Commit a33121e5 authored by Vladis Dronov's avatar Vladis Dronov Committed by David S. Miller
Browse files

ptp: fix the race between the release of ptp_clock and cdev 

In a case when a ptp chardev (like /dev/ptp0) is open but an underlying
device is removed, closing this file leads to a race. This reproduces
easily in a kvm virtual machine:

ts# cat openptp0.c
int main() { ... fp = fopen("/dev/ptp0", "r"); ... sleep(10); }
ts# uname -r
5.5.0-rc3-46cf053e
ts# cat /proc/cmdline
... slub_debug=FZP
ts# modprobe ptp_kvm
ts# ./openptp0 &
[1] 670
opened /dev/ptp0, sleeping 10s...
ts# rmmod ptp_kvm
ts# ls /dev/ptp*
ls: cannot access '/dev/ptp*': No such file or directory
ts# ...woken up
[   48.010809] general protection fault: 0000 [#1] SMP
[   48.012502] CPU: 6 PID: 658 Comm: openptp0 Not tainted 5.5.0-rc3-46cf053e #25
[   48.014624] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), ...
[   48.016270] RIP: 0010:module_put.part.0+0x7/0x80
[   48.017939] RSP: 0018:ffffb3850073be00 EFLAGS: 00010202
[   48.018339] RAX: 000000006b6b6b6b RBX: 6b6b6b6b6b6b6b6b RCX: ffff89a476c00ad0
[   48.018936] RDX: fffff65a08d3ea08 RSI: 0000000000000247 RDI: 6b6b6b6b6b6b6b6b
[   48.019470] ...                                              ^^^ a slub poison
[   48.023854] Call Trace:
[   48.024050]  __fput+0x21f/0x240
[   48.024288]  task_work_run+0x79/0x90
[   48.024555]  do_exit+0x2af/0xab0
[   48.024799]  ? vfs_write+0x16a/0x190
[   48.025082]  do_group_exit+0x35/0x90
[   48.025387]  __x64_sys_exit_group+0xf/0x10
[   48.025737]  do_syscall_64+0x3d/0x130
[   48.026056]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   48.026479] RIP: 0033:0x7f53b12082f6
[   48.026792] ...
[   48.030945] Modules linked in: ptp i6300esb watchdog [last unloaded: ptp_kvm]
[   48.045001] Fixing recursive fault but reboot is needed!

This happens in:

static void __fput(struct file *file)
{   ...
    if (file->f_op->release)
        file->f_op->release(inode, file); <<< cdev is kfree'd here
    if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL &&
             !(mode & FMODE_PATH))) {
        cdev_put(inode->i_cdev); <<< cdev fields are accessed here

Namely:

__fput()
  posix_clock_release()
    kref_put(&clk->kref, delete_clock) <<< the last reference
      delete_clock()
        delete_ptp_clock()
          kfree(ptp) <<< cdev is embedded in ptp
  cdev_put
    module_put(p->owner) <<< *p is kfree'd, bang!

Here cdev is embedded in posix_clock which is embedded in ptp_clock.
The race happens because ptp_clock's lifetime is controlled by two
refcounts: kref and cdev.kobj in posix_clock. This is wrong.

Make ptp_clock's sysfs device a parent of cdev with cdev_device_add()
created especially for such cases. This way the parent device with its
ptp_clock is not released until all references to the cdev are released.
This adds a requirement that an initialized but not exposed struct
device should be provided to posix_clock_register() by a caller instead
of a simple dev_t.

This approach was adopted from the commit 72139dfa ("watchdog: Fix
the race between the release of watchdog_core_data and cdev"). See
details of the implementation in the commit 233ed09d ("chardev: add
helper function to register char devs with a struct device").

