CVE-2026-53175

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.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/010c3313a4d178dc2d3ce958d2e5cb055e2864c1
• https://git.kernel.org/stable/c/0e823ca0e7391630784ae7dd0981b7ad170a93d9
• https://git.kernel.org/stable/c/32594b09854970d7ba83eb2dc8c69a2edd158c8e
• https://git.kernel.org/stable/c/89b909e9704587bfecc1aab1d37e98faee03b9f9
• https://git.kernel.org/stable/c/c22599cc90e1cd5f8129c8670bd68a02ff7177b4
• https://access.redhat.com/security/cve/CVE-2026-53175
• https://bugzilla.redhat.com/show_bug.cgi?id=2492840
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-53175.json