From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id AB147382394; Fri, 29 May 2026 17:31:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780075910; cv=none; b=T/lu1h84EKzT0l012hbddnHlSZtbh42SJz/RPEsaCzPpPA7rwAZunvWU35bhiBJHwm752a1eUrbDcBMzA7nyZbBOaGT/bpur3MLAw3g10WEA2Ma6+3poK8hIzNQOLf4Jubcrv6dE9FZSYp5ilPRCfCMUxJf1DghFrzAZ60F1bjY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780075910; c=relaxed/simple; bh=MQMSQmWEhJFkzfIswyfhs/Mb21Bj5GyrWCZq5ZbzJ68=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=ZQ5djSy19nE5OS35/BxNZXZ/hC164G/iRs0VJs7EEnvtpCle05Tj7r1sbiP+VvZ/QaT7L4Z6I1okkHGbB2qcmbTiltH2VjA4hluAURKspkKYpejB+3MgTsjQP9SIfn8GuOXk3hEz2cv2Up3w6OXHAY5IX8hFxPsSq2QdyJdUH60= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Be9D30zC; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Be9D30zC" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 166361F00899; Fri, 29 May 2026 17:31:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780075909; bh=HsS54jvB99rkZSl4J6teqaQqg24anYU2BWbCptKFrbA=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=Be9D30zCxz61RFKEIE+Sl+YIDRVAaj0TMFKQEfCT0lfGPdepSLWWYpicT3IQEubmk CDAmmzlQSqtUa1vUulTIfTS975VdLItoJSEJzgGHTZWAlGWgT0vGf1ijqbnP75rUhA dsnBn5R8d40HVNvk6s+hCzNjGziOJ0BUB6cUYYrYmEgH4fWLl+7GCbV1oBDWrCJ2Ew O2Yocun3qDv2NQyDx9k5r0pcFirTNA2cZqt+Pw2gf24If23gCEXiRU3xn66/7K5KfG BULoTjNUiunCVM8C1UfCrkDoxLmma2/ye7fJIB5iJyPH9QD9AZEYhStgnWrEWoPsjd xcKO8IhMDKWZw== Message-ID: Subject: Re: [PATCH 02/10] nfsd: drain callbacks and clear cl_cb_session From: Jeff Layton To: Chuck Lever , Chuck Lever , NeilBrown , Olga Kornievskaia , Dai Ngo , Tom Talpey , "J. Bruce Fields" , Scott Mayhew , Trond Myklebust , Andreas Gruenbacher , Mike Snitzer , Rick Macklem Cc: Chris Mason , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Date: Fri, 29 May 2026 13:31:46 -0400 In-Reply-To: References: <20260528-nfsd-fixes-v1-0-e78708eff77d@kernel.org> <20260528-nfsd-fixes-v1-2-e78708eff77d@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ 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CHqumUHRry+RF0lH00clzX/W8OrQJZtoBPXv9ahka/Vp7kEulcBJr1cH5Wz/WprhsIM7U9pse1f1g Yy9YbXtWctUz8uvDR7shsQxAhX3qO7DilMtuGo1v97I/Kx4gXQ52syh/w6EBny71CZrOgD6kJwPVV AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.1 (3.60.1-1.fc44) Precedence: bulk X-Mailing-List: linux-nfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2026-05-29 at 11:13 -0400, Chuck Lever wrote: >=20 > On Thu, May 28, 2026, at 5:55 PM, Jeff Layton wrote: > > From: Chris Mason > >=20 > > 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. After the flush returns, > > nfsd4_destroy_session() proceeds through nfsd4_put_session_locked() > > and free_session() kfree()s the slab while rpciod's > > nfsd4_cb_sequence_done(), grab_slot(), and nfsd41_cb_release_slot() > > are still dereferencing cb->cb_clp->cl_cb_session. > >=20 > > 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 */ > >=20 > > 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 > >=20 > > if (!conn->cb_xprt || !ses) > > return -EINVAL; > >=20 > > so clp->cl_cb_session =3D ses never fires and the field retains a > > pointer to the about-to-be-freed session. Symmetrically, if a later > > probe finds a different session's backchannel conn and that > > setup_callback_client() call fails, the error tail must still scrub > > any previously published cl_cb_session. > >=20 > > Fix by mirroring the two-stage drain that nfsd4_shutdown_callback() > > already performs: call nfsd41_cb_inflight_wait_complete() in > > nfsd4_probe_callback_sync() after flush_workqueue() so rpciod-side > > nfsd41_cb_inflight_end() decrements are observed before the caller > > releases the final session reference. The two direct callers, > > nfsd4_destroy_session() and nfsd4_init_conn() (itself invoked from > > nfsd4_create_session() and nfsd4_bind_conn_to_session()), run in > > sleepable process context and tolerate the wait_var_event() sleep: > >=20 > > nfsd4_destroy_session() (fs/nfsd/nfs4state.c): > > unhash_session(ses); > > spin_unlock(&nn->client_lock); /* spinlock dropped */ > > nfsd4_probe_callback_sync(ses->se_client); > >=20 > > nfsd4_init_conn() (fs/nfsd/nfs4state.c): > > acquires no locks in its body; calls nfsd4_hash_conn(), > > nfsd4_register_conn(), then