Link: https://lore.kernel.org/linux-fsdevel/20191125125342.6189-1-vdronov@redhat.com/T/#u


Analyzed-by: default avatarStephen Johnston <sjohnsto@redhat.com>
Analyzed-by: default avatarVern Lovejoy <vlovejoy@redhat.com>
Signed-off-by: default avatarVladis Dronov <vdronov@redhat.com>
Acked-by: default avatarRichard Cochran <richardcochran@gmail.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 54fa49ee

drivers/ptp/ptp_clock.c

+14 −17
Original line number Diff line number Diff line
@@ -166,9 +166,9 @@ static struct posix_clock_operations ptp_clock_ops = { 
	.read		= ptp_read,

};



static void delete_ptp_clock(struct posix_clock *pc)

static void ptp_clock_release(struct device *dev)

{

	struct ptp_clock *ptp = container_of(pc, struct ptp_clock, clock);

	struct ptp_clock *ptp = container_of(dev, struct ptp_clock, dev);



	mutex_destroy(&ptp->tsevq_mux);

	mutex_destroy(&ptp->pincfg_mux);
@@ -213,7 +213,6 @@ struct ptp_clock *ptp_clock_register(struct ptp_clock_info *info, 
	}



	ptp->clock.ops = ptp_clock_ops;

	ptp->clock.release = delete_ptp_clock;

	ptp->info = info;

	ptp->devid = MKDEV(major, index);

	ptp->index = index;
@@ -236,15 +235,6 @@ struct ptp_clock *ptp_clock_register(struct ptp_clock_info *info, 
	if (err)

		goto no_pin_groups;



	/* Create a new device in our class. */

	ptp->dev = device_create_with_groups(ptp_class, parent, ptp->devid,

					     ptp, ptp->pin_attr_groups,

					     "ptp%d", ptp->index);

	if (IS_ERR(ptp->dev)) {

		err = PTR_ERR(ptp->dev);

		goto no_device;

	}



	/* Register a new PPS source. */

	if (info->pps) {

		struct pps_source_info pps;
@@ -260,8 +250,18 @@ struct ptp_clock *ptp_clock_register(struct ptp_clock_info *info, 
		}

	}



	/* Create a posix clock. */

	err = posix_clock_register(&ptp->clock, ptp->devid);

	/* Initialize a new device of our class in our clock structure. */

	device_initialize(&ptp->dev);

	ptp->dev.devt = ptp->devid;

	ptp->dev.class = ptp_class;

	ptp->dev.parent = parent;

	ptp->dev.groups = ptp->pin_attr_groups;

	ptp->dev.release = ptp_clock_release;

	dev_set_drvdata(&ptp->dev, ptp);

	dev_set_name(&ptp->dev, "ptp%d", ptp->index);



	/* Create a posix clock and link it to the device. */

	err = posix_clock_register(&ptp->clock, &ptp->dev);

	if (err) {

		pr_err("failed to create posix clock\n");

		goto no_clock;
@@ -273,8 +273,6 @@ no_clock: 
	if (ptp->pps_source)

		pps_unregister_source(ptp->pps_source);

no_pps:

	device_destroy(ptp_class, ptp->devid);

no_device:

	ptp_cleanup_pin_groups(ptp);

no_pin_groups:

	if (ptp->kworker)
@@ -304,7 +302,6 @@ int ptp_clock_unregister(struct ptp_clock *ptp) 
	if (ptp->pps_source)

		pps_unregister_source(ptp->pps_source);



	device_destroy(ptp_class, ptp->devid);

	ptp_cleanup_pin_groups(ptp);



	posix_clock_unregister(&ptp->clock);

drivers/ptp/ptp_private.h

+1 −1
Original line number Diff line number Diff line
@@ -28,7 +28,7 @@ struct timestamp_event_queue { 


struct ptp_clock {

	struct posix_clock clock;

	struct device *dev;

	struct device dev;

	struct ptp_clock_info *info;

	dev_t devid;

	int index; /* index into clocks.map */

include/linux/posix-clock.h

+11 −8
Original line number Diff line number Diff line
@@ -69,29 +69,32 @@ struct posix_clock_operations { 
 *

 * @ops:     Functional interface to the clock

 * @cdev:    Character device instance for this clock

 * @kref:    Reference count.

 * @dev:     Pointer to the clock's device.

