Coordinating this is exactly
> > why we have driver_override and, building on that, match IDs that
> > require driver_override.  This allows drivers to describe
> > themselves as vfio-pci compatible drivers via modules.alias while
> > requiring userspace to enable the match via driver_override.  
> 
> And as said before, that's a horrid hack and one that is breaking
> things today in all sorts of ways, which is why the taint was added.

Are you aware of bind/unbind breaking things for PCI specifically?

Without also involving 'driver_override' or 'new_id'? Everyone agrees
that these APIs used for anything other than pci-stub or vfio are foot
guns. But your patch targets something different.

We can even bind PCI drivers to wrong devices *without* using bind and
hence, I presume but don't care to test, without tripping this taint.

> If you wish to override the kernel, then great, a taint flag is added
> and all is fine.  All this means is that now developers know what you
> did and can decide to ignore your bug report or not.  No functionality
> changed at all.

I believe people pointed out that it's a functional change for users of
panic_on_taint who also use bind without being idiots.

Does this taint even solve a "real" problem? Those bug reports weren't
coming from users, they were coming from "abusers" like syzbot looking
for trouble. They were quite obvious even without a taint message.

And at least on linux-usb, they seem to have stopped after Google and
one other copycat were told to stop sending them, and before this taint
reaches released kernels.

I even told them to block not only bind but also new_id, so there will
be no second wave of reports caused by syzbot discovering that it can
bypass the taint by using new_id intsead of driver_override.

> Why are you worried about a taint flag?  So many servers run with
> taint flags enabled due to out-of-tree or questionable licenses with
> no problems, what's one more informational bit here?

And what's the point of a flag which is always set on countless normal
systems in the wild? If this feature survives to April 1st, I will send
a patch to rename it to "VM passthrough taint" because everybody (or at
least those dealing with desktop users) knows what it *really* means.

Regards,
Michal