[PATCH v4] driver core: avoid klist_remove() on unattached knode_driver

[PATCH v4] driver core: avoid klist_remove() on unattached knode_driver

[Nguyen Quang Le Kien]

usb_driver_claim_interface() sets dev->driver directly and skips
device_bind_driver() when the interface is not yet registered, so the
device can reach teardown with dev->driver set but knode_driver never
added to the driver's klist_devices. __device_release_driver() then
unconditionally calls klist_remove() on the unattached node, which
dereferences a NULL klist pointer in klist_put() and crashes.

Only remove the node if the device is actually bound. Use
device_is_bound() rather than klist_node_attached() directly: the
latter is a raw klist API and does not NULL-check dev->p, while
device_is_bound() is the standard bound-state check used throughout
driver core (driver_bound(), __device_attach()).

Fixes: 94e7b1c5ff20 ("[PATCH] Add a klist to struct device_driver for the devices bound to it.")
Reported-by: syzbot+87188222c77c0dbbdb4d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=87188222c77c0dbbdb4d
Cc: stable@vger.kernel.org
Signed-off-by: Nguyen Quang Le Kien <khiemtranzo532001@gmail.com>
---
Changes in v4:
- add a changelog below the --- line, as required by submitting-patches.rst
- add Cc: stable@vger.kernel.org, Fixes: points to a released kernel

Changes in v3:
- document in the commit message why device_is_bound() is used instead
  of klist_node_attached()
- add Fixes: tag

Changes in v2:
- use device_is_bound() instead of klist_node_attached() as the bound
  check, per review
---
 drivers/base/dd.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 60c005223..4154b4499 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -1354,7 +1354,8 @@ static void __device_release_driver(struct device *dev, struct device *parent)
 		device_unbind_cleanup(dev);
 		device_links_driver_cleanup(dev);
 
-		klist_remove(&dev->p->knode_driver);
+		if (device_is_bound(dev))
+			klist_remove(&dev->p->knode_driver);
 		device_pm_check_callbacks(dev);
 
 		bus_notify(dev, BUS_NOTIFY_UNBOUND_DRIVER);
-- 
2.34.1

Re: [PATCH v4] driver core: avoid klist_remove() on unattached knode_driver

[Greg KH]

On Thu, Aug 20, 2026 at 04:45:57PM +0800, Nguyen Quang Le Kien wrote:
> usb_driver_claim_interface() sets dev->driver directly and skips
> device_bind_driver() when the interface is not yet registered, so the
> device can reach teardown with dev->driver set but knode_driver never
> added to the driver's klist_devices. __device_release_driver() then
> unconditionally calls klist_remove() on the unattached node, which
> dereferences a NULL klist pointer in klist_put() and crashes.
> 
> Only remove the node if the device is actually bound. Use
> device_is_bound() rather than klist_node_attached() directly: the
> latter is a raw klist API and does not NULL-check dev->p, while
> device_is_bound() is the standard bound-state check used throughout
> driver core (driver_bound(), __device_attach()).
> 
> Fixes: 94e7b1c5ff20 ("[PATCH] Add a klist to struct device_driver for the devices bound to it.")
> Reported-by: syzbot+87188222c77c0dbbdb4d@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=87188222c77c0dbbdb4d
> Cc: stable@vger.kernel.org
> Signed-off-by: Nguyen Quang Le Kien <khiemtranzo532001@gmail.com>
> ---
> Changes in v4:
> - add a changelog below the --- line, as required by submitting-patches.rst
> - add Cc: stable@vger.kernel.org, Fixes: points to a released kernel

Please slow down.  As my bot said, there is no rush, no deadline, and
it's the middle of the merge window and we can't even do anything with
this anyway.

At the least, wait a week between patch revisions.  If you wish to see
patches reviewed faster, then help out with actual reviews of patches
from others.  Constantly resending stuff like this doesn't make anyone
want to review this at all.

thanks,

greg k-h

Re: [PATCH v4] driver core: avoid klist_remove() on unattached knode_driver

[Danilo Krummrich]

(Cc: linux-usb)

On Thu Aug 20, 2026 at 10:45 AM CEST, Nguyen Quang Le Kien wrote:
> Fixes: 94e7b1c5ff20 ("[PATCH] Add a klist to struct device_driver for the devices bound to it.")

