CVE-2024-26921

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.

Notes