[PATCH] char: xillybus: Fix use-after-free in xillyusb_open()

[PATCH] char: xillybus: Fix use-after-free in xillyusb_open()

[Hyunwoo Kim]

A race condition may occur if the user physically removes
the USB device while calling open() for this device node.

This is a race condition between the xillyusb_open() function and
the xillyusb_disconnect() function, which may eventually result in UAF.

So, add a mutex to the xillyusb_open() and xillyusb_disconnect()
functions to avoid race contidion.

Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
---
 drivers/char/xillybus/xillyusb.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/drivers/char/xillybus/xillyusb.c b/drivers/char/xillybus/xillyusb.c
index 39bcbfd908b4..d615f74dcf36 100644
--- a/drivers/char/xillybus/xillyusb.c
+++ b/drivers/char/xillybus/xillyusb.c
@@ -63,6 +63,7 @@ static const struct usb_device_id xillyusb_table[] = {
 };
 
 MODULE_DEVICE_TABLE(usb, xillyusb_table);
+static DEFINE_MUTEX(disconnect_mutex);
 
 struct xillyusb_dev;
 
@@ -1237,9 +1238,13 @@ static int xillyusb_open(struct inode *inode, struct file *filp)
 	int rc;
 	int index;
 
+	mutex_lock(&disconnect_mutex);
+
 	rc = xillybus_find_inode(inode, (void **)&xdev, &index);
-	if (rc)
+	if (rc) {
+		mutex_unlock(&disconnect_mutex);
 		return rc;
+	}
 
 	chan = &xdev->channels[index];
 	filp->private_data = chan;
@@ -1379,6 +1384,8 @@ static int xillyusb_open(struct inode *inode, struct file *filp)
 			request_read_anything(chan, OPCODE_SET_PUSH);
 	}
 
+	mutex_unlock(&disconnect_mutex);
+
 	return 0;
 
 unfifo:
@@ -1410,10 +1417,14 @@ static int xillyusb_open(struct inode *inode, struct file *filp)
 
 	kref_put(&xdev->kref, cleanup_dev);
 
+	mutex_unlock(&disconnect_mutex);
+
 	return rc;
 
 unmutex_fail:
 	mutex_unlock(&chan->lock);
+	mutex_unlock(&disconnect_mutex);
+
 	return rc;
 }
 
@@ -1698,6 +1709,8 @@ static int xillyusb_release(struct inode *inode, struct file *filp)
 	struct xillyusb_dev *xdev = chan->xdev;
 	int rc_read = 0, rc_write = 0;
 
+	mutex_lock(&disconnect_mutex);
+
 	if (filp->f_mode & FMODE_READ) {
 		struct xillyfifo *in_fifo = chan->in_fifo;
 
@@ -1760,6 +1773,8 @@ static int xillyusb_release(struct inode *inode, struct file *filp)
 
 	kref_put(&xdev->kref, cleanup_dev);
 
+	mutex_unlock(&disconnect_mutex);
+
 	return rc_read ? rc_read : rc_write;
 }
 
@@ -2172,6 +2187,8 @@ static void xillyusb_disconnect(struct usb_interface *interface)
 	int rc;
 	int i;
 
+	mutex_lock(&disconnect_mutex);
+
 	xillybus_cleanup_chrdev(xdev, &interface->dev);
 
 	/*
@@ -2228,6 +2245,8 @@ static void xillyusb_disconnect(struct usb_interface *interface)
 	xdev->dev = NULL;
 
 	kref_put(&xdev->kref, cleanup_dev);
+
+	mutex_unlock(&disconnect_mutex);
 }
 
 static struct usb_driver xillyusb_driver = {
-- 
2.25.1


Dear,


This race condition can occur in the flow of:
```
                cpu0                                                cpu1
       1. xillyusb_open()
          xillybus_find_inode()
          mutex_lock(&unit_mutex);
          unit = iter;
          mutex_unlock(&unit_mutex);
                                                             2. xillyusb_disconnect()
                                                                xillybus_cleanup_chrdev()
                                                                mutex_lock(&unit_mutex);
                                                                kfree(unit);
                                                                mutex_unlock(&unit_mutex);
       3. *private_data = unit->private_data;   // UAF

```

Because the interval between 1 and 3 in xillyusb_open() is very short, this race condition is usually rare.

