Re: [PATCH] drivers: core: Make dev->driver usage safe in dev_uevent()

[Greg Kroah-Hartman]

On Tue, Apr 30, 2024 at 10:23:36AM +0200, Dirk Behme wrote:
> Hi Greg,
> 
> On 30.04.2024 09:20, Greg Kroah-Hartman wrote:
> > On Tue, Apr 30, 2024 at 06:55:31AM +0200, Dirk Behme wrote:
> > > Inspired by the function dev_driver_string() in the same file make sure
> > > in case of uninitialization dev->driver is used safely in dev_uevent(),
> > > as well.
> > 
> > I think you are racing and just getting "lucky" with your change here,
> > just like dev_driver_string() is doing there (that READ_ONCE() really
> > isn't doing much to protect you...)
> > 
> > > This change is based on the observation of the following race condition:
> > > 
> > > Thread #1:
> > > ==========
> > > 
> > > really_probe() {
> > > ...
> > > probe_failed:
> > > ...
> > > device_unbind_cleanup(dev) {
> > >        ...
> > >        dev->driver = NULL;   // <= Failed probe sets dev->driver to NULL
> > >        ...
> > >        }
> > > ...
> > > }
> > > 
> > > Thread #2:
> > > ==========
> > > 
> > > dev_uevent() {
> > 
> > Wait, how can dev_uevent() be called if probe fails?  Who is calling
> > that?
> > 
> > > ...
> > > if (dev->driver)
> > >        // If dev->driver is NULLed from really_probe() from here on,
> > >        // after above check, the system crashes
> > >        add_uevent_var(env, "DRIVER=%s", dev->driver->name);
> > > 
> > > dev_driver_string() can't be used here because we want NULL and not
> > > anything else in the non-init case.
> > > 
> > > Similar cases are reported by syzkaller in
> > > 
> > > https://syzkaller.appspot.com/bug?extid=ffa8143439596313a85a
> > > 
> > > But these are regarding the *initialization* of dev->driver
> > > 
> > > dev->driver = drv;
> > > 
> > > As this switches dev->driver to non-NULL these reports can be considered
> > > to be false-positives (which should be "fixed" by this commit, as well,
> > > though).
> > > 
> > > Fixes: 239378f16aa1 ("Driver core: add uevent vars for devices of a class")
> > > Cc: syzbot+ffa8143439596313a85a@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > Reviewed-by: Eugeniu Rosca <eugeniu.rosca@xxxxxxxxx>
> > > Tested-by: Eugeniu Rosca <eugeniu.rosca@xxxxxxxxx>
> > > Signed-off-by: Dirk Behme <dirk.behme@xxxxxxxxxxxx>
> > > ---
> > >   drivers/base/core.c | 9 +++++++--
> > >   1 file changed, 7 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/base/core.c b/drivers/base/core.c
> > > index 5f4e03336e68..99ead727c08f 100644
> > > --- a/drivers/base/core.c
> > > +++ b/drivers/base/core.c
> > > @@ -2639,6 +2639,7 @@ static const char *dev_uevent_name(const struct kobject *kobj)
> > >   static int dev_uevent(const struct kobject *kobj, struct kobj_uevent_env *env)
> > >   {
> > >   	const struct device *dev = kobj_to_dev(kobj);
> > > +	struct device_driver *drv;
> > >   	int retval = 0;
> > >   	/* add device node properties if present */
> > > @@ -2667,8 +2668,12 @@ static int dev_uevent(const struct kobject *kobj, struct kobj_uevent_env *env)
> > >   	if (dev->type && dev->type->name)
> > >   		add_uevent_var(env, "DEVTYPE=%s", dev->type->name);
> > > -	if (dev->driver)
> > > -		add_uevent_var(env, "DRIVER=%s", dev->driver->name);
> > > +	/* dev->driver can change to NULL underneath us because of unbinding
> > > +	 * or failing probe(), so be careful about accessing it.
> > > +	 */
> > > +	drv = READ_ONCE(dev->driver);
> > > +	if (drv)
> > > +		add_uevent_var(env, "DRIVER=%s", drv->name);
> > 
> > Again, you are just reducing the window here.  Maybe a bit, but not all
> > that much overall as there is no real lock present.
> > 
> > So how is this actually solving anything?
> 
> 
> Looking at dev_driver_string() I hoped that it just reads *once*. I.e. we
> don't care if we read NULL or any valid pointer, as long as this pointer
> read is done only once ("atomically"?). If READ_ONCE() doesn't do that, I
> agree, it's not the (race) fix we are looking for.

Yes, what if you read it, and then it is unloaded from the system before
you attempt to access drv->name?  not good.

> Initially, I was thinking about anything like [1] below. I.e. adding a mutex
> lock.  But not sure if that is better (acceptable?).

a proper lock is the only way to correctly solve this.

thanks,

greg k-h