This is not the correct commit to reference, this commit seems fine.

> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 60c005223..4154b4499 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -1354,7 +1354,8 @@ static void __device_release_driver(struct device *dev, struct device *parent)
>  		device_unbind_cleanup(dev);
>  		device_links_driver_cleanup(dev);
>  
> -		klist_remove(&dev->p->knode_driver);
> +		if (device_is_bound(dev))
> +			klist_remove(&dev->p->knode_driver);

This looks like band-aid for the underlying design tension in the USB core
(which we should address instead) and does not belong in the driver core.

It's not visible from the above diff, but with this patch the control flow
becomes:

	if (dev->driver) {
		if (device_is_bound(dev))
			klist_remove(&dev->p->knode_driver);
		...
	}

but dev->driver already indicates that the device is bound to dev->driver in
this context.

The reason we "need" this check regardless is that usb_driver_claim_interface()
(ab)uses dev->driver to indicate that a certain USB driver claimed, or rather
reserved, this device.

There are two cases in usb_driver_claim_interface():

  (1) The device to claim is already registered with the driver core, in which
      case device_bind_driver(dev) is called and dev->driver is correctly set
      during the bind attempt.

  (2) The device to claim was not yet registered with the driver core. This can
      happen when the USB interface the driver actually binds to is registered
      (and hence probed) before the additional interface the driver wants to
      claim is registered. The USB core sets dev->driver independent of the bind
      state to indicate it has reserved the interface.

The first case is perfectly fine, but the issue with the second case is that now
dev->driver is semantically overloaded:

The USB core treats it as "dev is reserved for dev->driver" and the driver core
treats it as "dev is currently binding or bound to dev->driver", but that's not
actually the case yet, since device_bind_driver() hasn't been called yet.

IOW, dev->driver should only be set under the device lock before calling
device_bind_driver(), and, in case of failure, should be cleared after
device_bind_driver() with the device lock still held.

Besides the reported crash, another implication of this is that all other
functions from device_release_driver() are called as well, even though
device_bind_driver() was never called before, and there is no guarantee that
this does not cause other unexpected side effects already or in the future.

I think one solution could be to add a new claimed field to struct usb_interface
to indicate that the interface is reserved for a certain driver and make
usb_device_match() reject the device if ever probed otherwise.

Another solution (but that's a bit more work) would be to separate interface
registration from interface probing in usb_set_configuration(). Of course that
needs help from the driver core as well, but it would also get us rid of the
slightly odd situation that a driver may operate a claimed device already, even
though it is not yet registered with the driver core.

I'd suggest going for a claimed field in struct usb_interface first to fix the
immediate problem and then take it from there.

That said, I wonder if there's more to think about with the
usb_driver_claim_interface() / usb_driver_release_interface() API.

After having a brief look it seems that drivers have invented various different
approaches to protect against the case where userspace could write the claimed
interface's name to:

	/sys/bus/usb/drivers/<driver>/unbind

I think this could be much cleaner if the driver core would support "claimed"
devices" natively. OTH, there's only ~20 drivers across USB and PnP though, so
probably not worth.

Re: [PATCH v4] driver core: avoid klist_remove() on unattached knode_driver

[Alan Stern]