However, if someone wants to trigger this UAF, they can use the technique presented in this paper:
https://www.usenix.org/system/files/sec21-lee-yoochan.pdf

This is a method to increase the execution time between No. 1 and No. 3 using the Reschedule IPI.
This means you will be able to trigger the UAF much more reliably.

Here is the KASAN log:
```
[   66.645661] ==================================================================
[   66.645678] BUG: KASAN: use-after-free in xillybus_find_inode+0x2c6/0x2ea [xillybus_class]
[   66.645712] Read of size 8 at addr ffff88811eb45f90 by task xillyusb_test/2143

[   66.645735] CPU: 2 PID: 2143 Comm: xillyusb_test Not tainted 6.1.0-rc1+ #10
[   66.645752] Hardware name: Gigabyte Technology Co., Ltd. B460MDS3H/B460M DS3H, BIOS F3 05/27/2020
[   66.645762] Call Trace:
[   66.645772]  <TASK>
[   66.645783]  dump_stack_lvl+0x49/0x63
[   66.645808]  print_report+0x177/0x46e
[   66.645828]  ? kasan_complete_mode_report_info+0x7c/0x210
[   66.645846]  ? xillybus_find_inode+0x2c6/0x2ea [xillybus_class]
[   66.645871]  kasan_report+0xb0/0x140
[   66.645891]  ? xillybus_find_inode+0x2c6/0x2ea [xillybus_class]
[   66.645917]  __asan_report_load8_noabort+0x14/0x20
[   66.645932]  xillybus_find_inode+0x2c6/0x2ea [xillybus_class]
[   66.645960]  xillyusb_open+0xa6/0x1030 [xillyusb]
[   66.645990]  ? endpoint_alloc+0x6d0/0x6d0 [xillyusb]
[   66.646014]  ? __kasan_check_write+0x14/0x20
[   66.646028]  ? _raw_spin_lock+0x88/0xe0
[   66.646044]  ? try_module_get.part.0+0xb8/0x1a0
[   66.646062]  chrdev_open+0x230/0x6d0
[   66.646082]  ? cdev_device_add+0x1f0/0x1f0
[   66.646099]  ? fsnotify_perm.part.0+0x1d9/0x4e0
[   66.646118]  do_dentry_open+0x449/0x1000
[   66.646132]  ? inode_permission+0x125/0x560
[   66.646148]  ? cdev_device_add+0x1f0/0x1f0
[   66.646167]  vfs_open+0x9f/0xd0
[   66.646181]  path_openat+0xd58/0x3eb0
[   66.646199]  ? kasan_save_alloc_info+0x1e/0x30
[   66.646212]  ? __kasan_slab_alloc+0x90/0xa0
[   66.646229]  ? kmem_cache_alloc+0x1f6/0x310
[   66.646244]  ? getname_flags.part.0+0x52/0x490
[   66.646260]  ? getname+0x7a/0xb0
[   66.646275]  ? __x64_sys_openat+0x128/0x210
[   66.646288]  ? do_syscall_64+0x59/0x90
[   66.646306]  ? path_lookupat+0x660/0x660
[   66.646328]  do_filp_open+0x1b1/0x3e0
[   66.646344]  ? debug_smp_processor_id+0x17/0x20
[   66.646363]  ? may_open_dev+0xd0/0xd0
[   66.646378]  ? getname_flags.part.0+0x52/0x490
[   66.646399]  ? _raw_spin_lock_bh+0xe0/0xe0
[   66.646422]  do_sys_openat2+0x132/0x450
[   66.646435]  ? recalc_sigpending+0x144/0x1c0
[   66.646450]  ? build_open_flags+0x450/0x450
[   66.646465]  ? sigprocmask+0x201/0x360
[   66.646478]  ? __ia32_sys_ssetmask+0x1d0/0x1d0
[   66.646495]  __x64_sys_openat+0x128/0x210
[   66.646509]  ? __ia32_compat_sys_open+0x1b0/0x1b0
[   66.646526]  ? debug_smp_processor_id+0x17/0x20
