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.

Cc: stable@vger.kernel.org
Fixes: 006a5035b495 ("inet: frags: flush pending skbs in fqdir_pre_exit()")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Link: https://patch.msgid.link/ah6ukYq5G98LshdA@v4bel
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions