Re: [PATCH] driver core: fix use-after-free of driver_override via driver_match_device()

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Nov 26, 2025 at 07:55:06PM +0800, Gui-Dong Han wrote:
> On Wed, Nov 26, 2025 at 7:39 PM Greg KH <gregkh@linuxfoundation.org> wrote:
> >
> > On Wed, Nov 26, 2025 at 07:30:06PM +0800, Gui-Dong Han wrote:
> > > driver_set_override() modifies and frees dev->driver_override while
> > > holding device_lock(dev). However, driver_match_device() reads
> > > dev->driver_override when calling bus match functions.
> > >
> > > Currently, driver_match_device() is called from three sites. One site
> > > (__device_attach_driver) holds device_lock(dev), but the other two
> > > (bind_store and __driver_attach) do not. This allows a concurrent
> > > driver_set_override() to free the string while driver_match_device() is
> > > using it, leading to a use-after-free (UAF).
> > >
> > > This issue affects at least 11 bus types (including PCI, AMBA, Platform)
> > > that rely on driver_override for matching.
> > >
> > > Fix this by holding device_lock(dev) around the driver_match_device() calls
> > > in bind_store() and __driver_attach(). This ensures all access to
> > > dev->driver_override via driver_match_device() is protected by the device
> > > lock.
> > >
> > > Tested with the PoCs from Bugzilla that trigger this UAF. Stress testing
> > > the two newly locked paths for 24 hours with CONFIG_PROVE_LOCKING and
> > > CONFIG_LOCKDEP enabled showed no UAF recurrence and no lockdep
> > > warnings.
> > >
> > > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220789
> > > Suggested-by: Qiu-ji Chen <chenqiuji666@gmail.com>
> > > Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
> > > ---
> > > The Bugzilla entry contains full KASAN reports and two PoCs that reliably
> > > reproduce the UAF on both unlocked paths using a standard QEMU setup
> > > (default e1000 device at 0000:00:03.0).
> > > I chose to fix this in the driver core for the following reasons:
> > > 1. Both racing functions are part of the driver core.
> > > 2. Fixing this per-driver/per-bus is tedious and would require careful
> > > ad-hoc locking that does not align with the existing device_lock(dev).
> > > 3. We cannot simply add device_lock(dev) inside bus match functions because
> > > one call path (__device_attach_driver) already holds this lock. Adding the
> > > lock inside the match callback would cause a deadlock on that path.
> > > ---
> > >  drivers/base/bus.c | 17 ++++++++++++-----
> > >  drivers/base/dd.c  |  3 +++
> > >  2 files changed, 15 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/base/bus.c b/drivers/base/bus.c
> > > index 5e75e1bce551..9e62d6009058 100644
> > > --- a/drivers/base/bus.c
> > > +++ b/drivers/base/bus.c
> > > @@ -261,13 +261,20 @@ static ssize_t bind_store(struct device_driver *drv, const char *buf,
> > >       const struct bus_type *bus = bus_get(drv->bus);
> > >       struct device *dev;
> > >       int err = -ENODEV;
> > > +     int ret;
> > >
> > >       dev = bus_find_device_by_name(bus, NULL, buf);
> > > -     if (dev && driver_match_device(drv, dev)) {
> > > -             err = device_driver_attach(drv, dev);
> > > -             if (!err) {
> > > -                     /* success */
> > > -                     err = count;
> > > +     if (dev) {
> > > +             /* Protects against driver_set_override() races */
> > > +             device_lock(dev);
> > > +             ret = driver_match_device(drv, dev);
> > > +             device_unlock(dev);
> >
> > Why not have driver_match_device() take the lock instead?  This way
> > looks like an "anti-pattern" that we will get wrong over time.
> 
> The reason I did not put the lock inside driver_match_device() is that
> one of its existing callers, __device_attach_driver(), already holds
> device_lock(dev). Unconditionally adding the lock inside
> driver_match_device() would cause a deadlock on that path.

Ok, then we should add a lockdep check that this specific lock is
grabbed in that function so that we don't forget that this is a
requirement now.

> To address this and move the locking inside as you suggested, I would
> need to modify the signature of driver_match_device() to accept a
> flag, for example:
> int driver_match_device(struct device_driver *drv, struct device *dev,
> bool locked)

No, that way lies madness, never do that :)

thanks,

greg k-h