[   66.646545]  do_syscall_64+0x59/0x90
[   66.646560]  ? syscall_exit_to_user_mode+0x26/0x50
[   66.646578]  ? do_syscall_64+0x69/0x90
[   66.646593]  ? debug_smp_processor_id+0x17/0x20
[   66.646610]  ? fpregs_assert_state_consistent+0x52/0xc0
[   66.646630]  ? exit_to_user_mode_prepare+0x49/0x1a0
[   66.646644]  ? syscall_exit_to_user_mode+0x26/0x50
[   66.646662]  ? do_syscall_64+0x69/0x90
[   66.646675]  ? exit_to_user_mode_prepare+0x16a/0x1a0
[   66.646689]  ? syscall_exit_to_user_mode+0x26/0x50
[   66.646706]  entry_SYSCALL_64_after_hwframe+0x63/0xcd
[   66.646722] RIP: 0033:0x4534e4
[   66.646736] Code: 24 20 eb 8f 66 90 44 89 54 24 0c e8 d6 ab 02 00 44 8b 54 24 0c 44 89 e2 48 89 ee 41 89 c0 bf 9c ff ff ff b8 01 01 00 00 0f 05 <48> 3d 00 f0 ff ff 77 34 44 89 c7 89 44 24 0c e8 18 ac 02 00 8b 44
[   66.646751] RSP: 002b:00007f8130000140 EFLAGS: 00000293 ORIG_RAX: 0000000000000101
[   66.646771] RAX: ffffffffffffffda RBX: 00007f8130000640 RCX: 00000000004534e4
[   66.646784] RDX: 0000000000000000 RSI: 00000000004b1008 RDI: 00000000ffffff9c
[   66.646795] RBP: 00000000004b1008 R08: 0000000000000000 R09: 00007ffc22c66baf
[   66.646806] R10: 0000000000000000 R11: 0000000000000293 R12: 0000000000000000
[   66.646817] R13: 0000000000000000 R14: 000000000041b2a0 R15: 00007f812f800000
[   66.646834]  </TASK>

[   66.646848] Allocated by task 2115:
[   66.646859]  kasan_save_stack+0x26/0x50
[   66.646875]  kasan_set_track+0x25/0x40
[   66.646888]  kasan_save_alloc_info+0x1e/0x30
[   66.646898]  __kasan_kmalloc+0xb4/0xc0
[   66.646911]  kmalloc_trace+0x4a/0xb0
[   66.646924]  xillybus_init_chrdev+0xf2/0x815 [xillybus_class]
[   66.646944]  xillyusb_probe.cold+0xa61/0xadf [xillyusb]
[   66.646966]  usb_probe_interface+0x266/0x740
[   66.646981]  really_probe+0x1fa/0xa80
[   66.646993]  __driver_probe_device+0x2cb/0x490
[   66.647005]  driver_probe_device+0x4e/0x140
[   66.647017]  __driver_attach+0x1a3/0x520
[   66.647028]  bus_for_each_dev+0x11e/0x1c0
[   66.647039]  driver_attach+0x3d/0x60
[   66.647050]  bus_add_driver+0x449/0x5a0
[   66.647061]  driver_register+0x219/0x390
[   66.647073]  usb_register_driver+0x228/0x400
[   66.647084]  0xffffffffc033202d
[   66.647094]  do_one_initcall+0x97/0x310
[   66.647109]  do_init_module+0x19a/0x630
[   66.647120]  load_module+0x6ca4/0x7d90
[   66.647131]  __do_sys_finit_module+0x134/0x1d0
[   66.647142]  __x64_sys_finit_module+0x72/0xb0
[   66.647153]  do_syscall_64+0x59/0x90
[   66.647164]  entry_SYSCALL_64_after_hwframe+0x63/0xcd

[   66.647182] Freed by task 2089:
[   66.647191]  kasan_save_stack+0x26/0x50
[   66.647205]  kasan_set_track+0x25/0x40
[   66.647218]  kasan_save_free_info+0x2e/0x50
[   66.647228]  ____kasan_slab_free+0x174/0x1e0
[   66.647241]  __kasan_slab_free+0x12/0x20
[   66.647255]  slab_free_freelist_hook+0xd0/0x1a0
