In the Linux kernel, the following vulnerability has been resolved:
nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown
After a DESTROY_SESSION the per-session teardown path can free a
session while rpciod still holds an inflight callback rpc_task that
dereferences clp->cl_cb_session. nfsd4_probe_callback_sync() flushes
cl_callback_wq, but once nfsd4_run_cb_work() has called
rpc_call_async() the rpc_task lives on rpciod; flushing the workqueue
does not wait for it. rpc_shutdown_client() does drain rpciod tasks,
but uses a 1-second wait_event_timeout — tasks stuck in rpc_delay()
(e.g. 2-second NFS4ERR_DELAY retries) can outlive the drain.
<pre>
destroy path rpciod
------------ ------
unhash_session(ses)
nfsd4_probe_callback_sync(clp)
flush_workqueue(cl_callback_wq)
/* returns; rpc_task still live */
nfsd4_put_session_locked(ses)
free_session(ses) -> kfree(ses)
nfsd4_cb_sequence_done()
reads cb_clp->cl_cb_session
/* freed slab */
</pre>
A second window exists in nfsd4_process_cb_update(). When
__nfsd4_find_backchannel() returns NULL because unhash_session() has
already removed the destroyed session from cl_sessions,
setup_callback_client() takes the v4.1 early return so
clp->cl_cb_session = ses never fires and the field retains a pointer
to the about-to-be-freed session.
Fix both by converting cl_cb_session to an RCU-protected pointer:
-
Move the cl_cb_session = ses assignment in setup_callback_client()
to after rpc_create() succeeds, so it is only published when a
working backchannel exists. Clear cl_cb_session on the error
return in nfsd4_process_cb_update(). Both stores use
rcu_assign_pointer().
-
Annotate cl_cb_session with __rcu. All rpciod-side readers use
rcu_read_lock()/rcu_dereference() and check for NULL, bailing to
the appropriate error or requeue path:
encode_cb_sequence4args(), decode_cb_sequence4resok(),
nfsd41_cb_get_slot(), nfsd41_cb_release_slot(),
nfsd4_cb_prepare(), and nfsd4_cb_sequence_done().
-
Switch __free_session() from kfree() to kfree_rcu() so the
session slab is not reclaimed until after an RCU grace period,
guaranteeing that rpciod readers inside rcu_read_lock() never
dereference freed memory.
-
Pass the session pointer to the nfsd_cb_seq_status and
nfsd_cb_free_slot tracepoints instead of having them re-read
cl_cb_session.
-
nfsd4_cb_prepare() calls rpc_exit() when the session is NULL,
routing through the done/release path to requeue the callback.
CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS Score: 9.8
AV:N - The UAF is in nfsd NFSv4.1 session teardown (nfsd4_destroy_session) and the rpciod callback path (nfsd4_cb_sequence_done). DESTROY_SESSION and the inflight CB_COMPOUND run over TCP/2049, so a remote NFS client reaches the bug with no local access.
AC:L - The attacker controls both sides: they keep a callback rpc_task on rpciod (e.g. NFS4ERR_DELAY so rpc_delay is 2s) and send DESTROY_SESSION. flush_workqueue does not wait for rpciod, and rpc_shutdown_client's 1s drain can expire first, so the window is attacker-driven and retryable.
PR:N - Typical nfsd exports use AUTH_SYS/AUTH_NULL, so RPC identities are self-asserted. EXCHANGE_ID, CREATE_SESSION with a backchannel, OPEN, and DESTROY_SESSION succeed for any host allowed by the export; nfsd4_mach_creds_match is a no-op under default SP4_NONE. No local account is required.
UI:N - The attacking NFS client issues CREATE_SESSION, OPEN/CB_RECALL traffic, NFS4ERR_DELAY replies, and DESTROY_SESSION itself. No victim on the server must mount a filesystem, open a file, or take any other action.
S:U - The use-after-free corrupts a kmalloc'd nfsd4_session in the host nfsd kernel. Impact stays in that kernel security authority and does not cross a VM, IOMMU, or sandbox boundary.
C:H - After free_session kfree's the session, rpciod still reads se_sessionid and se_cb_seq_nr in encode_cb_sequence4args and decode_cb_sequence4resok. Reclaim of that slab lets the attacker disclose kernel memory via the callback they receive. Per kernel guidance a UAF is C:H.
I:H - The same dangling cl_cb_session is written by update_cb_slot_table (attacker-controlled target_highest_slotid), grab_slot/nfsd41_cb_release_slot (se_cb_slot_avail and se_lock), and nfsd4_cb_sequence_done (se_cb_seq_nr++). That is a UAF write primitive for heap corruption and control-flow hijack.
A:H - Use-after-free of struct nfsd4_session from an rpciod callback oopses or panics the NFS server even without full exploitation, and the attacker can repeat DESTROY_SESSION against inflight callbacks to deny service.
| Attack Vector |
Network |
Scope |
Unchanged |
| Attack Complexity |
Low |
Confidentiality Impact |
High |
| Privileges Required |
None |
Integrity Impact |
High |
| User Interaction |
None |
Availability Impact |
High |
AV:N - The UAF is in nfsd NFSv4.1 session teardown (nfsd4_destroy_session) and the rpciod callback path (nfsd4_cb_sequence_done). DESTROY_SESSION and the inflight CB_COMPOUND run over TCP/2049, so a remote NFS client reaches the bug with no local access.
AC:L - The attacker controls both sides: they keep a callback rpc_task on rpciod (e.g. NFS4ERR_DELAY so rpc_delay is 2s) and send DESTROY_SESSION. flush_workqueue does not wait for rpciod, and rpc_shutdown_client's 1s drain can expire first, so the window is attacker-driven and retryable.
PR:N - Typical nfsd exports use AUTH_SYS/AUTH_NULL, so RPC identities are self-asserted. EXCHANGE_ID, CREATE_SESSION with a backchannel, OPEN, and DESTROY_SESSION succeed for any host allowed by the export; nfsd4_mach_creds_match is a no-op under default SP4_NONE. No local account is required.
UI:N - The attacking NFS client issues CREATE_SESSION, OPEN/CB_RECALL traffic, NFS4ERR_DELAY replies, and DESTROY_SESSION itself. No victim on the server must mount a filesystem, open a file, or take any other action.
S:U - The use-after-free corrupts a kmalloc'd nfsd4_session in the host nfsd kernel. Impact stays in that kernel security authority and does not cross a VM, IOMMU, or sandbox boundary.
C:H - After free_session kfree's the session, rpciod still reads se_sessionid and se_cb_seq_nr in encode_cb_sequence4args and decode_cb_sequence4resok. Reclaim of that slab lets the attacker disclose kernel memory via the callback they receive. Per kernel guidance a UAF is C:H.
I:H - The same dangling cl_cb_session is written by update_cb_slot_table (attacker-controlled target_highest_slotid), grab_slot/nfsd41_cb_release_slot (se_cb_slot_avail and se_lock), and nfsd4_cb_sequence_done (se_cb_seq_nr++). That is a UAF write primitive for heap corruption and control-flow hijack.
A:H - Use-after-free of struct nfsd4_session from an rpciod callback oopses or panics the NFS server even without full exploitation, and the attacker can repeat DESTROY_SESSION against inflight callbacks to deny service.
CVSS 3.1