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Message-ID: <fc5fe55c-422d-4e63-a5bd-8b6b2d3e6c62@enneenne.com>
Date: Mon, 16 Sep 2024 10:16:11 +0200
From: Rodolfo Giometti <giometti@...eenne.com>
To: Calvin Owens <calvin@...nvd.org>, linux-kernel@...r.kernel.org
Cc: George Spelvin <linux@...izon.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] pps: Remove embedded cdev to fix a use-after-free
On 14/09/24 02:24, Calvin Owens wrote:
> 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@...r.kernel.org
> Signed-off-by: Calvin Owens <calvin@...nvd.org>
> ---
> Changes in v2:
> - Don't move pr_debug() from pps_device_destruct() to pps_unregister_cdev()
> - Actually add stable@...r.kernel.org 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;
Acked-by: Rodolfo Giometti <giometti@...eenne.com>
--
GNU/Linux Solutions e-mail: giometti@...eenne.com
Linux Device Driver giometti@...ux.it
Embedded Systems phone: +39 349 2432127
UNIX programming
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