# \[PATCH\] usb: atm: cxacru: fix NULL pointer dereference on uninitialized atm\_dev

- From: syzbot \<syzbot@kernel.org\>
- Date: 2026-08-31 14:40:43 
- List: netdev
- Page: https://ratatoskr.run/netdev/2026/08/17485259
- Thread: https://ratatoskr.run/linux-usb/2026/08/17485259/t.md

```text
From: "Wang, Jie" <redacted>

The cxacru driver uses the usbatm framework, which defers the ATM device
registration to a kernel thread (usbatm_do_heavy_init()) but immediately
returns success from the USB probe. Because the probe returns success, the
driver core creates sysfs attribute files (like adsl_state and
adsl_config), making them accessible to userspace before the ATM device is
fully initialized.

If a user writes to these sysfs files before the initialization is complete
or if it fails, the driver attempts to send a command to the device. If the
command fails, the error handling path calls atm_err(), which
unconditionally dereferences instance->usbatm->atm_dev. Since atm_dev is
NULL, this leads to a NULL pointer dereference:

Oops: general protection fault, probably for non-canonical address
0xdffffc0000000002: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
RIP: 0010:adsl_state_store+0x5cc/0x770 drivers/usb/atm/cxacru.c:359
Call Trace:
 <TASK>
 kernfs_fop_write_iter+0x3a4/0x540 fs/kernfs/file.c:345
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x612/0xba0 fs/read_write.c:687
 ksys_write+0x150/0x270 fs/read_write.c:739
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
 </TASK>

To fix this, add checks for instance->usbatm->atm_dev == NULL in
adsl_state_store() and adsl_config_store(). This prevents the functions
from proceeding and dereferencing the uninitialized atm_dev, consistent
with how mac_address_show() handles the same race condition.

Fixes: e605c30977bb ("USB: atm: cxacru: convert to use dev_groups")
Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: <redacted>
Closes: https://syzkaller.appspot.com/bug?extid=9b195c4f412ea5c4e56a
Link: https://syzkaller.appspot.com/ai_job?id=b6352d63-d03a-4e87-92a4-b334c3ebcf97
Signed-off-by: "Wang, Jie" <redacted>

---
diff --git a/drivers/usb/atm/cxacru.c b/drivers/usb/atm/cxacru.c
index 429ac20a8..dee15f7b7 100644
--- a/drivers/usb/atm/cxacru.c
+++ b/drivers/usb/atm/cxacru.c
@@ -347,7 +347,7 @@ static ssize_t adsl_state_store(struct device *dev,
 		return -EINVAL;
 	ret = 0;
 
-	if (instance == NULL)
+	if (instance == NULL || instance->usbatm->atm_dev == NULL)
 		return -ENODEV;
 
 	if (mutex_lock_interruptible(&instance->adsl_state_serialize))
@@ -444,7 +444,7 @@ static ssize_t adsl_config_store(struct device *dev,
 	if (!capable(CAP_NET_ADMIN))
 		return -EACCES;
 
-	if (instance == NULL)
+	if (instance == NULL || instance->usbatm->atm_dev == NULL)
 		return -ENODEV;
 
 	pos = 0;


base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f
-- 
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```