CVE-2026-53175inet: frags: fix use-after-free caused by the fqdir_pre_exit() flush

Official description

In the Linux kernel, the following vulnerability has been resolved: inet: frags: fix use-after-free caused by the fqdir_pre_exit() flush On netns teardown, fqdir_pre_exit() walks the fqdir rhashtable and flushes every fragment queue that is not yet complete using inet_frag_queue_flush(). That helper frees all the skbs queued on the fragment queue but does not set INET_FRAG_COMPLETE, and leaves q->fragments_tail and q->last_run_head pointing at the freed skbs. The queue itself stays in the rhashtable. fqdir_pre_exit() first lowers high_thresh to 0 to stop new queue lookups, but it cannot stop a fragment that already obtained the queue through inet_frag_find() earlier and stalled just before taking the queue lock. Once that fragment resumes after the flush and takes the queue lock, it passes the INET_FRAG_COMPLETE check and then dereferences the freed fragments_tail. inet_frag_queue_insert() reads FRAG_CB() and ->len of that pointer and, on the append path, writes ->next_frag, causing a slab use-after-free. IPv6, nf_conntrack_reasm6 and 6lowpan reassembly share the same flush path and are affected as well. Reset rb_fragments, fragments_tail and last_run_head in inet_frag_queue_flush() so a flushed queue no longer points at the freed skbs. A fragment that resumes after the flush and takes the queue lock then finds an empty queue and starts a new run instead of dereferencing the freed fragments_tail. ip_frag_reinit() already performed this reset after its own flush, so drop the now duplicate code there.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/0e823ca0e7391630784ae7dd0981b7ad170a93d9
• https://git.kernel.org/stable/c/c22599cc90e1cd5f8129c8670bd68a02ff7177b4
• https://git.kernel.org/stable/c/89b909e9704587bfecc1aab1d37e98faee03b9f9
• https://git.kernel.org/stable/c/010c3313a4d178dc2d3ce958d2e5cb055e2864c1
• https://git.kernel.org/stable/c/32594b09854970d7ba83eb2dc8c69a2edd158c8e
• https://access.redhat.com/security/cve/CVE-2026-53175 vdb-entry, x_refsource_REDHAT
• RHBZ#2492840 issue-tracking, x_refsource_REDHAT
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-53175.json x_sadp-csaf-vex