pps: Fix a use-after-free

On a board running ntpd and gpsd, I'm seeing a consistent use-after-free
in sys_exit() from gpsd when rebooting:

pps pps1: removed
------------[ cut here ]------------
kobject: '(null)' (00000000db4bec24): is not initialized, yet kobject_put() is being called.
WARNING: CPU: 2 PID: 440 at lib/kobject.c:734 kobject_put+0x120/0x150
CPU: 2 UID: 299 PID: 440 Comm: gpsd Not tainted 6.11.0-rc6-00308-gb31c44928842 #1
Hardware name: Raspberry Pi 4 Model B Rev 1.1 (DT)
pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : kobject_put+0x120/0x150
lr : kobject_put+0x120/0x150
sp : ffffffc0803d3ae0
x29: ffffffc0803d3ae0 x28: ffffff8042dc9738 x27: 0000000000000001
x26: 0000000000000000 x25: ffffff8042dc9040 x24: ffffff8042dc9440
x23: ffffff80402a4620 x22: ffffff8042ef4bd0 x21: ffffff80405cb600
x20: 000000000008001b x19: ffffff8040b3b6e0 x18: 0000000000000000
x17: 0000000000000000 x16: 0000000000000000 x15: 696e6920746f6e20
x14: 7369203a29343263 x13: 205d303434542020 x12: 0000000000000000
x11: 0000000000000000 x10: 0000000000000000 x9 : 0000000000000000
x8 : 0000000000000000 x7 : 0000000000000000 x6 : 0000000000000000
x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
x2 : 0000000000000000 x1 : 0000000000000000 x0 : 0000000000000000
Call trace:
kobject_put+0x120/0x150
cdev_put+0x20/0x3c
__fput+0x2c4/0x2d8
____fput+0x1c/0x38
task_work_run+0x70/0xfc
do_exit+0x2a0/0x924
do_group_exit+0x34/0x90
get_signal+0x7fc/0x8c0
do_signal+0x128/0x13b4
do_notify_resume+0xdc/0x160
el0_svc+0xd4/0xf8
el0t_64_sync_handler+0x140/0x14c
el0t_64_sync+0x190/0x194
---[ end trace 0000000000000000 ]---

...followed by more symptoms of corruption, with similar stacks:

refcount_t: underflow; use-after-free.
kernel BUG at lib/list_debug.c:62!
Kernel panic - not syncing: Oops - BUG: Fatal exception

This happens because pps_device_destruct() frees the pps_device with the
embedded cdev immediately after calling cdev_del(), but, as the comment
above cdev_del() notes, fops for previously opened cdevs are still
callable even after cdev_del() returns. I think this bug has always
been there: I can't explain why it suddenly started happening every time
I reboot this particular board.

In commit d953e0e837e6 ("pps: Fix a use-after free bug when
unregistering a source."), George Spelvin suggested removing the
embedded cdev. That seems like the simplest way to fix this, so I've
implemented his suggestion, using __register_chrdev() with pps_idr
becoming the source of truth for which minor corresponds to which
device.

But now that pps_idr defines userspace visibility instead of cdev_add(),
we need to be sure the pps->dev refcount can't reach zero while
userspace can still find it again. So, the idr_remove() call moves to
pps_unregister_cdev(), and pps_idr now holds a reference to pps->dev.

pps_core: source serial1 got cdev (251:1)
<...>
pps pps1: removed
pps_core: unregistering pps1
pps_core: deallocating pps1

Fixes: d953e0e837e6 ("pps: Fix a use-after free bug when unregistering a source.")
Cc: [e-mail]
Signed-off-by: Calvin Owens <[e-mail]>
Reviewed-by: Michal Schmidt <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/pps/clients/pps-gpio.c

···
 		return -EINVAL;
 	}
 
-	dev_info(data->pps->dev, "Registered IRQ %d as PPS source\n",
-		 data->irq);
+	dev_dbg(&data->pps->dev, "Registered IRQ %d as PPS source\n",
+		data->irq);
 
 	return 0;
 }

drivers/pps/clients/pps-ktimer.c

···
 
 static void __exit pps_ktimer_exit(void)
 {
-	dev_info(pps->dev, "ktimer PPS source unregistered\n");
+	dev_dbg(&pps->dev, "ktimer PPS source unregistered\n");
 
 	del_timer_sync(&ktimer);
 	pps_unregister_source(pps);
···
 	timer_setup(&ktimer, pps_ktimer_event, 0);
 	mod_timer(&ktimer, jiffies + HZ);
 
