vxlan: keep the last remote linked during FDB flush

Overview

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

vxlan: keep the last remote linked during FDB flush

A non-nexthop FDB entry is expected to have at least one remote while it remains reachable through the FDB hash table. A filtered bulk flush violates this invariant when every remote matches: It unlinks the last remote in vxlan_fdb_dst_destroy() and only afterwards tells vxlan_flush() to destroy the parent FDB entry.

An RCU reader can find the parent during this interval. first_remote_rcu() then applies list_entry_rcu() to the empty list head, producing an invalid remote pointer that the receive learning path can read from and write to.

When a matching remote is the sole remaining remote, leave it linked and ask the caller to destroy the entire FDB entry. vxlan_fdb_destroy() keeps the remote attached while sending the deletion notification and removing the parent from the lookup structures.

Affected

• Linux >= c499fccb71cb85902b5c5b9ce9c9ae6683e54a8f < 2a7c2f00843225d5f037676bca649321f3d024c7
• Linux >= c499fccb71cb85902b5c5b9ce9c9ae6683e54a8f < a8820c8a7718327e96849782033e7c85a0f6bcfe
• Linux >= c499fccb71cb85902b5c5b9ce9c9ae6683e54a8f < 8ba68fd6cdd1e3c92b92f25c7e48bf7bd51a183c
• Linux >= c499fccb71cb85902b5c5b9ce9c9ae6683e54a8f < 4bbc76ee1b21d3bd045d6d819b2bacd30a6372ab
• Linux >= c499fccb71cb85902b5c5b9ce9c9ae6683e54a8f < d5d4a7b538b52db63927773a8905fcd9f78a42e2
• Linux 6.7

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.