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

From: Greg Kroah-Hartman <[email-masked]>

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.

The Linux kernel CVE team has assigned CVE-2024-26921 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 4.1 with commit 7026b1ddb6b8 and fixed in 5.10.227 with commit 9705f447bf9a
	Issue introduced in 4.1 with commit 7026b1ddb6b8 and fixed in 5.15.168 with commit 4318608dc28e
	Issue introduced in 4.1 with commit 7026b1ddb6b8 and fixed in 6.1.85 with commit 7d0567842b78
	Issue introduced in 4.1 with commit 7026b1ddb6b8 and fixed in 6.6.26 with commit f4877225313d
	Issue introduced in 4.1 with commit 7026b1ddb6b8 and fixed in 6.8.5 with commit e09cbe017311
	Issue introduced in 4.1 with commit 7026b1ddb6b8 and fixed in 6.9 with commit 18685451fc4e

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-26921
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	include/linux/skbuff.h
	net/ipv4/inet_fragment.c
	net/ipv4/ip_fragment.c
	net/ipv6/netfilter/nf_conntrack_reasm.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	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