Messages in this thread | | | Date | Tue, 30 Apr 2024 10:41:57 +0200 | From | Greg Kroah-Hartman <> | Subject | Re: [PATCH] drivers: core: Make dev->driver usage safe in dev_uevent() |
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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@syzkaller.appspotmail.com > > > Reviewed-by: Eugeniu Rosca <eugeniu.rosca@bosch.com> > > > Tested-by: Eugeniu Rosca <eugeniu.rosca@bosch.com> > > > Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com> > > > --- > > > 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
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