Re: [PATCH RFC v2] nfsd: prevent hung task in nfsd_nl_listener_set_doit()

#syz upstream

On sie 19, 2026 08:30, syzbot 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-3.6-flash Gemini:gemini-3.1-pro-preview syzbot
> Reported-by: [email protected]
> Closes: https://syzkaller.appspot.com/bug?extid=41bc60511c2884783c27
> Link: https://syzkaller.appspot.com/ai_job?id=30871f87-7a83-46b8-ab83-919894b0787b
> To: "Chuck Lever" <[email protected]>
> To: "Jeff Layton" <[email protected]>
> To: <[email protected]>
> To: "Lorenzo Bianconi" <[email protected]>
> Cc: "Dai Ngo" <[email protected]>
> Cc: <[email protected]>
> Cc: "NeilBrown" <[email protected]>
> Cc: "Olga Kornievskaia" <[email protected]>
> Cc: "Tom Talpey" <[email protected]>
> 
> ---
> v2:
> - Defined NFSD_MAX_LISTENERS macro instead of using a hardcoded constant.
> - Changed type of count variable to unsigned int.
> 
> v1:
> https://lore.kernel.org/all/[email protected]/T/
> ---
> 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, struct genl_info *info)
>  	return err;
>  }
>  
> +#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 = genl_info_net(info);
>  	struct svc_xprt *xprt, *tmp;
>  	const struct nlattr *attr;
> +	unsigned int count = 0;
>  	struct svc_serv *serv;
>  	LIST_HEAD(permsocks);
>  	struct nfsd_net *nn;
>  	bool delete = false;
>  	int err, rem;
>  
> +	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);
>  
>  	err = nfsd_create_serv(net);
> @@ -2073,8 +2082,10 @@ 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