[   66.647267]  __kmem_cache_free+0x193/0x2c0
[   66.647280]  kfree+0x79/0x120
[   66.647291]  xillybus_cleanup_chrdev+0x3c1/0x570 [xillybus_class]
[   66.647311]  xillyusb_disconnect+0xfe/0x790 [xillyusb]
[   66.647332]  usb_unbind_interface+0x187/0x7c0
[   66.647345]  device_remove+0x117/0x170
[   66.647357]  device_release_driver_internal+0x418/0x660
[   66.647369]  device_release_driver+0x12/0x20
[   66.647381]  bus_remove_device+0x28f/0x540
[   66.647392]  device_del+0x501/0xc30
[   66.647407]  usb_disable_device+0x2a5/0x660
[   66.647418]  usb_disconnect.cold+0x1f9/0x620
[   66.647431]  hub_event+0x16d3/0x3d20
[   66.647446]  process_one_work+0x778/0x11c0
[   66.647459]  worker_thread+0x544/0x1180
[   66.647471]  kthread+0x280/0x320
[   66.647481]  ret_from_fork+0x1f/0x30

[   66.647501] Last potentially related work creation:
[   66.647510]  kasan_save_stack+0x26/0x50
[   66.647524]  __kasan_record_aux_stack+0xb6/0xd0
[   66.647534]  kasan_record_aux_stack_noalloc+0xb/0x20
[   66.647544]  kvfree_call_rcu+0xaf/0xa50
[   66.647555]  memcg_reparent_list_lrus+0x477/0x790
[   66.647566]  mem_cgroup_css_offline+0x1ea/0x310
[   66.647579]  css_killed_work_fn+0xe4/0x380
[   66.647590]  process_one_work+0x778/0x11c0
[   66.647602]  worker_thread+0x544/0x1180
[   66.647614]  kthread+0x280/0x320
[   66.647623]  ret_from_fork+0x1f/0x30

[   66.647642] The buggy address belongs to the object at ffff88811eb45f80
                which belongs to the cache kmalloc-64 of size 64
[   66.647656] The buggy address is located 16 bytes inside of
                64-byte region [ffff88811eb45f80, ffff88811eb45fc0)

[   66.647677] The buggy address belongs to the physical page:
[   66.647686] page:000000009bdb1cfb refcount:1 mapcount:0 mapping:0000000000000000 index:0xffff88811eb45a00 pfn:0x11eb45
[   66.647701] flags: 0x17ffffc0000200(slab|node=0|zone=2|lastcpupid=0x1fffff)
[   66.647720] raw: 0017ffffc0000200 ffffea000481fec8 ffffea0004544b48 ffff888100042640
[   66.647731] raw: ffff88811eb45a00 000000000020001e 00000001ffffffff 0000000000000000
[   66.647738] page dumped because: kasan: bad access detected

[   66.647751] Memory state around the buggy address:
[   66.647760]  ffff88811eb45e80: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
[   66.647771]  ffff88811eb45f00: 00 00 00 00 00 fc fc fc fc fc fc fc fc fc fc fc
[   66.647781] >ffff88811eb45f80: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
[   66.647790]                          ^
[   66.647800]  ffff88811eb46000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[   66.647810]  ffff88811eb46080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[   66.647818] ==================================================================
[   66.647826] Disabling lock debugging due to kernel taint
```


