Overview
About vulnerability
In the Linux kernel, the following vulnerability has been resolved:
ax25: Fix refcount imbalance on inbound connections
When releasing a socket in ax25_release(), we call netdev_put() to decrease the refcount on the associated ax.25 device. However, the execution path for accepting an incoming connection never calls netdev_hold(). This imbalance leads to refcount errors, and ultimately to kernel crashes.
A typical call trace for the above situation will start with one of the following errors:
refcount_t: decrement hit 0; leaking memory. refcount_t: underflow; use-after-free.
And will then have a trace like:
Call Trace:
On reboot (or any attempt to remove the interface), the kernel gets stuck in an infinite loop:
unregister_netdevice: waiting for ax0 to become free. Usage count = 0
This patch corrects these issues by ensuring that we call netdev_hold() and ax25_dev_hold() for new connections in ax25_accept(). This makes the logic leading to ax25_accept() match the logic for ax25_bind(): in both cases we increment the refcount, which is ultimately decremented in ax25_release().
Details
- Affected product:
- Ubuntu 16.04 ELS , Ubuntu 18.04 ELS
- Affected packages:
- linux @ 4.15.0 (+2 more)
In the Linux kernel, the following vulnerability has been resolved:
ax25: Fix refcount imbalance on inbound connections
When releasing a socket in ax25_release(), we call netdev_put() to decrease the refcount on the associated ax.25 device. However, the execution path for accepting an incoming connection never calls netdev_hold(). This imbalance leads to refcount errors, and ultimately to kernel crashes.
A typical call trace for the above situation will start with one of the following errors:
refcount_t: decrement hit 0; leaking memory. refcount_t: underflow; use-after-free.
And will then have a trace like:
Call Trace:
On reboot (or any attempt to remove the interface), the kernel gets stuck in an infinite loop:
unregister_netdevice: waiting for ax0 to become free. Usage count = 0
This patch corrects these issues by ensuring that we call netdev_hold() and ax25_dev_hold() for new connections in ax25_accept(). This makes the logic leading to ax25_accept() match the logic for ax25_bind(): in both cases we increment the refcount, which is ultimately decremented in ax25_release().