WARNING in svc

Based on kernel commit 05441d697f7f:

Technical Root Cause Analysis

Involved Components

Source Files: fs/nfs/callback.c, net/sunrpc/svc.c, net/sunrpc/svc_xprt.c
Functions: nfs4_callback_up_net(), nfs_callback_up_net(), nfs_callback_up(), svc_destroy(), svc_xprt_create(), svc_add_new_perm_xprt()
Data Structures and Fields:
- struct svc_serv: fields sv_permsocks, sv_pools
- struct svc_xprt: fields xpt_list, xpt_server, xpt_flags

Root Cause and Trigger Mechanism

Callback Service Initialization: During NFSv4.0 mount/client setup, nfs_callback_up() is invoked to bring up the callback service. It creates a server instance (struct svc_serv) via nfs_callback_create_svc(minorversion) and attempts to configure per-network-namespace callback transports by calling nfs_callback_up_net(minorversion, serv, net, xprt).
Creation of Listeners in nfs4_callback_up_net(): For NFSv4.0 (minorversion == 0), nfs_callback_up_net() invokes nfs4_callback_up_net():
```
ret = svc_xprt_create(serv, "tcp", net, PF_INET,
                      nfs_callback_set_tcpport, SVC_SOCK_ANONYMOUS, cred);
if (ret <= 0)
    goto out_err;
nn->nfs_callback_tcpport = ret;

ret = svc_xprt_create(serv, "tcp", net, PF_INET6,
                      nfs_callback_set_tcpport, SVC_SOCK_ANONYMOUS, cred);
if (ret > 0) {
    nn->nfs_callback_tcpport6 = ret;
} else if (ret != -EAFNOSUPPORT)
    goto out_err;
```
- The first call to svc_xprt_create() for PF_INET succeeds. The created struct svc_xprt is attached to serv->sv_permsocks via svc_add_new_perm_xprt(), and its xpt_server pointer is set to serv.
- The second call to svc_xprt_create() for PF_INET6 fails with an error other than -EAFNOSUPPORT (for example, -EINVAL, -EADDRINUSE, -ENOBUFS, or other socket creation/bind errors).
- nfs4_callback_up_net() takes the goto out_err; branch and returns the error code without cleaning up or closing the previously created PF_INET transport.
Incomplete Error Unwind:
- In nfs_callback_up_net(), on ret < 0, it prints "NFS: callback service start failed", decrements nn->cb_users[minorversion], and returns ret without destroying any transports associated with serv.
- Control returns to nfs_callback_up(), which branches to err_net:. Because cb_info->users is 0, it calls:
```
xprt_svc_shutdown_bc(xprt);
svc_set_num_threads(cb_info->serv, 0, 0);
xprt_svc_destroy_nullify_bc(xprt, &cb_info->serv);
```
- xprt_svc_destroy_nullify_bc() forwards directly to svc_destroy(&cb_info->serv).
Premature Destruction of svc_serv: In svc_destroy() (net/sunrpc/svc.c):
```
WARN_ONCE(!list_empty(&serv->sv_permsocks),
          "SVC: permsocks remain for %s\n", serv->sv_programs->pg_name);
WARN_ONCE(!list_empty(&serv->sv_tempsocks),
          "SVC: tempsocks remain for %s\n", serv->sv_programs->pg_name);
...
kfree(serv->sv_pools);
kfree(serv);
```
Because the IPv4 transport was never torn down (unlike during graceful shutdown or in fs/lockd/svc.c:make_socks() where svc_xprt_destroy_all(serv, net, ...) is called on error), serv->sv_permsocks is not empty. This triggers the reported WARN_ONCE.
Dangling Pointer and Use-After-Free: Following the warning, svc_destroy() proceeds to free serv->sv_pools and serv. The still-active IPv4 listener transport (struct svc_xprt) remains registered in the network stack with its xpt_server field holding a dangling pointer to the freed struct svc_serv.

ID	Workflow	Result	Correct	Bug	Created	Started	Finished	Revision
1182a445	assessment-security	DenialOfService: ✅ Exploitable: ✅ FilesystemTrigger: ❌ FuzzingArtifact: ❌ MemoryCorruption: ✅ NetworkNeedsSetup: ❌ NetworkTrigger: ❌ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ❌ UserNamespace: ❌ VMGuestTrigger: ❌ VMHostTrigger: ❌	❓	WARNING in svc_destroy	2026/11/05 18:23	2026/11/05 18:23	2026/11/05 19:23	5cecaf1f

assessment-security

DenialOfService: ✅ Exploitable: ✅ FilesystemTrigger: ❌ FuzzingArtifact: ❌ MemoryCorruption: ✅ NetworkNeedsSetup: ❌ NetworkTrigger: ❌ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ❌ UserNamespace: ❌ VMGuestTrigger: ❌ VMHostTrigger: ❌

❓

WARNING in svc_destroy

2026/11/05 18:23

2026/11/05 19:23

5cecaf1f

Kernel	Title	Rank 🛈	Repro	Cause bisect	Fix bisect	Count	Last	Reported	Patched	Status
linux-6.6	WARNING in svc_destroy origin:lts-only	-1	syz, C			452	1d10h	101d	0/2	upstream: reported C repro on 2026/07/27 20:31

