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.
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: stable@xxxxxxxxxxxxxxx
Signed-off-by: Calvin Owens <calvin@xxxxxxxxxx>
---
Changes in v2:
- Don't move pr_debug() from pps_device_destruct() to pps_unregister_cdev()
- Actually add stable@xxxxxxxxxxxxxxx to CC
---
drivers/pps/pps.c | 83 ++++++++++++++++++++------------------
include/linux/pps_kernel.h | 1 -
2 files changed, 44 insertions(+), 40 deletions(-)
diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
index 5d19baae6a38..6980ab17f314 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 +348,7 @@ 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 +373,26 @@ 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 +404,12 @@ 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 */
+ 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;