-	dev_info(pps->dev, "ktimer PPS source registered\n");
+	dev_dbg(&pps->dev, "ktimer PPS source registered\n");
 
 	return 0;
 }

drivers/pps/clients/pps-ldisc.c

···
 	pps_event(pps, &ts, active ? PPS_CAPTUREASSERT :
 			PPS_CAPTURECLEAR, NULL);
 
-	dev_dbg(pps->dev, "PPS %s at %lu\n",
+	dev_dbg(&pps->dev, "PPS %s at %lu\n",
 			active ? "assert" : "clear", jiffies);
 }
 
···
 		goto err_unregister;
 	}
 
-	dev_info(pps->dev, "source \"%s\" added\n", info.path);
+	dev_dbg(&pps->dev, "source \"%s\" added\n", info.path);
 
 	return 0;
 
···
 	if (WARN_ON(!pps))
 		return;
 
-	dev_info(pps->dev, "removed\n");
+	dev_info(&pps->dev, "removed\n");
 	pps_unregister_source(pps);
 }
 

drivers/pps/clients/pps_parport.c

···
 	/* check the signal (no signal means the pulse is lost this time) */
 	if (!signal_is_set(port)) {
 		local_irq_restore(flags);
-		dev_err(dev->pps->dev, "lost the signal\n");
+		dev_err(&dev->pps->dev, "lost the signal\n");
 		goto out_assert;
 	}
 
···
 	/* timeout */
 	dev->cw_err++;
 	if (dev->cw_err >= CLEAR_WAIT_MAX_ERRORS) {
-		dev_err(dev->pps->dev, "disabled clear edge capture after %d"
+		dev_err(&dev->pps->dev, "disabled clear edge capture after %d"
 				" timeouts\n", dev->cw_err);
 		dev->cw = 0;
 		dev->cw_err = 0;

drivers/pps/kapi.c

···
 static void pps_echo_client_default(struct pps_device *pps, int event,
 		void *data)
 {
-	dev_info(pps->dev, "echo %s %s\n",
+	dev_info(&pps->dev, "echo %s %s\n",
 		event & PPS_CAPTUREASSERT ? "assert" : "",
 		event & PPS_CAPTURECLEAR ? "clear" : "");
 }
···
 		goto kfree_pps;
 	}
 
-	dev_info(pps->dev, "new PPS source %s\n", info->name);
+	dev_dbg(&pps->dev, "new PPS source %s\n", info->name);
 
 	return pps;
 
···
 	/* check event type */
 	BUG_ON((event & (PPS_CAPTUREASSERT | PPS_CAPTURECLEAR)) == 0);
 
-	dev_dbg(pps->dev, "PPS event at %lld.%09ld\n",
+	dev_dbg(&pps->dev, "PPS event at %lld.%09ld\n",
 			(s64)ts->ts_real.tv_sec, ts->ts_real.tv_nsec);
 
 	timespec_to_pps_ktime(&ts_real, ts->ts_real);
···
 		/* Save the time stamp */
 		pps->assert_tu = ts_real;
 		pps->assert_sequence++;
-		dev_dbg(pps->dev, "capture assert seq #%u\n",
+		dev_dbg(&pps->dev, "capture assert seq #%u\n",
 			pps->assert_sequence);
 
 		captured = ~0;
···
 		/* Save the time stamp */
 		pps->clear_tu = ts_real;
 		pps->clear_sequence++;
-		dev_dbg(pps->dev, "capture clear seq #%u\n",
+		dev_dbg(&pps->dev, "capture clear seq #%u\n",
 			pps->clear_sequence);
 
 		captured = ~0;

