CVE-2021-47634 ubi: Fix race condition between ctrl_cdev_ioctl and ubi_cdev_ioctl
A race condition in the Linux kernel UBI driver allows a local attacker to trigger a use-after-free, potentially leading to code execution or privilege escalation.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2021-47634?
CVE-2021-47634 is a high-severity use after free vulnerability in Linux kernel, published on 26 Feb 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
ubi: Fix race condition between ctrl_cdev_ioctl and ubi_cdev_ioctl
Hulk Robot reported a KASAN report about use-after-free: ================================================================== BUG: KASAN: use-after-free in __list_del_entry_valid+0x13d/0x160 Read of size 8 at addr ffff888035e37d98 by task ubiattach/1385 [...] Call Trace: klist_dec_and_del+0xa7/0x4a0 klist_put+0xc7/0x1a0 device_del+0x4d4/0xed0 cdev_device_del+0x1a/0x80 ubi_attach_mtd_dev+0x2951/0x34b0 [ubi] ctrl_cdev_ioctl+0x286/0x2f0 [ubi]
Allocated by task 1414: device_add+0x60a/0x18b0 cdev_device_add+0x103/0x170 ubi_create_volume+0x1118/0x1a10 [ubi] ubi_cdev_ioctl+0xb7f/0x1ba0 [ubi]
Freed by task 1385: cdev_device_del+0x1a/0x80 ubi_remove_volume+0x438/0x6c0 [ubi] ubi_cdev_ioctl+0xbf4/0x1ba0 [ubi] [...] ==================================================================
The lock held by ctrl_cdev_ioctl is ubi_devices_mutex, but the lock held by ubi_cdev_ioctl is ubi->device_mutex. Therefore, the two locks can be concurrent.
ctrl_cdev_ioctl contains two operations: ubi_attach and ubi_detach. ubi_detach is bug-free because it uses reference counting to prevent concurrency. However, uif_init and uif_close in ubi_attach may race with ubi_cdev_ioctl.
uif_init will race with ubi_cdev_ioctl as in the following stack. cpu1 cpu2 cpu3 _______________________|________________________|______________________ ctrl_cdev_ioctl ubi_attach_mtd_dev uif_init ubi_cdev_ioctl ubi_create_volume cdev_device_add ubi_add_volume // sysfs exist kill_volumes ubi_cdev_ioctl ubi_remove_volume cdev_device_del // first free ubi_free_volume cdev_del // double free cdev_device_del
And uif_close will race with ubi_cdev_ioctl as in the following stack. cpu1 cpu2 cpu3 _______________________|________________________|______________________ ctrl_cdev_ioctl ubi_attach_mtd_dev uif_init ubi_cdev_ioctl ubi_create_volume cdev_device_add ubi_debugfs_init_dev //error goto out_uif; uif_close kill_volumes ubi_cdev_ioctl ubi_remove_volume cdev_device_del // first free ubi_free_volume // double free
The cause of this problem is that commit 714fb87e8bc0 make device "available" before it becomes accessible via sysfs. Therefore, we roll back the modification. We will fix the race condition between ubi device creation and udev by removing ubi_get_device in vol_attribute_show and dev_attribute_show.This avoids accessing uninitialized ubi_devices[ubi_num].
ubi_get_device is used to prevent devices from being deleted during sysfs execution. However, now kernfs ensures that devices will not be deleted before all reference counting are released. The key process is shown in the following stack.
device_del device_remove_attrs device_remove_groups sysfs_remove_groups sysfs_remove_group remove_files kernfs_remove_by_name kernfs_remove_by_name_ns __kernfs_remove kernfs_drain
Weakness: CWE-416
Which versions are affected by CVE-2021-47634?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2021-47634?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2021-47634 being exploited?
There is no known exploitation of CVE-2021-47634 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as DEBIAN-CVE-2021-47634, GHSA-j4hc-7ppm-rrh4, UBUNTU-CVE-2021-47634.