Executive brief
The Linux kernel was triggering unnecessary warning messages when certain types of network sockets (non-IPv4/IPv6 sockets) used the sk_mc_loop() function. These warnings were logged even though the behavior was correct and expected. Removing the spurious warning reduces unnecessary system log noise and prevents potential issues in systems that treat kernel warnings as alerts.
Technical details
The vulnerability is an incorrect WARN_ON_ONCE(1) macro in the sk_mc_loop() function in net/core/sock.c. When sk_mc_loop() is called on non-IPv4/non-IPv6 sockets (such as AF_PACKET sockets), the function falls through the switch statement and triggers the warning unconditionally. This is not a true vulnerability but rather a logic error: non-INET sockets do not support IP_MULTICAST_LOOP or IPV6_MULTICAST_LOOP socket options, so defaulting to true loopback behavior without warning is correct. The fix simply removes the spurious WARN_ON_ONCE() line, allowing the function to return true silently for unsupported socket families.
Affected products
- Linux Linux kernel multiple versions (identified in stable trees linux-5.x through linux-7.x and older branches)
Timeline
- 2026-08-04: other: Patch authored by Eric Dumazet
- 2026-08-19: other: Patch merged into stable branches
- 2026-09-03: disclosed: CVE-2026-80733 published