drivers/pps/kc.c

···
 			pps_kc_hardpps_mode = 0;
 			pps_kc_hardpps_dev = NULL;
 			spin_unlock_irq(&pps_kc_hardpps_lock);
-			dev_info(pps->dev, "unbound kernel"
+			dev_info(&pps->dev, "unbound kernel"
 					" consumer\n");
 		} else {
 			spin_unlock_irq(&pps_kc_hardpps_lock);
-			dev_err(pps->dev, "selected kernel consumer"
+			dev_err(&pps->dev, "selected kernel consumer"
 					" is not bound\n");
 			return -EINVAL;
 		}
···
 			pps_kc_hardpps_mode = bind_args->edge;
 			pps_kc_hardpps_dev = pps;
 			spin_unlock_irq(&pps_kc_hardpps_lock);
-			dev_info(pps->dev, "bound kernel consumer: "
+			dev_info(&pps->dev, "bound kernel consumer: "
 				"edge=0x%x\n", bind_args->edge);
 		} else {
 			spin_unlock_irq(&pps_kc_hardpps_lock);
-			dev_err(pps->dev, "another kernel consumer"
+			dev_err(&pps->dev, "another kernel consumer"
 					" is already bound\n");
 			return -EINVAL;
 		}
···
 		pps_kc_hardpps_mode = 0;
 		pps_kc_hardpps_dev = NULL;
 		spin_unlock_irq(&pps_kc_hardpps_lock);
-		dev_info(pps->dev, "unbound kernel consumer"
+		dev_info(&pps->dev, "unbound kernel consumer"
 				" on device removal\n");
 	} else
 		spin_unlock_irq(&pps_kc_hardpps_lock);

drivers/pps/pps.c

···
  * Local variables
  */
 
-static dev_t pps_devt;
+static int pps_major;
 static struct class *pps_class;
 
 static DEFINE_MUTEX(pps_idr_lock);
···
 	else {
 		unsigned long ticks;
 
-		dev_dbg(pps->dev, "timeout %lld.%09d\n",
+		dev_dbg(&pps->dev, "timeout %lld.%09d\n",
 				(long long) fdata->timeout.sec,
 				fdata->timeout.nsec);
 		ticks = fdata->timeout.sec * HZ;
···
 
 	/* Check for pending signals */
 	if (err == -ERESTARTSYS) {
-		dev_dbg(pps->dev, "pending signal caught\n");
+		dev_dbg(&pps->dev, "pending signal caught\n");
 		return -EINTR;
 	}
 
···
 
 	switch (cmd) {
 	case PPS_GETPARAMS:
-		dev_dbg(pps->dev, "PPS_GETPARAMS\n");
+		dev_dbg(&pps->dev, "PPS_GETPARAMS\n");
 
 		spin_lock_irq(&pps->lock);
 
···
 		break;
 
 	case PPS_SETPARAMS:
-		dev_dbg(pps->dev, "PPS_SETPARAMS\n");
+		dev_dbg(&pps->dev, "PPS_SETPARAMS\n");
 
 		/* Check the capabilities */
 		if (!capable(CAP_SYS_TIME))
···
 		if (err)
 			return -EFAULT;
 		if (!(params.mode & (PPS_CAPTUREASSERT | PPS_CAPTURECLEAR))) {
-			dev_dbg(pps->dev, "capture mode unspecified (%x)\n",
+			dev_dbg(&pps->dev, "capture mode unspecified (%x)\n",
 								params.mode);
 			return -EINVAL;
 		}
 
 		/* Check for supported capabilities */
 		if ((params.mode & ~pps->info.mode) != 0) {
-			dev_dbg(pps->dev, "unsupported capabilities (%x)\n",
+			dev_dbg(&pps->dev, "unsupported capabilities (%x)\n",
 								params.mode);
 			return -EINVAL;
 		}
···
 		/* Restore the read only parameters */
 		if ((params.mode & (PPS_TSFMT_TSPEC | PPS_TSFMT_NTPFP)) == 0) {
 			/* section 3.3 of RFC 2783 interpreted */
-			dev_dbg(pps->dev, "time format unspecified (%x)\n",
+			dev_dbg(&pps->dev, "time format unspecified (%x)\n",
 								params.mode);
 			pps->params.mode |= PPS_TSFMT_TSPEC;
 		}
···
 		break;
 
 	case PPS_GETCAP:
-		dev_dbg(pps->dev, "PPS_GETCAP\n");
+		dev_dbg(&pps->dev, "PPS_GETCAP\n");
 
 		err = put_user(pps->info.mode, iuarg);
 		if (err)
···
 	case PPS_FETCH: {
 		struct pps_fdata fdata;
 