Rank 🛈

linux-6.6

WARNING in svc_destroy origin:lts-only

-1

syz, C

452

1d10h

101d

0/2

upstream: reported C repro on 2026/07/27 20:31

NFS: callback service start failed ------------[ cut here ]------------ SVC: permsocks remain for NFSv4 callback WARNING: net/sunrpc/svc.c:532 at svc_destroy+0x299/0x330 net/sunrpc/svc.c:532, CPU#1: syz.2.321/6994 Modules linked in: CPU: 1 UID: 0 PID: 6994 Comm: syz.2.321 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/15/2026 RIP: 0010:svc_destroy+0x29d/0x330 net/sunrpc/svc.c:532 Code: 48 b8 00 00 00 00 00 fc ff df 49 8b 5d 00 48 8d 7b 18 48 89 fa 48 c1 ea 03 80 3c 02 00 75 67 48 8d 3d 17 5e 78 07 48 8b 73 18 <67> 48 0f b9 3a e9 d0 fd ff ff 48 8b 7c 24 08 e8 bf 13 86 f7 e9 a5 RSP: 0018:ffffc900042af860 EFLAGS: 00010246 RAX: dffffc0000000000 RBX: ffffffff9040ba80 RCX: ffffc900063d9000 RDX: 1ffffffff2081753 RSI: ffffffff8c37e800 RDI: ffffffff927aee30 RBP: 0000000000000000 R08: 00000000ffffffff R09: 0000000000000000 R10: 0000000000000001 R11: 0000000000000000 R12: ffff888035744000 R13: ffff888055f04c00 R14: ffff88802de30000 R15: ffffffff9cd5b540 FS: 00007ff81118f6c0(0000) GS:ffff88812257c000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 000000110c29af61 CR3: 0000000056a04000 CR4: 00000000003526f0 Call Trace: <TASK> nfs_callback_up+0x511/0xad0 fs/nfs/callback.c:236 nfs4_init_callback fs/nfs/nfs4client.c:327 [inline] nfs4_init_client_minor_version fs/nfs/nfs4client.c:379 [inline] nfs4_init_client+0x236/0x420 fs/nfs/nfs4client.c:429 nfs_get_client+0xf38/0x1230 fs/nfs/client.c:467 nfs4_set_client+0x3f6/0x640 fs/nfs/nfs4client.c:763 nfs4_init_server.isra.0+0x5be/0xf80 fs/nfs/nfs4client.c:1014 nfs4_create_server+0x185/0x2b0 fs/nfs/nfs4client.c:1063 nfs4_try_get_tree+0x93/0x140 fs/nfs/nfs4super.c:228 nfs_get_tree+0xa61/0x1440 fs/nfs/fs_context.c:1621 vfs_get_tree+0x92/0x320 fs/super.c:1881 fc_mount fs/namespace.c:1282 [inline] do_new_mount_fc fs/namespace.c:3910 [inline] do_new_mount fs/namespace.c:3986 [inline] path_mount+0x7d0/0x2850 fs/namespace.c:4337 do_mount fs/namespace.c:4350 [inline] __do_sys_mount fs/namespace.c:4565 [inline] __se_sys_mount fs/namespace.c:4542 [inline] __x64_sys_mount+0x293/0x310 fs/namespace.c:4542 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline] do_syscall_64+0x123/0x790 arch/x86/entry/syscall_64.c:84 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7ff81039fb19 Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007ff81118f028 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5 RAX: ffffffffffffffda RBX: 00007ff810625fa0 RCX: 00007ff81039fb19 RDX: 0000200000000480 RSI: 0000200000000440 RDI: 0000200000000400 RBP: 00007ff810437b70 R08: 00002000000004c0 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 R13: 00007ff810626038 R14: 00007ff810625fa0 R15: 00007ffd2a563db8 </TASK> ---------------- Code disassembly (best guess): 0: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax 7: fc ff df a: 49 8b 5d 00 mov 0x0(%r13),%rbx e: 48 8d 7b 18 lea 0x18(%rbx),%rdi 12: 48 89 fa mov %rdi,%rdx 15: 48 c1 ea 03 shr $0x3,%rdx 19: 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1) 1d: 75 67 jne 0x86 1f: 48 8d 3d 17 5e 78 07 lea 0x7785e17(%rip),%rdi # 0x7785e3d 26: 48 8b 73 18 mov 0x18(%rbx),%rsi * 2a: 67 48 0f b9 3a ud1 (%edx),%rdi <-- trapping instruction 2f: e9 d0 fd ff ff jmp 0xfffffe04 34: 48 8b 7c 24 08 mov 0x8(%rsp),%rdi 39: e8 bf 13 86 f7 call 0xf78613fd 3e: e9 .byte 0xe9 3f: a5 movsl %ds:(%rsi),%es:(%rdi)

Crashes (1):
Time	Kernel	Commit	Syzkaller	Config	Log	Report	Syz repro	C repro	VM info	Assets (help?)	Manager	Title
2026/11/05 18:08	upstream	05441d697f7f	5cecaf1f	.config	console log	report			info	[disk image] [vmlinux] [kernel image]	ci-upstream-kasan-gce-selinux-root	WARNING in svc_destroy

Crashes (1):

Assets (help?)