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 DC617438012; Thu, 20 Aug 2026 11:37:15 +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=1787225847; cv=none; b=T6HuaP21FXgdZLF3/VCOUHnH3bTKrIEF2aF1Zmw5GR5F2qyR4/bW4Ze5k4CJlLxpc6LZRB69BkRvtTdVcEOivtSyZ1S4bQWgiSTde6abdPghaJbq67CLPTQfnAYYpUt8loGiYwGMBkJm2f1vut9ACeA5Fr+c+r79QAdPRvamLDo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787225847; c=relaxed/simple; bh=oqTEbapJ7m7OJal1cTCQ+CBSUzwlIduS9wikKb3wDiw=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=H6PV8N0kwd4Bweb5sBJz4FtvcC2x6iAmyuCLObU10DRskWLfI1/Oy5Ey4hq3uLIS10Gf1a5YWopd1GeuXo6kA10ZK0lDYMs6miF9mt8JNJRzBCFZEA8a/oM0mFZPvfMpA2Bh3GNVD3JuEdi4JVtiCqYdhhwQ4mbWjsngoEupJ3E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GfgXJkGA; 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="GfgXJkGA" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F19F61F00A3F; Thu, 20 Aug 2026 11:37:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1787225834; bh=blFnMC+DqJzzIMmp8d38EvkiAJPI2DffUzalk7L15Ro=; h=Subject:From:To:Cc:Date:In-Reply-To:References; b=GfgXJkGATjfRyfwTXEMZKd41dS9xb5UI9heYlSXnKuZEceP6aEPnrYhD1aXzDHVcU GF4D/ER/Q3o3dcm3ePSJVy8InDBw0TG/mv5vZ6+lK7LPyJ0TKOqik0fuKsZzUxS0pm 4hdCaUqWvK/cNsuMgpbu4inZ95BIe/zVIjEa5DvzD7r/4kF/SdsFgpwITMFOpRj9pY +qOJ5EGG/gxrJrEW1B/i4A82pvvEAuSZjT9H7q3MoACauagf6YsGjHY1KtVrCXiXdz qE+/yuelGTzQhe7Sm3N1S8CFsozf3YOMdgjV1wutGq2Q3EdizfcfGriQ8aq985wRRA SWdYyRx6eKC3w== Message-ID: <699bacb503758fbe2ebf01f3e8aca95e3eee4128.camel@kernel.org> Subject: Re: [PATCH] nfsd: prevent hung task in nfsd_nl_listener_set_doit() From: Jeff Layton To: syzbot , syzkaller-bugs@googlegroups.com, Slawomir Stepien , Chuck Lever , linux-nfs@vger.kernel.org, Lorenzo Bianconi Cc: Dai.Ngo@oracle.com, linux-kernel@vger.kernel.org, neil@brown.name, okorniev@redhat.com, syzbot@lists.linux.dev, tom@talpey.com Date: Thu, 20 Aug 2026 07:37:12 -0400 In-Reply-To: <4601ffd5-c662-4ab1-ab2f-50a67a0a990e@mail.kernel.org> References: <4601ffd5-c662-4ab1-ab2f-50a67a0a990e@mail.kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Thu, 2026-08-20 at 06:55 +0000, syzbot wrote: > From: Slawomir Stepien >=20 > In nfsd_nl_listener_set_doit(), the kernel iterates over all > NFSD_A_SERVER_SOCK_ADDR attributes provided in a netlink message to > configure NFS server listeners. There is currently no limit on the number > of attributes a user can send. >=20 > For each attribute, svc_xprt_create_from_sa() is called, which may > synchronously invoke request_module() to load the corresponding transport > module. If a user provides a large number of invalid transport names, > request_module() is called sequentially for each, taking a massive amount > of time. Since this entire process occurs while holding the global > nfsd_mutex, it blocks other tasks attempting to acquire the mutex and > triggers a hung task timeout: >=20 My LLM latched onto the same explanation, but the request module upcalls are actually quite quick and have some other gates on them that make this problem hard to hit in practice. The real problem is rpcbind registration, which can be quite slow. The fix for that is to make the registration asynchronous, and to not trigger errors on rpcbind registration failure. That's a bigger project, but let us know if you're interested! In any case, I've sent a series that includes a similar fix to this one, along with some others: https://lore.kernel.org/linux-nfs/20260811-nfsd-nl-hang-v2-0-c0c92b3953= c3@kernel.org/ I'll try to get the follow-on series out later today or tomorrow. > INFO: task blocked for more than 10 seconds. > ... > Call Trace: > > __schedule+0x17e7/0x5630 kernel/sched/core.c:7234 > schedule+0x164/0x2b0 kernel/sched/core.c:7326 > schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7383 > __mutex_lock_common kernel/locking/mutex.c:726 [inline] > __mutex_lock+0x7bf/0x1550 kernel/locking/mutex.c:821 > nfsd_nl_version_get_doit+0x17c/0xd20 fs/nfsd/nfsctl.c:1889 > genl_family_rcv_msg_doit+0x233/0x340 net/netlink/genetlink.c:1114 > genl_family_rcv_msg net/netlink/genetlink.c:1194 [inline] > genl_rcv_msg+0x614/0x7a0 net/netlink/genetlink.c:1209 > ... > 2 locks held by task/5864: > #0: ffffffff90114168 (cb_lock){++++}-{4:4}, at: genl_rcv+0x19/0x40 > net/netlink/genetlink.c:1217 > #1: ffffffff8eeb1a60 (nfsd_mutex){+.+.}-{4:4}, at: > nfsd_nl_listener_set_doit+0x135/0x1750 fs/nfsd/nfsctl.c:1964 >=20 > Furthermore, the function does not break out of the loop if > svc_xprt_create_from_sa() fails, and it suffers from an O(N^2) complexity > issue because svc_find_listener() iterates over the serv->sv_permsocks li= st > for each attribute. >=20 > Address this by introducing a hard limit of 128 NFSD_A_SERVER_SOCK_ADDR > attributes per netlink message. The limit is enforced before acquiring th= e > nfsd_mutex to prevent lock contention. Additionally, modify the loop to > break immediately if listener creation fails, avoiding needless sequentia= l > request_module() calls for invalid configurations. >=20 > Fixes: 16a471177496 ("NFSD: add listener-{set,get} netlink command") > Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot > Reported-by: syzbot+41bc60511c2884783c27@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=3D41bc60511c2884783c27 > Link: https://syzkaller.appspot.com/ai_job?id=3D30871f87-7a83-46b8-ab83-9= 19894b0787b > Signed-off-by: Slawomir Stepien > > --- > diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c > index fa92e31d1..516c04c14 100644 > --- a/fs/nfsd/nfsctl.c > +++ b/fs/nfsd/nfsctl.c > @@ -1943,6 +1943,8 @@ int nfsd_nl_version_get_doit(struct sk_buff *skb, s= truct genl_info *info) > return err; > } > =20 > +#define NFSD_MAX_LISTENERS 128 > + > /** > * nfsd_nl_listener_set_doit - set the nfs running sockets > * @skb: reply buffer > @@ -1955,12 +1957,19 @@ int nfsd_nl_listener_set_doit(struct sk_buff *skb= , struct genl_info *info) > struct net *net =3D genl_info_net(info); > struct svc_xprt *xprt, *tmp; > const struct nlattr *attr; > + unsigned int count =3D 0; > struct svc_serv *serv; > LIST_HEAD(permsocks); > struct nfsd_net *nn; > bool delete =3D false; > int err, rem; > =20 > + nlmsg_for_each_attr_type(attr, NFSD_A_SERVER_SOCK_ADDR, info->nlhdr, > + GENL_HDRLEN, rem) { > + if (++count > NFSD_MAX_LISTENERS) > + return -EINVAL; > + } > + > mutex_lock(&nfsd_mutex); > =20 > err =3D nfsd_create_serv(net); > @@ -2073,8 +2082,10 @@ int nfsd_nl_listener_set_doit(struct sk_buff *skb,= struct genl_info *info) > ret =3D svc_xprt_create_from_sa(serv, xcl_name, net, sa, 0, > current_cred()); > /* always save the latest error */ > - if (ret < 0) > + if (ret < 0) { > err =3D ret; > + break; > + } > } > =20 > if (!serv->sv_nrthreads && list_empty(&nn->nfsd_serv->sv_permsocks)) >=20 >=20 > base-commit: 075b74841bd0065a3bda3440873c747938e69b68 I have a patch series in progress for this already, which also=20 --=20 Jeff Layton