-		dev_dbg(pps->dev, "PPS_FETCH\n");
+		dev_dbg(&pps->dev, "PPS_FETCH\n");
 
 		err = copy_from_user(&fdata, uarg, sizeof(struct pps_fdata));
 		if (err)
···
 	case PPS_KC_BIND: {
 		struct pps_bind_args bind_args;
 
-		dev_dbg(pps->dev, "PPS_KC_BIND\n");
+		dev_dbg(&pps->dev, "PPS_KC_BIND\n");
 
 		/* Check the capabilities */
 		if (!capable(CAP_SYS_TIME))
···
 
 		/* Check for supported capabilities */
 		if ((bind_args.edge & ~pps->info.mode) != 0) {
-			dev_err(pps->dev, "unsupported capabilities (%x)\n",
+			dev_err(&pps->dev, "unsupported capabilities (%x)\n",
 					bind_args.edge);
 			return -EINVAL;
 		}
···
 		if (bind_args.tsformat != PPS_TSFMT_TSPEC ||
 				(bind_args.edge & ~PPS_CAPTUREBOTH) != 0 ||
 				bind_args.consumer != PPS_KC_HARDPPS) {
-			dev_err(pps->dev, "invalid kernel consumer bind"
+			dev_err(&pps->dev, "invalid kernel consumer bind"
 					" parameters (%x)\n", bind_args.edge);
 			return -EINVAL;
 		}
···
 		struct pps_fdata fdata;
 		int err;
 
-		dev_dbg(pps->dev, "PPS_FETCH\n");
+		dev_dbg(&pps->dev, "PPS_FETCH\n");
 
 		err = copy_from_user(&compat, uarg, sizeof(struct pps_fdata_compat));
 		if (err)
···
 #define pps_cdev_compat_ioctl	NULL
 #endif
 
+static struct pps_device *pps_idr_get(unsigned long id)
+{
+	struct pps_device *pps;
+
+	mutex_lock(&pps_idr_lock);
+	pps = idr_find(&pps_idr, id);
+	if (pps)
+		get_device(&pps->dev);
+
+	mutex_unlock(&pps_idr_lock);
+	return pps;
+}
+
 static int pps_cdev_open(struct inode *inode, struct file *file)
 {
-	struct pps_device *pps = container_of(inode->i_cdev,
-						struct pps_device, cdev);
+	struct pps_device *pps = pps_idr_get(iminor(inode));
+
+	if (!pps)
+		return -ENODEV;
+
 	file->private_data = pps;
-	kobject_get(&pps->dev->kobj);
 	return 0;
 }
 
 static int pps_cdev_release(struct inode *inode, struct file *file)
 {
-	struct pps_device *pps = container_of(inode->i_cdev,
-						struct pps_device, cdev);
-	kobject_put(&pps->dev->kobj);
+	struct pps_device *pps = file->private_data;
+
+	WARN_ON(pps->id != iminor(inode));
+	put_device(&pps->dev);
 	return 0;
 }
 
···
 {
 	struct pps_device *pps = dev_get_drvdata(dev);
 
-	cdev_del(&pps->cdev);
-
-	/* Now we can release the ID for re-use */
 	pr_debug("deallocating pps%d\n", pps->id);
-	mutex_lock(&pps_idr_lock);
-	idr_remove(&pps_idr, pps->id);
-	mutex_unlock(&pps_idr_lock);
-
-	kfree(dev);
 	kfree(pps);
 }
 
 int pps_register_cdev(struct pps_device *pps)
 {
 	int err;
-	dev_t devt;
 
 	mutex_lock(&pps_idr_lock);
 	/*
···
 		goto out_unlock;
 	}
 	pps->id = err;
-	mutex_unlock(&pps_idr_lock);
 