 * @rwsem:   Protects the 'zombie' field from concurrent access.

 * @zombie:  If 'zombie' is true, then the hardware has disappeared.

 * @release: A function to free the structure when the reference count reaches

 *           zero. May be NULL if structure is statically allocated.

 *

 * Drivers should embed their struct posix_clock within a private

 * structure, obtaining a reference to it during callbacks using

 * container_of().

 *

 * Drivers should supply an initialized but not exposed struct device

 * to posix_clock_register(). It is used to manage lifetime of the

 * driver's private structure. It's 'release' field should be set to

 * a release function for this private structure.

 */

struct posix_clock {

	struct posix_clock_operations ops;

	struct cdev cdev;

	struct kref kref;

	struct device *dev;

	struct rw_semaphore rwsem;

	bool zombie;

	void (*release)(struct posix_clock *clk);

};



/**

 * posix_clock_register() - register a new clock

 * @clk:   Pointer to the clock. Caller must provide 'ops' and 'release'

 * @devid: Allocated device id

 * @clk:   Pointer to the clock. Caller must provide 'ops' field

 * @dev:   Pointer to the initialized device. Caller must provide

 *         'release' field

 *

 * A clock driver calls this function to register itself with the

 * clock device subsystem. If 'clk' points to dynamically allocated
@@ -100,7 +103,7 @@ struct posix_clock { 
 *

 * Returns zero on success, non-zero otherwise.

 */

int posix_clock_register(struct posix_clock *clk, dev_t devid);

int posix_clock_register(struct posix_clock *clk, struct device *dev);



/**

 * posix_clock_unregister() - unregister a clock

kernel/time/posix-clock.c

+13 −18
Original line number Diff line number Diff line
@@ -14,8 +14,6 @@ 


#include "posix-timers.h"



static void delete_clock(struct kref *kref);



/*

 * Returns NULL if the posix_clock instance attached to 'fp' is old and stale.

 */
@@ -125,7 +123,7 @@ static int posix_clock_open(struct inode *inode, struct file *fp) 
		err = 0;



	if (!err) {

		kref_get(&clk->kref);

		get_device(clk->dev);

		fp->private_data = clk;

	}

out:
@@ -141,7 +139,7 @@ static int posix_clock_release(struct inode *inode, struct file *fp) 
	if (clk->ops.release)

		err = clk->ops.release(clk);



	kref_put(&clk->kref, delete_clock);

	put_device(clk->dev);



	fp->private_data = NULL;
@@ -161,38 +159,35 @@ static const struct file_operations posix_clock_file_operations = { 
#endif

};



int posix_clock_register(struct posix_clock *clk, dev_t devid)

int posix_clock_register(struct posix_clock *clk, struct device *dev)

{

	int err;



	kref_init(&clk->kref);

	init_rwsem(&clk->rwsem);



	cdev_init(&clk->cdev, &posix_clock_file_operations);

	clk->cdev.owner = clk->ops.owner;

	err = cdev_add(&clk->cdev, devid, 1);



	err = cdev_device_add(&clk->cdev, dev);

	if (err) {

		pr_err("%s unable to add device %d:%d\n",

			dev_name(dev), MAJOR(dev->devt), MINOR(dev->devt));

		return err;

	}

EXPORT_SYMBOL_GPL(posix_clock_register);



static void delete_clock(struct kref *kref)

{

	struct posix_clock *clk = container_of(kref, struct posix_clock, kref);

	clk->cdev.owner = clk->ops.owner;

	clk->dev = dev;



	if (clk->release)

		clk->release(clk);

	return 0;

}

EXPORT_SYMBOL_GPL(posix_clock_register);



void posix_clock_unregister(struct posix_clock *clk)

{

	cdev_del(&clk->cdev);

	cdev_device_del(&clk->cdev, clk->dev);



	down_write(&clk->rwsem);

	clk->zombie = true;

	up_write(&clk->rwsem);



	kref_put(&clk->kref, delete_clock);

	put_device(clk->dev);

}

EXPORT_SYMBOL_GPL(posix_clock_unregister);

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