nfsd4_probe_callback_sync() -- > > entirely in sleepable process context. > >=20 > > Also clear clp->cl_cb_session unconditionally on the > > nfsd4_process_cb_update() error return so every > > setup_callback_client() failure -- whether c is NULL or points at a > > different session whose probe failed -- leaves the field NULL rather > > than pointing at a session that may subsequently be freed. > >=20 > > Fixes: dcbeaa68dbbd ("nfsd4: allow backchannel recovery") > > Assisted-by: kres:claude-opus-4-7 > > Signed-off-by: Chris Mason > > --- > > fs/nfsd/nfs4callback.c | 21 +++++++++++++++++---- > > 1 file changed, 17 insertions(+), 4 deletions(-) > >=20 > > diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c > > index 1964a213f80e..1cf6b6100357 100644 > > --- a/fs/nfsd/nfs4callback.c > > +++ b/fs/nfsd/nfs4callback.c > > @@ -1205,9 +1205,8 @@ static int setup_callback_client(struct=20 > > nfs4_client *clp, struct nfs4_cb_conn *c > > } else { > > if (!conn->cb_xprt || !ses) > > return -EINVAL; > > - clp->cl_cb_session =3D ses; > > args.bc_xprt =3D conn->cb_xprt; > > - args.prognumber =3D clp->cl_cb_session->se_cb_prog; > > + args.prognumber =3D ses->se_cb_prog; > > args.protocol =3D conn->cb_xprt->xpt_class->xcl_ident | > > XPRT_TRANSPORT_BC; > > args.authflavor =3D ses->se_cb_sec.flavor; > > @@ -1225,8 +1224,10 @@ static int setup_callback_client(struct=20 > > nfs4_client *clp, struct nfs4_cb_conn *c > > return -ENOMEM; > > } > >=20 > > - if (clp->cl_minorversion !=3D 0) > > + if (clp->cl_minorversion !=3D 0) { > > clp->cl_cb_conn.cb_xprt =3D conn->cb_xprt; > > + clp->cl_cb_session =3D ses; > > + } > > clp->cl_cb_client =3D client; > > clp->cl_cb_cred =3D cred; > > rcu_read_lock(); > > @@ -1299,6 +1300,7 @@ void nfsd4_probe_callback_sync(struct nfs4_client= *clp) > > { > > nfsd4_probe_callback(clp); > > flush_workqueue(clp->cl_callback_wq); > > + nfsd41_cb_inflight_wait_complete(clp); > > } > >=20 > > void nfsd4_change_callback(struct nfs4_client *clp, struct=20 > > nfs4_cb_conn *conn) > > @@ -1679,7 +1681,17 @@ static struct nfsd4_conn *=20 > > __nfsd4_find_backchannel(struct nfs4_client *clp) > > * Note there isn't a lot of locking in this code; instead we depend o= n > > * the fact that it is run from clp->cl_callback_wq, which won't run= =20 > > two > > * work items at once. So, for example, clp->cl_callback_wq handles= =20 > > all > > - * access of cl_cb_client and all calls to rpc_create or=20 > > rpc_shutdown_client. > > + * access of cl_cb_client and cl_cb_session, and all calls to=20 > > rpc_create > > + * or rpc_shutdown_client. > > + * > > + * rpciod-side readers of cl_cb_session (encode_cb_sequence4args(), > > + * nfsd4_cb_sequence_done(), the cb-slot helpers, and the cb_sequence > > + * tracepoints) run outside cl_callback_wq. The > > + * nfsd41_cb_inflight_wait_complete() drain in=20 > > nfsd4_probe_callback_sync() > > + * waits until cl_cb_inflight reaches zero before the caller proceeds= =20 > > with > > + * session teardown; any rpc_task that reads cl_cb_session must hold a= n > > + * inflight pin (via nfsd41_cb_inflight_begin) for this fence to be > > + * effective. > > */ > > static void nfsd4_process_cb_update(struct nfsd4_callback *cb) > > { > > @@ -1731,6 +1743,7 @@ static void nfsd4_process_cb_update(struct=20 > > nfsd4_callback *cb) > > nfsd4_mark_cb_down(clp); > > if (c) > > svc_xprt_put(c->cn_xprt); > > + clp->cl_cb_session =3D NULL; > > return; > > } > > } > >=20 > > --=20 > > 2.54.0 >=20 > Several NFSD callback done handlers retry indefinitely on > NFS4ERR_DELAY via rpc_delay(), so a client that keeps > replying DELAY leaves this per-client counter nonzero and > blocks the foreground CREATE/BIND/DESTROY_SESSION request > even though the callback no longer references the session > being torn down. >=20 > Although partly due to the way callbacks are structured > currently, this patch potentially introduces a client- > controlled DoS vector. >=20 Good point. I'm currently looking at reworking this so that each stage of the callback state machine can tolerate a NULL cl_cb_session pointer. If I can make that work, then we can fix the UAF without blocking. I'll definitely be sending a v2 of the series. --=20 Jeff Layton