drivers: core: synchronize really_probe() and dev_uevent()

Synchronize the dev->driver usage in really_probe() and dev_uevent().
These can run in different threads, what can result in the following
race condition for dev->driver uninitialization:

Thread #1:
==========

really_probe() {
...
probe_failed:
...
device_unbind_cleanup(dev) {
    ...
    dev->driver = NULL;   // <= Failed probe sets dev->driver to NULL
    ...
    }
...
}

Thread Rust-for-Linux#2:
==========

dev_uevent() {
...
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);
...
}

really_probe() holds the lock, already. So nothing needs to be done
there. dev_uevent() is called with lock held, often, too. But not
always. What implies that we can't add any locking in dev_uevent()
itself. So fix this race by adding the lock to the non-protected
path. This is the path where above race is observed:

 dev_uevent+0x235/0x380
 uevent_show+0x10c/0x1f0  <= Add lock here
 dev_attr_show+0x3a/0xa0
 sysfs_kf_seq_show+0x17c/0x250
 kernfs_seq_show+0x7c/0x90
 seq_read_iter+0x2d7/0x940
 kernfs_fop_read_iter+0xc6/0x310
 vfs_read+0x5bc/0x6b0
 ksys_read+0xeb/0x1b0
 __x64_sys_read+0x42/0x50
 x64_sys_call+0x27ad/0x2d30
 do_syscall_64+0xcd/0x1d0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

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).

The same issue was reported and tried to be fixed back in 2015 in

https://lore.kernel.org/lkml/[email protected]/

already.

Fixes: 239378f ("Driver core: add uevent vars for devices of a class")
Cc: stable <[email protected]>
Cc: [email protected]
Cc: Ashish Sangwan <[email protected]>
Cc: Namjae Jeon <[email protected]>
Signed-off-by: Dirk Behme <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/base/core.c

@@ -2739,8 +2739,11 @@ static ssize_t uevent_show(struct device *dev, struct device_attribute *attr,
 	if (!env)
 		return -ENOMEM;
 
+	/* Synchronize with really_probe() */
+	device_lock(dev);
 	/* let the kset specific function add its keys */
 	retval = kset->uevent_ops->uevent(&dev->kobj, env);
+	device_unlock(dev);
 	if (retval)
 		goto out;