Re: [PATCH RFC] nfsd: prevent hung task in nfsd_nl_listener_set_doit()

[Slawomir Stepien <sst@poczta.fm>]

On sie 07, 2026 13:35, 'syzbot' via syzkaller-upstream-moderation wrote:
> 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.
> 
> 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:
> 
>   INFO: task blocked for more than 10 seconds.
>   ...
>   Call Trace:
>    <TASK>
>    __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
> 
> 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 list
> for each attribute.
> 
> Address this by introducing a hard limit of 128 NFSD_A_SERVER_SOCK_ADDR
> attributes per netlink message. The limit is enforced before acquiring the
> nfsd_mutex to prevent lock contention. Additionally, modify the loop to
> break immediately if listener creation fails, avoiding needless sequential
> request_module() calls for invalid configurations.
> 
> Fixes: 16a471177496 ("NFSD: add listener-{set,get} netlink command")
> Assisted-by: Gemini:gemini-3.5-flash Gemini:gemini-3.1-pro-preview syzbot
> Reported-by: syzbot+41bc60511c2884783c27@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=41bc60511c2884783c27
> Link: https://syzkaller.appspot.com/ai_job?id=e7446d8c-5a41-495a-8af7-9f0d7feddff1
> To: "Chuck Lever" <cel@kernel.org>
> To: "Jeff Layton" <jlayton@kernel.org>
> To: <linux-nfs@vger.kernel.org>
> To: "Lorenzo Bianconi" <lorenzo@kernel.org>
> Cc: "Dai Ngo" <Dai.Ngo@oracle.com>
> Cc: <linux-kernel@vger.kernel.org>
> Cc: "NeilBrown" <neil@brown.name>
> Cc: "Olga Kornievskaia" <okorniev@redhat.com>
> Cc: "Tom Talpey" <tom@talpey.com>
> 
> ---
> diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
> index fa92e31d1..9c1cc02fc 100644
> --- a/fs/nfsd/nfsctl.c
> +++ b/fs/nfsd/nfsctl.c
> @@ -1960,6 +1960,13 @@ int nfsd_nl_listener_set_doit(struct sk_buff *skb, struct genl_info *info)
>  	struct nfsd_net *nn;
>  	bool delete = false;
>  	int err, rem;
> +	int count = 0;

The count can't be negative, so maybe use some unsigned type here?

> +
> +	nlmsg_for_each_attr_type(attr, NFSD_A_SERVER_SOCK_ADDR, info->nlhdr,
> +				 GENL_HDRLEN, rem) {
> +		if (++count > 128)

Can you make some nicely named #define with this 128 value?

> +			return -EINVAL;
> +	}
>  
>  	mutex_lock(&nfsd_mutex);
>  
> @@ -2073,8 +2080,10 @@ int nfsd_nl_listener_set_doit(struct sk_buff *skb, struct genl_info *info)
>  		ret = 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 = ret;
> +			break;
> +		}
>  	}
>  
>  	if (!serv->sv_nrthreads && list_empty(&nn->nfsd_serv->sv_permsocks))
> 
> 
> base-commit: 075b74841bd0065a3bda3440873c747938e69b68

-- 
Slawomir Stepien