CVE-2024-35910 tcp: properly terminate timers for kernel sockets
A use-after-free vulnerability in Linux kernel TCP timers allows attackers to cause a denial of service by triggering timer execution after network namespace teardown.
- 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-2024-35910?
CVE-2024-35910 is a high-severity use after free vulnerability in Debian Debian Linux, published on 19 May 2024. 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:
tcp: properly terminate timers for kernel sockets
We had various syzbot reports about tcp timers firing after the corresponding netns has been dismantled.
Fortunately Josef Bacik could trigger the issue more often, and could test a patch I wrote two years ago.
When TCP sockets are closed, we call inet_csk_clear_xmit_timers() to 'stop' the timers.
inet_csk_clear_xmit_timers() can be called from any context, including when socket lock is held. This is the reason it uses sk_stop_timer(), aka del_timer(). This means that ongoing timers might finish much later.
For user sockets, this is fine because each running timer holds a reference on the socket, and the user socket holds a reference on the netns.
For kernel sockets, we risk that the netns is freed before timer can complete, because kernel sockets do not hold reference on the netns.
This patch adds inet_csk_clear_xmit_timers_sync() function that using sk_stop_timer_sync() to make sure all timers are terminated before the kernel socket is released. Modules using kernel sockets close them in their netns exit() handler.
Also add sock_not_owned_by_me() helper to get LOCKDEP support : inet_csk_clear_xmit_timers_sync() must not be called while socket lock is held.
It is very possible we can revert in the future commit 3a58f13a881e ("net: rds: acquire refcount on TCP sockets") which attempted to solve the issue in rds only. (net/smc/af_smc.c and net/mptcp/subflow.c have similar code)
We probably can remove the check_net() tests from tcp_out_of_resources() and __tcp_close() in the future.
Which versions are affected by CVE-2024-35910?
It affects 2 products.
Products include Debian Debian Linux, Linux kernel.
How do I fix CVE-2024-35910?
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-2024-35910 being exploited?
There is no known exploitation of CVE-2024-35910 in the wild and no public exploit code.
Other vulnerabilities in Debian Debian Linux
Sources
Also known as BELL-CVE-2024-35910, DEBIAN-CVE-2024-35910, GHSA-f8qw-j52x-qgrm, UBUNTU-CVE-2024-35910.