[syzbot] [net?] [nfs?] KASAN: null-ptr-deref Read in xprt_reserve

Hello,

syzbot found the following issue on:

HEAD commit:    6c377d19d4a5 Merge tag 'ext4_for_linus-7.3-rc7' of git://g..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1223300d580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4118aaa2d227be26
dashboard link: https://syzkaller.appspot.com/bug?extid=ae55a0fac80c73c938ac
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/c5f7458da164/disk-6c377d19.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/8ae69062b78e/vmlinux-6c377d19.xz
kernel image: https://storage.googleapis.com/syzbot-assets/602017f8f9bb/bzImage-6c377d19.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: <redacted>

==================================================================
BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:82 [inline]
BUG: KASAN: null-ptr-deref in _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
BUG: KASAN: null-ptr-deref in xprt_throttle_congested net/sunrpc/xprt.c:1708 [inline]
BUG: KASAN: null-ptr-deref in xprt_reserve+0xe2/0x2c0 net/sunrpc/xprt.c:1948
Read of size 8 at addr 0000000000000628 by task syz.8.3118/20390

CPU: 1 UID: 0 PID: 20390 Comm: syz.8.3118 Tainted: G S           L      syzkaller #0 PREEMPT(full) 
Tainted: [S]=CPU_OUT_OF_SPEC, [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/26/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 kasan_report+0x117/0x150 mm/kasan/report.c:595
 check_region_inline mm/kasan/generic.c:-1 [inline]
 kasan_check_range+0x264/0x2c0 mm/kasan/generic.c:200
 instrument_atomic_read include/linux/instrumented.h:82 [inline]
 _test_bit include/asm-generic/bitops/instrumented-non-atomic.h:141 [inline]
 xprt_throttle_congested net/sunrpc/xprt.c:1708 [inline]
 xprt_reserve+0xe2/0x2c0 net/sunrpc/xprt.c:1948
 __rpc_execute+0x526/0x1280 net/sunrpc/sched.c:947
 rpc_execute+0x1c1/0x390 net/sunrpc/sched.c:1023
 rpc_run_task+0x4a6/0x660 net/sunrpc/clnt.c:1269
 rpc_ping_noreply net/sunrpc/clnt.c:2944 [inline]
 rpc_create_xprt+0x452/0xa90 net/sunrpc/clnt.c:494
 rpc_create+0x649/0x900 net/sunrpc/clnt.c:619
 gssp_rpc_create net/sunrpc/auth_gss/gss_rpc_upcall.c:107 [inline]
 set_gssp_clnt+0x188/0x2d0 net/sunrpc/auth_gss/gss_rpc_upcall.c:131
 write_gssp+0x21e/0x310 net/sunrpc/auth_gss/svcauth_gss.c:1423
 pde_write fs/proc/inode.c:330 [inline]
 proc_reg_write+0x1e9/0x2e0 fs/proc/inode.c:342
 vfs_write+0x296/0xba0 fs/read_write.c:685
 ksys_write+0x150/0x270 fs/read_write.c:739
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f44e699e1d9
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:00007f44e78be028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f44e6c25fa0 RCX: 00007f44e699e1d9
RDX: 0000000000000001 RSI: 0000200000000300 RDI: 0000000000000005
RBP: 00007f44e6a35888 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f44e6c26038 R14: 00007f44e6c25fa0 R15: 00007ffdb88808e8
 </TASK>
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup