Linux kernel af_unix garbage collection race with MSG_PEEK
CVE-2026-23394 is a race condition in the Linux kernel AF_UNIX subsystem's garbage-collection logic. In af_unix, the garbage collector can incorrectly conclude that a socket strongly connected component (SCC) is dead when a concurrent recv() using MSG_PEEK temporarily increments a file reference count without properly synchronizing with garbage collection. The bug was reintroduced after earlier fixes when locking previously used in unix_peek_fds() was removed following changes to the GC algorithm. In the reported scenario, one socket in an SCC is closed but still reachable via another socket's receive queue; a concurrent MSG_PEEK on that queue bumps the referenced socket's file refcount, and a subsequent close() on the peer socket changes refcounts while GC is evaluating unix_vertex_dead() for both sockets. Because GC does not observe the MSG_PEEK refcount transition in time, it can misclassify both sockets as dead and purge the receive queue of a still-live socket. The fix uses seqcount_t and memory-ordering to signal MSG_PEEK interference so GC abandons collection of the SCC and defers it to a later run.
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