CVE-2024-56644

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

net/ipv6: release expired exception dst cached in socket

Dst objects get leaked in ip6_negative_advice() when this function is

executed for an expired IPv6 route located in the exception table. There

are several conditions that must be fulfilled for the leak to occur:

* an ICMPv6 packet indicating a change of the MTU for the path is received,

resulting in an exception dst being created

* a TCP connection that uses the exception dst for routing packets must

start timing out so that TCP begins retransmissions

* after the exception dst expires, the FIB6 garbage collector must not run

before TCP executes ip6_negative_advice() for the expired exception dst

When TCP executes ip6_negative_advice() for an exception dst that has

expired and if no other socket holds a reference to the exception dst, the

refcount of the exception dst is 2, which corresponds to the increment

made by dst_init() and the increment made by the TCP socket for which the

connection is timing out. The refcount made by the socket is never

released. The refcount of the dst is decremented in sk_dst_reset() but

that decrement is counteracted by a dst_hold() intentionally placed just

before the sk_dst_reset() in ip6_negative_advice(). After

ip6_negative_advice() has finished, there is no other object tied to the

dst. The socket lost its reference stored in sk_dst_cache and the dst is

no longer in the exception table. The exception dst becomes a leaked

object.

As a result of this dst leak, an unbalanced refcount is reported for the

loopback device of a net namespace being destroyed under kernels that do

not contain e5f80fcf869a ("ipv6: give an IPv6 dev to blackhole_netdev"):

unregister_netdevice: waiting for lo to become free. Usage count = 2

Fix the dst leak by removing the dst_hold() in ip6_negative_advice(). The

patch that introduced the dst_hold() in ip6_negative_advice() was

92f1655aa2b22 ("net: fix __dst_negative_advice() race"). But 92f1655aa2b22

merely refactored the code with regards to the dst refcount so the issue

was present even before 92f1655aa2b22. The bug was introduced in

54c1a859efd9f ("ipv6: Don't drop cache route entry unless timer actually

expired.") where the expired cached route is deleted and the sk_dst_cache

member of the socket is set to NULL by calling dst_negative_advice() but

the refcount belonging to the socket is left unbalanced.

The IPv4 version - ipv4_negative_advice() - is not affected by this bug.

When the TCP connection times out ipv4_negative_advice() merely resets the

sk_dst_cache of the socket while decrementing the refcount of the

exception dst.