Regards,
Hyunwoo Kim.

Re: [PATCH] char: xillybus: Fix use-after-free in xillyusb_open()

[Eli Billauer]

Hello, Hyunwoo.

> A race condition may occur if the user physically removes
> the USB device while calling open() for this device node.
> 
> This is a race condition between the xillyusb_open() function and
> the xillyusb_disconnect() function, which may eventually result in UAF.

Thanks a lot for pointing that out. In fact, this reveals two problems 
in the existing code:

(1) unit->private_data is accessed after the mutex has been released in 
xillybus_find_inode(), so there's no guarantee that it will be valid. 
This is what the test caught. This can however be fixed just by moving 
the release of the lock a few rows down.

(2) xillyusb_open() accesses @xdev without ensuring that it won't get freed.

Both of these two issues have a negligible probability of causing a 
visible problem, but this must be fixed, of course.

> 
> So, add a mutex to the xillyusb_open() and xillyusb_disconnect()
> functions to avoid race contidion.

I'm not very fond of this solution, partially because this mutex 
protects code and not data (There's this "Lock data, not code" rule, see 
[1]). Also, xillyusb_disconnect() can take a significant time to run, 
during which xillybus_open() for another (unrelated and still connected) 
XillyUSB device has to wait. I guess this demonstrates why protecting 
code with a mutex is considered bad practice.

Besides, there are already three mechanisms in place for preventing 
premature release of memory:

(1) @unit_mutex in xillybus_class.c, which protects @unit_list.
(2) @kref inside struct xillyusb_dev (xillyusb.c), which protects the 
structure it resides in.
(3) @error inside struct xillyusb_dev, which prevents xillybus_open() 
from opening a file that belongs to a device that is about to be released.

It's now apparent that they're not working well enough. Rather than 
adding another mutex, the existing mechanisms should be fixed.  Would 
you like to do this, or should I?

Thanks again,
   Eli

[1] Documentation/kernel-hacking/locking.rst in the kernel tree

Re: [PATCH] char: xillybus: Fix use-after-free in xillyusb_open()

[Hyunwoo Kim]

On Sun, Oct 23, 2022 at 05:19:40PM +0300, Eli Billauer wrote:
> Hello, Hyunwoo.
> 
> > A race condition may occur if the user physically removes
> > the USB device while calling open() for this device node.
> > 
> > This is a race condition between the xillyusb_open() function and
> > the xillyusb_disconnect() function, which may eventually result in UAF.
> 
> Thanks a lot for pointing that out. In fact, this reveals two problems in
> the existing code:
> 
> (1) unit->private_data is accessed after the mutex has been released in
> xillybus_find_inode(), so there's no guarantee that it will be valid. This
> is what the test caught. This can however be fixed just by moving the
> release of the lock a few rows down.
> 
> (2) xillyusb_open() accesses @xdev without ensuring that it won't get freed.
> 
> Both of these two issues have a negligible probability of causing a visible
> problem, but this must be fixed, of course.
> 
> > 
> > So, add a mutex to the xillyusb_open() and xillyusb_disconnect()
> > functions to avoid race contidion.
> 
> I'm not very fond of this solution, partially because this mutex protects
> code and not data (There's this "Lock data, not code" rule, see [1]). Also,
> xillyusb_disconnect() can take a significant time to run, during which
> xillybus_open() for another (unrelated and still connected) XillyUSB device
> has to wait. I guess this demonstrates why protecting code with a mutex is
> considered bad practice.
> 
> Besides, there are already three mechanisms in place for preventing
> premature release of memory:
> 
> (1) @unit_mutex in xillybus_class.c, which protects @unit_list.
> (2) @kref inside struct xillyusb_dev (xillyusb.c), which protects the
> structure it resides in.
> (3) @error inside struct xillyusb_dev, which prevents xillybus_open() from
> opening a file that belongs to a device that is about to be released.
> 
> It's now apparent that they're not working well enough. Rather than adding
> another mutex, the existing mechanisms should be fixed.  Would you like to
> do this, or should I?

Thanks for the detailed feedback.
It's probably better for you to work on it.


Regards,
Hyunwoo Kim.

Re: [PATCH] char: xillybus: Fix use-after-free in xillyusb_open()

[Eli Billauer]

On 23/10/2022 17:26, Hyunwoo Kim wrote:
> It's probably better for you to work on it.

OK. I'll be back with a patch soon.

Regards,
    Eli