CVE-2024-26921 inet: inet_defrag: prevent sk release while still in use
A use-after-free vulnerability in the Linux kernel's IP defragmentation allows attackers to cause denial of service or potentially execute code.
- 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-2024-26921?
CVE-2024-26921 is a high-severity use after free vulnerability in Linux kernel, published on 18 Apr 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:
inet: inet_defrag: prevent sk release while still in use
ip_local_out() and other functions can pass skb->sk as function argument.
If the skb is a fragment and reassembly happens before such function call returns, the sk must not be released.
This affects skb fragments reassembled via netfilter or similar modules, e.g. openvswitch or ct_act.c, when run as part of tx pipeline.
Eric Dumazet made an initial analysis of this bug. Quoting Eric: Calling ip_defrag() in output path is also implying skb_orphan(), which is buggy because output path relies on sk not disappearing.
A relevant old patch about the issue was : 8282f27449bf ("inet: frag: Always orphan skbs inside ip_defrag()")
[..]
net/ipv4/ip_output.c depends on skb->sk being set, and probably to an inet socket, not an arbitrary one.
If we orphan the packet in ipvlan, then downstream things like FQ packet scheduler will not work properly.
We need to change ip_defrag() to only use skb_orphan() when really needed, ie whenever frag_list is going to be used.
Eric suggested to stash sk in fragment queue and made an initial patch. However there is a problem with this:
If skb is refragmented again right after, ip_do_fragment() will copy head->sk to the new fragments, and sets up destructor to sock_wfree. IOW, we have no choice but to fix up sk_wmem accouting to reflect the fully reassembled skb, else wmem will underflow.
This change moves the orphan down into the core, to last possible moment. As ip_defrag_offset is aliased with sk_buff->sk member, we must move the offset into the FRAG_CB, else skb->sk gets clobbered.
This allows to delay the orphaning long enough to learn if the skb has to be queued or if the skb is completing the reasm queue.
In the former case, things work as before, skb is orphaned. This is safe because skb gets queued/stolen and won't continue past reasm engine.
In the latter case, we will steal the skb->sk reference, reattach it to the head skb, and fix up wmem accouting when inet_frag inflates truesize.
Which versions are affected by CVE-2024-26921?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2024-26921?
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-2024-26921 being exploited?
There is no known exploitation of CVE-2024-26921 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2024-26921, DEBIAN-CVE-2024-26921, GHSA-4vg2-58j7-gmw7, UBUNTU-CVE-2024-26921.