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 ...

Classification

CVE ID: CVE-2024-26921

Affected Products

Vendor: Linux

Product: Linux

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.04% (probability of being exploited)

EPSS Percentile: 12.38% (scored less or equal to compared to others)

EPSS Date: 2025-02-03 (when was this score calculated)

References

https://git.kernel.org/stable/c/1b6de5e6575b56502665c65cf93b0ae6aa0f51ab
https://git.kernel.org/stable/c/9705f447bf9a6cd088300ad2c407b5e1c6591091
https://git.kernel.org/stable/c/4318608dc28ef184158b4045896740716bea23f0
https://git.kernel.org/stable/c/7d0567842b78390dd9b60f00f1d8f838d540e325
https://git.kernel.org/stable/c/f4877225313d474659ee53150ccc3d553a978727
https://git.kernel.org/stable/c/e09cbe017311508c21e0739e97198a8388b98981
https://git.kernel.org/stable/c/18685451fc4e546fc0e718580d32df3c0e5c8272

Timeline