-	devt = MKDEV(MAJOR(pps_devt), pps->id);
-
-	cdev_init(&pps->cdev, &pps_cdev_fops);
-	pps->cdev.owner = pps->info.owner;
-
-	err = cdev_add(&pps->cdev, devt, 1);
-	if (err) {
-		pr_err("%s: failed to add char device %d:%d\n",
-				pps->info.name, MAJOR(pps_devt), pps->id);
+	pps->dev.class = pps_class;
+	pps->dev.parent = pps->info.dev;
+	pps->dev.devt = MKDEV(pps_major, pps->id);
+	dev_set_drvdata(&pps->dev, pps);
+	dev_set_name(&pps->dev, "pps%d", pps->id);
+	err = device_register(&pps->dev);
+	if (err)
 		goto free_idr;
-	}
-	pps->dev = device_create(pps_class, pps->info.dev, devt, pps,
-							"pps%d", pps->id);
-	if (IS_ERR(pps->dev)) {
-		err = PTR_ERR(pps->dev);
-		goto del_cdev;
-	}
 
 	/* Override the release function with our own */
-	pps->dev->release = pps_device_destruct;
+	pps->dev.release = pps_device_destruct;
 
-	pr_debug("source %s got cdev (%d:%d)\n", pps->info.name,
-			MAJOR(pps_devt), pps->id);
+	pr_debug("source %s got cdev (%d:%d)\n", pps->info.name, pps_major,
+		 pps->id);
 
+	get_device(&pps->dev);
+	mutex_unlock(&pps_idr_lock);
 	return 0;
 
-del_cdev:
-	cdev_del(&pps->cdev);
-
 free_idr:
-	mutex_lock(&pps_idr_lock);
 	idr_remove(&pps_idr, pps->id);
+	put_device(&pps->dev);
 out_unlock:
 	mutex_unlock(&pps_idr_lock);
 	return err;
···
 {
 	pr_debug("unregistering pps%d\n", pps->id);
 	pps->lookup_cookie = NULL;
-	device_destroy(pps_class, pps->dev->devt);
+	device_destroy(pps_class, pps->dev.devt);
+
+	/* Now we can release the ID for re-use */
+	mutex_lock(&pps_idr_lock);
+	idr_remove(&pps_idr, pps->id);
+	put_device(&pps->dev);
+	mutex_unlock(&pps_idr_lock);
 }
 
 /*
···
  * so that it will not be used again, even if the pps device cannot
  * be removed from the idr due to pending references holding the minor
  * number in use.
+ *
+ * Since pps_idr holds a reference to the device, the returned
+ * pps_device is guaranteed to be valid until pps_unregister_cdev() is
+ * called on it. But after calling pps_unregister_cdev(), it may be
+ * freed at any time.
  */
 struct pps_device *pps_lookup_dev(void const *cookie)
 {
···
 static void __exit pps_exit(void)
 {
 	class_destroy(pps_class);
-	unregister_chrdev_region(pps_devt, PPS_MAX_SOURCES);
+	__unregister_chrdev(pps_major, 0, PPS_MAX_SOURCES, "pps");
 }
 
 static int __init pps_init(void)
 {
-	int err;
-
 	pps_class = class_create("pps");
 	if (IS_ERR(pps_class)) {
 		pr_err("failed to allocate class\n");
···
 	}
 	pps_class->dev_groups = pps_groups;
 
-	err = alloc_chrdev_region(&pps_devt, 0, PPS_MAX_SOURCES, "pps");
-	if (err < 0) {
+	pps_major = __register_chrdev(0, 0, PPS_MAX_SOURCES, "pps",
+				      &pps_cdev_fops);
+	if (pps_major < 0) {
 		pr_err("failed to allocate char device region\n");
 		goto remove_class;
 	}
···
 
 remove_class:
 	class_destroy(pps_class);
-
-	return err;
+	return pps_major;
 }
 
 subsys_initcall(pps_init);

drivers/ptp/ptp_ocp.c

···
 
 	pps = pps_lookup_dev(bp->ptp);
 	if (pps)
-		ptp_ocp_symlink(bp, pps->dev, "pps");
+		ptp_ocp_symlink(bp, &pps->dev, "pps");
 
 	ptp_ocp_debugfs_add_device(bp);
 

include/linux/pps_kernel.h

···
 
 	unsigned int id;			/* PPS source unique ID */
 	void const *lookup_cookie;		/* For pps_lookup_dev() only */
-	struct cdev cdev;
-	struct device *dev;
+	struct device dev;
 	struct fasync_struct *async_queue;	/* fasync method */
 	spinlock_t lock;
 };