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: __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" To: "Jeff Layton" To: To: "Lorenzo Bianconi" Cc: "Dai Ngo" Cc: Cc: "NeilBrown" Cc: "Olga Kornievskaia" Cc: "Tom Talpey" --- 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; + + nlmsg_for_each_attr_type(attr, NFSD_A_SERVER_SOCK_ADDR, info->nlhdr, + GENL_HDRLEN, rem) { + if (++count > 128) + 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 -- This is an AI-generated patch subject to moderation. Reply with '#syz upstream' to Sign-off the patch as a human author and send it to the upstream kernel mailing lists. Reply with '#syz reject' to reject it ('#syz unreject' to undo). See https://goo.gle/syzbot-ai-patches for information about AI-generated patches. The person who has signed off on the patch is responsible for addressing comments. syzbot engineers can be reached at syzkaller@googlegroups.com.