CVE-2024-26921 — inet: inet_defrag: prevent sk release while still in use

Affected: Linux

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 ch…

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CVE-2024-26921 — inet: inet_defrag: prevent sk release while still in use