linux-cve-announce - CVE-2023-53585: bpf: reject unhashed sockets in bpf_sk

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Message-ID: <2025100425-CVE-2023-53585-b855@gregkh>
Date: Sat,  4 Oct 2025 17:51:39 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2023-53585: bpf: reject unhashed sockets in bpf_sk_assign

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

bpf: reject unhashed sockets in bpf_sk_assign

The semantics for bpf_sk_assign are as follows:

    sk = some_lookup_func()
    bpf_sk_assign(skb, sk)
    bpf_sk_release(sk)

That is, the sk is not consumed by bpf_sk_assign. The function
therefore needs to make sure that sk lives long enough to be
consumed from __inet_lookup_skb. The path through the stack for a
TCPv4 packet is roughly:

  netif_receive_skb_core: takes RCU read lock
    __netif_receive_skb_core:
      sch_handle_ingress:
        tcf_classify:
          bpf_sk_assign()
      deliver_ptype_list_skb:
        deliver_skb:
          ip_packet_type->func == ip_rcv:
            ip_rcv_core:
            ip_rcv_finish_core:
              dst_input:
                ip_local_deliver:
                  ip_local_deliver_finish:
                    ip_protocol_deliver_rcu:
                      tcp_v4_rcv:
                        __inet_lookup_skb:
                          skb_steal_sock

The existing helper takes advantage of the fact that everything
happens in the same RCU critical section: for sockets with
SOCK_RCU_FREE set bpf_sk_assign never takes a reference.
skb_steal_sock then checks SOCK_RCU_FREE again and does sock_put
if necessary.

This approach assumes that SOCK_RCU_FREE is never set on a sk
between bpf_sk_assign and skb_steal_sock, but this invariant is
violated by unhashed UDP sockets. A new UDP socket is created
in TCP_CLOSE state but without SOCK_RCU_FREE set. That flag is only
added in udp_lib_get_port() which happens when a socket is bound.

When bpf_sk_assign was added it wasn't possible to access unhashed
UDP sockets from BPF, so this wasn't a problem. This changed
in commit 0c48eefae712 ("sock_map: Lift socket state restriction
for datagram sockets"), but the helper wasn't adjusted accordingly.
The following sequence of events will therefore lead to a refcount
leak:

1. Add socket(AF_INET, SOCK_DGRAM) to a sockmap.
2. Pull socket out of sockmap and bpf_sk_assign it. Since
   SOCK_RCU_FREE is not set we increment the refcount.
3. bind() or connect() the socket, setting SOCK_RCU_FREE.
4. skb_steal_sock will now set refcounted = false due to
   SOCK_RCU_FREE.
5. tcp_v4_rcv() skips sock_put().

Fix the problem by rejecting unhashed sockets in bpf_sk_assign().
This matches the behaviour of __inet_lookup_skb which is ultimately
the goal of bpf_sk_assign().

The Linux kernel CVE team has assigned CVE-2023-53585 to this issue.


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

	Issue introduced in 5.7 with commit cf7fbe660f2dbd738ab58aea8e9b0ca6ad232449 and fixed in 5.10.195 with commit 791a12102e5191dcb6ce0b3a99d71b5a2802d12a
	Issue introduced in 5.7 with commit cf7fbe660f2dbd738ab58aea8e9b0ca6ad232449 and fixed in 5.15.132 with commit 7dcbc0bb0e5cc1823923744befce59ac353135e6
	Issue introduced in 5.7 with commit cf7fbe660f2dbd738ab58aea8e9b0ca6ad232449 and fixed in 6.1.53 with commit c0ce0fb76610d5fad31f56f2ca8241a2a6717a1b
	Issue introduced in 5.7 with commit cf7fbe660f2dbd738ab58aea8e9b0ca6ad232449 and fixed in 6.4.16 with commit 8aa43cfbb68b25119d2ced14ec717173e2901fa2
	Issue introduced in 5.7 with commit cf7fbe660f2dbd738ab58aea8e9b0ca6ad232449 and fixed in 6.5.3 with commit 3d4522f59fb748a54446846522941a4f09da63e9
	Issue introduced in 5.7 with commit cf7fbe660f2dbd738ab58aea8e9b0ca6ad232449 and fixed in 6.6 with commit 67312adc96b5a585970d03b62412847afe2c6b01

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-2023-53585
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:
	net/core/filter.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/791a12102e5191dcb6ce0b3a99d71b5a2802d12a
	https://git.kernel.org/stable/c/7dcbc0bb0e5cc1823923744befce59ac353135e6
	https://git.kernel.org/stable/c/c0ce0fb76610d5fad31f56f2ca8241a2a6717a1b
	https://git.kernel.org/stable/c/8aa43cfbb68b25119d2ced14ec717173e2901fa2
	https://git.kernel.org/stable/c/3d4522f59fb748a54446846522941a4f09da63e9
	https://git.kernel.org/stable/c/67312adc96b5a585970d03b62412847afe2c6b01