CVE-2024-40910

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:
<TASK>
? show_regs+0x64/0x70
? __warn+0x83/0x120
? refcount_warn_saturate+0xb2/0x100
? report_bug+0x158/0x190
? prb_read_valid+0x20/0x30
? handle_bug+0x3e/0x70
? exc_invalid_op+0x1c/0x70
? asm_exc_invalid_op+0x1f/0x30
? refcount_warn_saturate+0xb2/0x100
? refcount_warn_saturate+0xb2/0x100
ax25_release+0x2ad/0x360
__sock_release+0x35/0xa0
sock_close+0x19/0x20
[...]

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().

Affected Software

References

• https://git.kernel.org/stable/c/3c34fb0bd4a4237592c5ecb5b2e2531900c55774
• https://git.kernel.org/stable/c/52100fd74ad07b53a4666feafff1cd11436362d3
• https://git.kernel.org/stable/c/a723a6c8d4831cc8e2c7b0c9f3f0c010d4671964
• https://git.kernel.org/stable/c/f4df9d6c8d4e4c818252b0419c2165d66eabd4eb
• https://git.kernel.org/stable/c/3c34fb0bd4a4237592c5ecb5b2e2531900c55774
• https://git.kernel.org/stable/c/52100fd74ad07b53a4666feafff1cd11436362d3
• https://git.kernel.org/stable/c/a723a6c8d4831cc8e2c7b0c9f3f0c010d4671964
• https://git.kernel.org/stable/c/f4df9d6c8d4e4c818252b0419c2165d66eabd4eb
• https://lists.debian.org/debian-lts-announce/2025/01/msg00001.html