[v6.1] general protection fault in xprt_iter_get_next

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: fb28aa725e05 Linux 6.1.180
git tree: linux-6.1.y
console output: https://syzkaller.appspot.com/x/log.txt?x=17655bb9580000
kernel config: https://syzkaller.appspot.com/x/.config?x=31dfefc4a14efea3
dashboard link: https://syzkaller.appspot.com/bug?extid=c373d96a094c2bf074c5
compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/9a6fe2a54d0f/disk-fb28aa72.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/742b23a1d931/vmlinux-fb28aa72.xz
kernel image: https://storage.googleapis.com/syzbot-assets/b3f257c16d2e/bzImage-fb28aa72.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c373d9...@syzkaller.appspotmail.com

general protection fault, probably for non-canonical address 0xdffffc000000003d: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x00000000000001e8-0x00000000000001ef]
CPU: 1 PID: 4762 Comm: syz-executor Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:xprt_iter_ops net/sunrpc/xprtmultipath.c:218 [inline]
RIP: 0010:xprt_iter_get_next+0xcb/0x2a0 net/sunrpc/xprtmultipath.c:634
Code: c0 b9 a0 8b be 19 03 00 00 48 c7 c2 00 ba a0 8b e8 3a 00 0a f8 49 bc 00 00 00 00 00 fc ff df 4d 8d 7e 10 4c 89 f8 48 c1 e8 03 <42> 80 3c 20 00 74 08 4c 89 ff e8 36 b1 78 f8 4d 8b 3f 4d 85 ff 74
RSP: 0018:ffffc900065ff908 EFLAGS: 00010202
RAX: 000000000000003d RBX: ffffffff895aa8ec RCX: ffff8880184d9e00
RDX: 0000000000000000 RSI: ffffffff8adf5dc0 RDI: ffffffff8adf5d80
RBP: 0000000000000001 R08: dffffc0000000000 R09: 1ffffffff2162048
R10: dffffc0000000000 R11: fffffbfff2162049 R12: dffffc0000000000
R13: dffffc0000000000 R14: 00000000000001d8 R15: 00000000000001e8
FS: 0000555592168500(0000) GS:ffff8880b8f00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fcb039f6000 CR3: 000000004971e000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
rpc_task_get_next_xprt net/sunrpc/clnt.c:1139 [inline]
rpc_task_set_transport+0x149/0x3c0 net/sunrpc/clnt.c:1155
rpc_task_set_client net/sunrpc/clnt.c:1161 [inline]
rpc_run_task+0xcd/0x6a0 net/sunrpc/clnt.c:1216
rpc_call_sync+0x10c/0x1f0 net/sunrpc/clnt.c:1252
rpcb_register_call net/sunrpc/rpcb_clnt.c:389 [inline]
rpcb_register+0x2fd/0x5d0 net/sunrpc/rpcb_clnt.c:453
__svc_unregister net/sunrpc/svc.c:1118 [inline]
svc_unregister+0x1f9/0x580 net/sunrpc/svc.c:1145
svc_rpcb_cleanup+0x19/0x30 net/sunrpc/svc.c:407
nfsd_last_thread+0x100/0x2e0 fs/nfsd/nfssvc.c:560
nfsd_shutdown_threads+0xd6/0x1a0 fs/nfsd/nfssvc.c:643
nfsd_umount+0x3e/0xc0 fs/nfsd/nfsctl.c:1409
deactivate_locked_super+0x93/0xf0 fs/super.c:332
cleanup_mnt+0x471/0x4f0 fs/namespace.c:1191
task_work_run+0x1d0/0x260 kernel/task_work.c:203
resume_user_mode_work include/linux/resume_user_mode.h:49 [inline]
exit_to_user_mode_loop+0xe6/0x110 kernel/entry/common.c:177
exit_to_user_mode_prepare+0xee/0x180 kernel/entry/common.c:210
__syscall_exit_to_user_mode_work kernel/entry/common.c:292 [inline]
syscall_exit_to_user_mode+0x16/0x40 kernel/entry/common.c:303
do_syscall_64+0x58/0xa0 arch/x86/entry/common.c:82
entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7f508f59f257
Code: a2 c7 05 9c 44 25 00 00 00 00 00 eb 96 e8 e1 12 00 00 90 31 f6 e9 09 00 00 00 66 0f 1f 84 00 00 00 00 00 b8 a6 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 01 c3 48 c7 c2 e8 ff ff ff f7 d8 64 89 02 b8
RSP: 002b:00007ffe54946ed8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 00007f508f59f257
RDX: 0000000000000000 RSI: 0000000000000009 RDI: 00007ffe54946f90
RBP: 00007ffe54946f90 R08: 00007ffe54947f90 R09: 00000000ffffffff
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffe54948020
R13: 00007f508f63445c R14: 000000000001cc7e R15: 00007ffe54948060
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:xprt_iter_ops net/sunrpc/xprtmultipath.c:218 [inline]
RIP: 0010:xprt_iter_get_next+0xcb/0x2a0 net/sunrpc/xprtmultipath.c:634
Code: c0 b9 a0 8b be 19 03 00 00 48 c7 c2 00 ba a0 8b e8 3a 00 0a f8 49 bc 00 00 00 00 00 fc ff df 4d 8d 7e 10 4c 89 f8 48 c1 e8 03 <42> 80 3c 20 00 74 08 4c 89 ff e8 36 b1 78 f8 4d 8b 3f 4d 85 ff 74
RSP: 0018:ffffc900065ff908 EFLAGS: 00010202
RAX: 000000000000003d RBX: ffffffff895aa8ec RCX: ffff8880184d9e00
RDX: 0000000000000000 RSI: ffffffff8adf5dc0 RDI: ffffffff8adf5d80
RBP: 0000000000000001 R08: dffffc0000000000 R09: 1ffffffff2162048
R10: dffffc0000000000 R11: fffffbfff2162049 R12: dffffc0000000000
R13: dffffc0000000000 R14: 00000000000001d8 R15: 00000000000001e8
FS: 0000555592168500(0000) GS:ffff8880b8e00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000110c43885c CR3: 000000004971e000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
----------------
Code disassembly (best guess):
0: c0 b9 a0 8b be 19 03 sarb $0x3,0x19be8ba0(%rcx)
7: 00 00 add %al,(%rax)
9: 48 c7 c2 00 ba a0 8b mov $0xffffffff8ba0ba00,%rdx
10: e8 3a 00 0a f8 call 0xf80a004f
15: 49 bc 00 00 00 00 00 movabs $0xdffffc0000000000,%r12
1c: fc ff df
1f: 4d 8d 7e 10 lea 0x10(%r14),%r15
23: 4c 89 f8 mov %r15,%rax
26: 48 c1 e8 03 shr $0x3,%rax
* 2a: 42 80 3c 20 00 cmpb $0x0,(%rax,%r12,1) <-- trapping instruction
2f: 74 08 je 0x39
31: 4c 89 ff mov %r15,%rdi
34: e8 36 b1 78 f8 call 0xf878b16f
39: 4d 8b 3f mov (%r15),%r15
3c: 4d 85 ff test %r15,%r15
3f: 74 .byte 0x74


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