On Thu, Aug 20, 2026 at 06:14:37PM +0200, Danilo Krummrich wrote:
> (Cc: linux-usb)
> 
> On Thu Aug 20, 2026 at 10:45 AM CEST, Nguyen Quang Le Kien wrote:
> > Fixes: 94e7b1c5ff20 ("[PATCH] Add a klist to struct device_driver for the devices bound to it.")
> 
> This is not the correct commit to reference, this commit seems fine.
> 
> > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> > index 60c005223..4154b4499 100644
> > --- a/drivers/base/dd.c
> > +++ b/drivers/base/dd.c
> > @@ -1354,7 +1354,8 @@ static void __device_release_driver(struct device *dev, struct device *parent)
> >  		device_unbind_cleanup(dev);
> >  		device_links_driver_cleanup(dev);
> >  
> > -		klist_remove(&dev->p->knode_driver);
> > +		if (device_is_bound(dev))
> > +			klist_remove(&dev->p->knode_driver);
> 
> This looks like band-aid for the underlying design tension in the USB core
> (which we should address instead) and does not belong in the driver core.
> 
> It's not visible from the above diff, but with this patch the control flow
> becomes:
> 
> 	if (dev->driver) {
> 		if (device_is_bound(dev))
> 			klist_remove(&dev->p->knode_driver);
> 		...
> 	}
> 
> but dev->driver already indicates that the device is bound to dev->driver in
> this context.
> 
> The reason we "need" this check regardless is that usb_driver_claim_interface()
> (ab)uses dev->driver to indicate that a certain USB driver claimed, or rather
> reserved, this device.
> 
> There are two cases in usb_driver_claim_interface():
> 
>   (1) The device to claim is already registered with the driver core, in which
>       case device_bind_driver(dev) is called and dev->driver is correctly set
>       during the bind attempt.
> 
>   (2) The device to claim was not yet registered with the driver core. This can
>       happen when the USB interface the driver actually binds to is registered
>       (and hence probed) before the additional interface the driver wants to
>       claim is registered. The USB core sets dev->driver independent of the bind
>       state to indicate it has reserved the interface.
> 
> The first case is perfectly fine, but the issue with the second case is that now
> dev->driver is semantically overloaded:
> 
> The USB core treats it as "dev is reserved for dev->driver" and the driver core
> treats it as "dev is currently binding or bound to dev->driver", but that's not
> actually the case yet, since device_bind_driver() hasn't been called yet.
> 
> IOW, dev->driver should only be set under the device lock before calling
> device_bind_driver(), and, in case of failure, should be cleared after
> device_bind_driver() with the device lock still held.
> 
> Besides the reported crash, another implication of this is that all other
> functions from device_release_driver() are called as well, even though
> device_bind_driver() was never called before, and there is no guarantee that
> this does not cause other unexpected side effects already or in the future.
> 
> I think one solution could be to add a new claimed field to struct usb_interface
> to indicate that the interface is reserved for a certain driver and make
> usb_device_match() reject the device if ever probed otherwise.
> 
> Another solution (but that's a bit more work) would be to separate interface
> registration from interface probing in usb_set_configuration(). Of course that
> needs help from the driver core as well, but it would also get us rid of the
> slightly odd situation that a driver may operate a claimed device already, even
> though it is not yet registered with the driver core.
> 
> I'd suggest going for a claimed field in struct usb_interface first to fix the
> immediate problem and then take it from there.
> 
> That said, I wonder if there's more to think about with the
> usb_driver_claim_interface() / usb_driver_release_interface() API.
> 
> After having a brief look it seems that drivers have invented various different
> approaches to protect against the case where userspace could write the claimed
> interface's name to:
> 
> 	/sys/bus/usb/drivers/<driver>/unbind
> 
> I think this could be much cleaner if the driver core would support "claimed"
> devices" natively. OTH, there's only ~20 drivers across USB and PnP though, so
> probably not worth.

I don't object to the idea of adding a "claimed" field to usb_interface.  

However, isn't it true that the driver core has always supported the 
idea of a driver being associated with a device before binding?  In 
particular, __device_attach() specifically checks for dev->driver being 
already set.  If it is, the match and probe steps are skipped.

Are you saying that this code path should be removed as well?

As it is, the driver core does sort of support "claimed devices".  
Namely, a device is claimed (or probing) if dev->driver is set and 
device_is_bound(dev) is false.

Alan Stern

Re: [PATCH v4] driver core: avoid klist_remove() on unattached knode_driver

[Danilo Krummrich]

On Thu Aug 20, 2026 at 7:22 PM CEST, Alan Stern wrote:
> On Thu, Aug 20, 2026 at 06:14:37PM +0200, Danilo Krummrich wrote:
>> (Cc: linux-usb)
>> 
>> On Thu Aug 20, 2026 at 10:45 AM CEST, Nguyen Quang Le Kien wrote:
>> > Fixes: 94e7b1c5ff20 ("[PATCH] Add a klist to struct device_driver for the devices bound to it.")
>> 
>> This is not the correct commit to reference, this commit seems fine.
>> 
>> > diff --git a/drivers/base/dd.c b/drivers/base/dd.c
>> > index 60c005223..4154b4499 100644
>> > --- a/drivers/base/dd.c
>> > +++ b/drivers/base/dd.c
>> > @@ -1354,7 +1354,8 @@ static void __device_release_driver(struct device *dev, struct device *parent)
>> >  		device_unbind_cleanup(dev);
>> >  		device_links_driver_cleanup(dev);
>> >  
>> > -		klist_remove(&dev->p->knode_driver);
>> > +		if (device_is_bound(dev))
>> > +			klist_remove(&dev->p->knode_driver);
>> 
>> This looks like band-aid for the underlying design tension in the USB core
>> (which we should address instead) and does not belong in the driver core.
>> 
>> It's not visible from the above diff, but with this patch the control flow
>> becomes:
>> 
>> 	if (dev->driver) {
>> 		if (device_is_bound(dev))
>> 			klist_remove(&dev->p->knode_driver);
>> 		...
>> 	}
>> 
>> but dev->driver already indicates that the device is bound to dev->driver in
>> this context.
>> 
>> The reason we "need" this check regardless is that usb_driver_claim_interface()
>> (ab)uses dev->driver to indicate that a certain USB driver claimed, or rather
>> reserved, this device.
>> 
>> There are two cases in usb_driver_claim_interface():
>> 
>>   (1) The device to claim is already registered with the driver core, in which
>>       case device_bind_driver(dev) is called and dev->driver is correctly set
>>       during the bind attempt.
>> 
>>   (2) The device to claim was not yet registered with the driver core. This can
>>       happen when the USB interface the driver actually binds to is registered
>>       (and hence probed) before the additional interface the driver wants to
>>       claim is registered. The USB core sets dev->driver independent of the bind
>>       state to indicate it has reserved the interface.
>> 
>> The first case is perfectly fine, but the issue with the second case is that now
>> dev->driver is semantically overloaded:
>> 
>> The USB core treats it as "dev is reserved for dev->driver" and the driver core
>> treats it as "dev is currently binding or bound to dev->driver", but that's not
>> actually the case yet, since device_bind_driver() hasn't been called yet.
>> 
>> IOW, dev->driver should only be set under the device lock before calling
>> device_bind_driver(), and, in case of failure, should be cleared after
>> device_bind_driver() with the device lock still held.
>> 
>> Besides the reported crash, another implication of this is that all other
>> functions from device_release_driver() are called as well, even though
>> device_bind_driver() was never called before, and there is no guarantee that
>> this does not cause other unexpected side effects already or in the future.
>> 
>> I think one solution could be to add a new claimed field to struct usb_interface
>> to indicate that the interface is reserved for a certain driver and make
>> usb_device_match() reject the device if ever probed otherwise.
>> 
>> Another solution (but that's a bit more work) would be to separate interface
>> registration from interface probing in usb_set_configuration(). Of course that
>> needs help from the driver core as well, but it would also get us rid of the
>> slightly odd situation that a driver may operate a claimed device already, even
>> though it is not yet registered with the driver core.
>> 
>> I'd suggest going for a claimed field in struct usb_interface first to fix the
>> immediate problem and then take it from there.
>> 
>> That said, I wonder if there's more to think about with the
>> usb_driver_claim_interface() / usb_driver_release_interface() API.
>> 
>> After having a brief look it seems that drivers have invented various different
>> approaches to protect against the case where userspace could write the claimed
>> interface's name to:
>> 
>> 	/sys/bus/usb/drivers/<driver>/unbind
>> 
>> I think this could be much cleaner if the driver core would support "claimed"
>> devices" natively. OTH, there's only ~20 drivers across USB and PnP though, so
>> probably not worth.
>
> I don't object to the idea of adding a "claimed" field to usb_interface.  
>
> However, isn't it true that the driver core has always supported the 
> idea of a driver being associated with a device before binding?  In 
> particular, __device_attach() specifically checks for dev->driver being 
> already set.  If it is, the match and probe steps are skipped.

Ah, I also wanted to add a comment about this, but forgot about it, sorry.

This was added ~25 years ago to be able to hardwire a system device to a
specific driver, which less than a year after was replaced entirely by something
else and later converted back to a bus with proper match() logic. But the
dev->driver special case in __device_attach() was never removed.

So, I guess it is fair to say it was always there, but the driver core does not
really handle dev->driver being set "randomly" outside of the bind/unbind
lifecycle in general. It is expected that it is only set when the driver is
actually bound (or binding).

One example is __device_release_driver(), if device_bind_driver() (or
really_probe()) wasn't called, we should also not land within the
if (dev->driver) conditional of __device_release_driver().

Also note that usb_driver_claim_interface() is the only callsite in the kernel
that does set dev->driver without a subsequent device_bind_driver() while
holding the device lock.

(Well, actually, after a thorough check, there's w1, which hardwires the
w1_slave_driver, where it should actually use a trivial match() callback.)

In general, matching should be done through the match() callback.

> Are you saying that this code path should be removed as well?

Yes, I think we should remove it. It's a rather error prone special case that
can easily be handled with normal match() logic.

Re: [PATCH v4] driver core: avoid klist_remove() on unattached knode_driver

[Danilo Krummrich]

On Mon Aug 24, 2026 at 6:47 AM CEST, Akihiko Kai wrote:
> On Fri, Aug 21, 2026 at 03:54:44AM +0200, Danilo Krummrich wrote:
>> usb_driver_claim_interface() is the only callsite in the kernel that
>> does set dev->driver without a subsequent device_bind_driver().
>
> Small correction, I think there are a few more. usb_port
> (drivers/usb/core/port.c:782) sets dev->driver before device_register(),
> and usb_port_driver has no ->match and no ->probe, so it depends entirely
> on the __device_attach() dev->driver path to get bound. ccwgroup
> (drivers/s390/cio/ccwgroup.c:385) and pata_parport
> (drivers/ata/pata_parport/pata_parport.c:515) do the same. So that path
> doesn't look USB-claim-specific, and if we ever want to remove it,
> usb_port would need a ->match() first.

Good catch, I think those should be fixed as well.

I had a brief look at the usb_port one and it should be exactly the same case as
w1, which I already fixed [1]. Actually, it is even simpler, usb_device_match()
can just handle the usb_port_device_type, as the USB core already does for other
device types. I can send a patch for this later on.

[1] https://lore.kernel.org/driver-core/20260820225430.1847356-1-dakr@kernel.org/

> For the root cause, what about this: in usb_driver_claim_interface(),
> when the interface isn't registered yet, just don't set dev->driver --
> record the claim on struct usb_interface instead. Then, once the
> interface gets added in usb_set_configuration(), bind it explicitly with
> device_bind_driver(). That skips ->probe, which is exactly what
> usb_audio's claim-now-bind-later needs, and it also attaches
> knode_driver so teardown is safe. The __device_attach() path can stay
> for w1/ccwgroup/pata_parport.
>
> Does that sound reasonable?

I'd prefer to get rid of the dev->driver path in __device_attach(), I think
there's no reason not to use the common match() path instead.

So, for a fix that also goes into stable trees what you propose sounds
reasonable, but eventually we should use the match() infrastructure instead.

Re: [PATCH v4] driver core: avoid klist_remove() on unattached knode_driver

[Nguyen Quang Le Kien]

On Tue, Aug 26, 2026 at 09:14:00PM +0000, Danilo Krummrich wrote:
  > So, for a fix that also goes into stable trees what you propose sounds
  > reasonable, but eventually we should use the match() infrastructure instead.

  I've implemented the approach you described:

    - In usb_driver_claim_interface(), when !device_is_registered(dev),
      skip setting dev->driver. Instead set two new fields on struct
      usb_interface: claimed:1 and claimed_driver.

    - In usb_set_configuration(), after device_add() succeeds, check
      intf->claimed and call device_bind_driver() explicitly. This skips
      ->probe() and attaches knode_driver to the driver's
      klist, making teardown safe.

    - The __device_attach() dev->driver path in dd.c is left untouched
      for w1 / ccwgroup / pata_parport.

  syzbot confirmed the fix; the reproducer no longer triggers:

    Tested-by: syzbot+87188222c77c0dbbdb4d@syzkaller.appspotmail.com
    Patch:      https://syzkaller.appspot.com/x/patch.diff?x=12af8b49580000
    Dashboard:  https://syzkaller.appspot.com/bug?extid=87188222c77c0dbbdb4d

  Will send v5 shortly with the syzbot Tested-by added and the subject
  retitled to "usb: core: ..." since the change is scoped to USB only.

  Could I get a Reviewed-by from you?

  Thanks,
  Kiennql

Re: [PATCH v4] driver core: avoid klist_remove() on unattached knode_driver

[Nguyen Quang Le Kien]

In-Reply-To: <DKU2J83B6IX6.2CG1WOB66WC39@kernel.org>
References: <DKU2J83B6IX6.2CG1WOB66WC39@kernel.org>

On Fri, Aug 21, 2026 at 03:54:44AM +0200, Danilo Krummrich wrote:
> usb_driver_claim_interface() is the only callsite in the kernel that
> does set dev->driver without a subsequent device_bind_driver().

Small correction, I think there are a few more. usb_port
(drivers/usb/core/port.c:782) sets dev->driver before device_register(),
and usb_port_driver has no ->match and no ->probe, so it depends entirely
on the __device_attach() dev->driver path to get bound. ccwgroup
(drivers/s390/cio/ccwgroup.c:385) and pata_parport
(drivers/ata/pata_parport/pata_parport.c:515) do the same. So that path
doesn't look USB-claim-specific, and if we ever want to remove it,
usb_port would need a ->match() first.

For the root cause, what about this: in usb_driver_claim_interface(),
when the interface isn't registered yet, just don't set dev->driver --
record the claim on struct usb_interface instead. Then, once the
interface gets added in usb_set_configuration(), bind it explicitly with
device_bind_driver(). That skips ->probe, which is exactly what
usb_audio's claim-now-bind-later needs, and it also attaches
knode_driver so teardown is safe. The __device_attach() path can stay
for w1/ccwgroup/pata_parport.

Does that sound reasonable?

Re: [PATCH v4] driver core: avoid klist_remove() on unattached knode_driver

[Nguyen Quang Le Kien]

In-Reply-To: <DKU2J83B6IX6.2CG1WOB66WC39@kernel.org>
References: <DKU2J83B6IX6.2CG1WOB66WC39@kernel.org>

On Fri, Aug 21, 2026 at 03:54:44AM +0200, Danilo Krummrich wrote:
> usb_driver_claim_interface() is the only callsite in the kernel that
> does set dev->driver without a subsequent device_bind_driver().

Small correction, I think there are a few more. usb_port
(drivers/usb/core/port.c:782) sets dev->driver before device_register(),
and usb_port_driver has no ->match and no ->probe, so it depends entirely
on the __device_attach() dev->driver path to get bound. ccwgroup
(drivers/s390/cio/ccwgroup.c:385) and pata_parport
(drivers/ata/pata_parport/pata_parport.c:515) do the same. So that path
doesn't look USB-claim-specific, and if we ever want to remove it,
usb_port would need a ->match() first.

For the root cause, what about this: in usb_driver_claim_interface(),
when the interface isn't registered yet, just don't set dev->driver --
record the claim on struct usb_interface instead. Then, once the
interface gets added in usb_set_configuration(), bind it explicitly with
device_bind_driver(). That skips ->probe, which is exactly what
usb_audio's claim-now-bind-later needs, and it also attaches
knode_driver so teardown is safe. The __device_attach() path can stay
for w1/ccwgroup/pata_parport.

Does that sound reasonable?