Message-ID: <ed73319e062495a56f5c8fd95c1674f891871882.1725820970.git.calvin@wbinvd.org>
Date: Sun,  8 Sep 2024 12:04:13 -0700
From: Calvin Owens <calvin@...nvd.org>
To: linux-kernel@...r.kernel.org
Cc: Rodolfo Giometti <giometti@...eenne.com>,
	George Spelvin <linux@...izon.com>,
	Calvin Owens <calvin@...nvd.org>
Subject: [PATCH] pps: Remove embedded cdev to 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, 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 kobject 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 the pps->dev
kobject.

Fixes: d953e0e837e6 ("pps: Fix a use-after free bug when unregistering a source.")
Cc: stable@...r.kernel.org
Signed-off-by: Calvin Owens <calvin@...nvd.org>
---
 drivers/pps/pps.c          | 85 ++++++++++++++++++++------------------
 include/linux/pps_kernel.h |  1 -
 2 files changed, 45 insertions(+), 41 deletions(-)

diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
index 5d19baae6a38..72606d10f5f5 100644
--- a/drivers/pps/pps.c
+++ b/drivers/pps/pps.c
@@ -25,7 +25,7 @@
  * Local variables
  */
 
-static dev_t pps_devt;
+static int pps_major;
 static struct class *pps_class;
 
 static DEFINE_MUTEX(pps_idr_lock);
@@ -296,19 +296,35 @@ static long pps_cdev_compat_ioctl(struct file *file,
 #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)
+		kobject_get(&pps->dev->kobj);
+
+	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);
+	struct pps_device *pps = file->private_data;
+
+	WARN_ON(pps->id != iminor(inode));
 	kobject_put(&pps->dev->kobj);
 	return 0;
 }
@@ -332,14 +348,6 @@ static void pps_device_destruct(struct device *dev)
 {
 	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);
 }
@@ -364,39 +372,26 @@ int pps_register_cdev(struct pps_device *pps)
 		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);
-		goto free_idr;
-	}
+	devt = MKDEV(pps_major, pps->id);
 	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;
+		goto free_idr;
 	}
 
 	/* Override the release function with our own */
 	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);
 
+	kobject_get(&pps->dev->kobj);
+	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);
 out_unlock:
 	mutex_unlock(&pps_idr_lock);
@@ -408,6 +403,13 @@ void pps_unregister_cdev(struct pps_device *pps)
 	pr_debug("unregistering pps%d\n", pps->id);
 	pps->lookup_cookie = NULL;
 	device_destroy(pps_class, pps->dev->devt);
+
+	/* 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);
+	kobject_put(&pps->dev->kobj);
+	mutex_unlock(&pps_idr_lock);
 }
 
 /*
@@ -427,6 +429,11 @@ void pps_unregister_cdev(struct pps_device *pps)
  * 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 kobject, 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)
 {
@@ -449,13 +456,11 @@ EXPORT_SYMBOL(pps_lookup_dev);
 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");
@@ -463,8 +468,9 @@ static int __init pps_init(void)
 	}
 	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;
 	}
@@ -477,8 +483,7 @@ static int __init pps_init(void)
 
 remove_class:
 	class_destroy(pps_class);
-
-	return err;
+	return pps_major;
 }
 
 subsys_initcall(pps_init);
diff --git a/include/linux/pps_kernel.h b/include/linux/pps_kernel.h
index 78c8ac4951b5..8ee312788118 100644
--- a/include/linux/pps_kernel.h
+++ b/include/linux/pps_kernel.h
@@ -56,7 +56,6 @@ struct pps_device {
 
 	unsigned int id;			/* PPS source unique ID */
 	void const *lookup_cookie;		/* For pps_lookup_dev() only */
-	struct cdev cdev;
 	struct device *dev;
 	struct fasync_struct *async_queue;	/* fasync method */
 	spinlock_t lock;
-- 
2.45.2

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