In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown<\/p>\n
After a DESTROY_SESSION the per-session teardown path can free a
\nsession while rpciod still holds an inflight callback rpc_task that
\ndereferences clp->cl_cb_session. nfsd4_probe_callback_sync() flushes
\ncl_callback_wq, but once nfsd4_run_cb_work() has called
\nrpc_call_async() the rpc_task lives on rpciod; flushing the workqueue
\ndoes not wait for it. rpc_shutdown_client() does drain rpciod tasks,
\nbut uses a 1-second wait_event_timeout \u2014 tasks stuck in rpc_delay()
\n(e.g. 2-second NFS4ERR_DELAY retries) can outlive the drain.<\/p>\n
destroy path rpciod
\n ———— ——
\n unhash_session(ses)
\n nfsd4_probe_callback_sync(clp)
\n flush_workqueue(cl_callback_wq)
\n \/* returns; rpc_task still live *\/
\n nfsd4_put_session_locked(ses)
\n free_session(ses) -> kfree(ses)
\n nfsd4_cb_sequence_done()
\n reads cb_clp->cl_cb_session
\n \/* freed slab *\/<\/p>\n
A second window exists in nfsd4_process_cb_update(). When
\n__nfsd4_find_backchannel() returns NULL because unhash_session() has
\nalready removed the destroyed session from cl_sessions,
\nsetup_callback_client() takes the v4.1 early return so
\nclp->cl_cb_session = ses never fires and the field retains a pointer
\nto the about-to-be-freed session.<\/p>\n
Fix both by converting cl_cb_session to an RCU-protected pointer:<\/p>\n
– Move the cl_cb_session = ses assignment in setup_callback_client()
\n to after rpc_create() succeeds, so it is only published when a
\n working backchannel exists. Clear cl_cb_session on the error
\n return in nfsd4_process_cb_update(). Both stores use
\n rcu_assign_pointer().<\/p>\n
– Annotate cl_cb_session with __rcu. All rpciod-side readers use
\n rcu_read_lock()\/rcu_dereference() and check for NULL, bailing to
\n the appropriate error or requeue path:
\n encode_cb_sequence4args(), decode_cb_sequence4resok(),
\n nfsd41_cb_get_slot(), nfsd41_cb_release_slot(),
\n nfsd4_cb_prepare(), and nfsd4_cb_sequence_done().<\/p>\n
– Switch __free_session() from kfree() to kfree_rcu() so the
\n session slab is not reclaimed until after an RCU grace period,
\n guaranteeing that rpciod readers inside rcu_read_lock() never
\n dereference freed memory.<\/p>\n
– Pass the session pointer to the nfsd_cb_seq_status and
\n nfsd_cb_free_slot tracepoints instead of having them re-read
\n cl_cb_session.<\/p>\n
– nfsd4_cb_prepare() calls rpc_exit() when the session is NULL,
\n routing through the done\/release path to requeue the callback.<\/p>\n","protected":false},"excerpt":{"rendered":"
In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown<\/p>\n
After a DESTROY_SESSION the per-session teardown path ca<\/p>\n","protected":false},"template":"","meta":[],"categories":[],"tags":[],"class_list":["post-21630","vuln","type-vuln","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln\/21630","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln"}],"about":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/types\/vuln"}],"wp:attachment":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=21630"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=21630"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=21630"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}