Message-ID: <CACT4Y+aWzUkry7wtSib2YC-MsH=txHOOgMMy-dsMmVhgQrJEgQ@mail.gmail.com>
Date: Thu, 24 Oct 2024 11:27:21 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Chuck Lever III <chuck.lever@...cle.com>
Cc: Jeff Layton <jlayton@...nel.org>, Neil Brown <neilb@...e.de>, 
	syzbot <syzbot+24cd636199753ac5e0ca@...kaller.appspotmail.com>, 
	Dai Ngo <dai.ngo@...cle.com>, 
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, 
	Linux NFS Mailing List <linux-nfs@...r.kernel.org>, Olga Kornievskaia <okorniev@...hat.com>, 
	"syzkaller-bugs@...glegroups.com" <syzkaller-bugs@...glegroups.com>, Tom Talpey <tom@...pey.com>
Subject: Re: [syzbot] [nfs?] KASAN: slab-use-after-free Read in rhashtable_walk_enter

On Thu, 17 Oct 2024 at 15:16, Chuck Lever III <chuck.lever@...cle.com> wrote:
>
> > On Oct 17, 2024, at 4:34 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> >
> > On Sun, 22 Sept 2024 at 21:30, 'Chuck Lever III' via syzkaller-bugs
> > <syzkaller-bugs@...glegroups.com> wrote:
> >>
> >>
> >>
> >>> On Sep 20, 2024, at 3:13 PM, Jeff Layton <jlayton@...nel.org> wrote:
> >>>
> >>> On Fri, 2024-09-20 at 18:51 +0000, Chuck Lever III wrote:
> >>>>
> >>>>> On Sep 19, 2024, at 10:57 PM, NeilBrown <neilb@...e.de> wrote:
> >>>>>
> >>>>> On Thu, 19 Sep 2024, Jeff Layton wrote:
> >>>>>> On Wed, 2024-09-18 at 08:38 -0700, syzbot wrote:
> >>>>>>> Hello,
> >>>>>>>
> >>>>>>> syzbot found the following issue on:
> >>>>>>>
> >>>>>>> HEAD commit:    a430d95c5efa Merge tag 'lsm-pr-20240911' of git://git.kern..
> >>>>>>> git tree:       upstream
> >>>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=17c7469f980000
> >>>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=a69c66e868285a9d
> >>>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=24cd636199753ac5e0ca
> >>>>>>> compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> >>>>>>>
> >>>>>>> Unfortunately, I don't have any reproducer for this issue yet.
> >>>>>>>
> >>>>>>> Downloadable assets:
> >>>>>>> disk image: https://storage.googleapis.com/syzbot-assets/7f3aff905e91/disk-a430d95c.raw.xz
> >>>>>>> vmlinux: https://storage.googleapis.com/syzbot-assets/a468ce8431f0/vmlinux-a430d95c.xz
> >>>>>>> kernel image: https://storage.googleapis.com/syzbot-assets/80d4f1150155/bzImage-a430d95c.xz
> >>>>>>>
> >>>>>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> >>>>>>> Reported-by: syzbot+24cd636199753ac5e0ca@...kaller.appspotmail.com
> >>>>>>>
> >>>>>>> svc: failed to register nfsdv3 RPC service (errno 111).
> >>>>>>> svc: failed to register nfsaclv3 RPC service (errno 111).
> >>>>>>> ==================================================================
> >>>>>>> BUG: KASAN: slab-use-after-free in list_add include/linux/list.h:169 [inline]
> >>>>>>> BUG: KASAN: slab-use-after-free in rhashtable_walk_enter+0x333/0x370 lib/rhashtable.c:684
> >>>>>>> Read of size 8 at addr ffff8880773fa010 by task syz.2.11924/9970
> >>>>>>>
> >>>>>>> CPU: 0 UID: 0 PID: 9970 Comm: syz.2.11924 Not tainted 6.11.0-syzkaller-02574-ga430d95c5efa #0
> >>>>>>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/06/2024
> >>>>>>> Call Trace:
> >>>>>>> <TASK>
> >>>>>>> __dump_stack lib/dump_stack.c:93 [inline]
> >>>>>>> dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:119
> >>>>>>> print_address_description mm/kasan/report.c:377 [inline]
> >>>>>>> print_report+0xc3/0x620 mm/kasan/report.c:488
> >>>>>>> kasan_report+0xd9/0x110 mm/kasan/report.c:601
> >>>>>>> list_add include/linux/list.h:169 [inline]
> >>>>>>> rhashtable_walk_enter+0x333/0x370 lib/rhashtable.c:684
> >>>>>>> rhltable_walk_enter include/linux/rhashtable.h:1262 [inline]
> >>>>>>> __nfsd_file_cache_purge+0xad/0x490 fs/nfsd/filecache.c:805
> >>>>>>> nfsd_file_cache_shutdown+0xcf/0x480 fs/nfsd/filecache.c:897
> >>>>>>> nfsd_shutdown_generic fs/nfsd/nfssvc.c:329 [inline]
> >>>>>>> nfsd_shutdown_generic fs/nfsd/nfssvc.c:323 [inline]
> >>>>>>> nfsd_startup_net fs/nfsd/nfssvc.c:444 [inline]
> >>>>>>> nfsd_svc+0x6d4/0x970 fs/nfsd/nfssvc.c:817
> >>>>>>> nfsd_nl_threads_set_doit+0x52c/0xbc0 fs/nfsd/nfsctl.c:1714
> >>>>>>> genl_family_rcv_msg_doit+0x202/0x2f0 net/netlink/genetlink.c:1115
> >>>>>>> genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
> >>>>>>> genl_rcv_msg+0x565/0x800 net/netlink/genetlink.c:1210
> >>>>>>> netlink_rcv_skb+0x165/0x410 net/netlink/af_netlink.c:2550
> >>>>>>> genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
> >>>>>>> netlink_unicast_kernel net/netlink/af_netlink.c:1331 [inline]
> >>>>>>> netlink_unicast+0x53c/0x7f0 net/netlink/af_netlink.c:1357
> >>>>>>> netlink_sendmsg+0x8b8/0xd70 net/netlink/af_netlink.c:1901
> >>>>>>> sock_sendmsg_nosec net/socket.c:730 [inline]
> >>>>>>> __sock_sendmsg net/socket.c:745 [inline]
> >>>>>>> ____sys_sendmsg+0x9ae/0xb40 net/socket.c:2603
> >>>>>>> ___sys_sendmsg+0x135/0x1e0 net/socket.c:2657
> >>>>>>> __sys_sendmsg+0x117/0x1f0 net/socket.c:2686
> >>>>>>> do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> >>>>>>> do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
> >>>>>>> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >>>>>>> RIP: 0033:0x7fd947f7def9
> >>>>>>> Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
> >>>>>>> RSP: 002b:00007fd948e38038 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
> >>>>>>> RAX: ffffffffffffffda RBX: 00007fd948135f80 RCX: 00007fd947f7def9
> >>>>>>> RDX: 0000000000000004 RSI: 0000000020000280 RDI: 0000000000000003
> >>>>>>> RBP: 00007fd947ff0b76 R08: 0000000000000000 R09: 0000000000000000
> >>>>>>> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> >>>>>>> R13: 0000000000000000 R14: 00007fd948135f80 R15: 00007ffc6cab9d78
> >>>>>>> </TASK>
> >>>>>>>
> >>>>>>> Allocated by task 8716:
> >>>>>>> kasan_save_stack+0x33/0x60 mm/kasan/common.c:47
> >>>>>>> kasan_save_track+0x14/0x30 mm/kasan/common.c:68
> >>>>>>> poison_kmalloc_redzone mm/kasan/common.c:370 [inline]
> >>>>>>> __kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:387
> >>>>>>> kasan_kmalloc include/linux/kasan.h:211 [inline]
> >>>>>>> __do_kmalloc_node mm/slub.c:4159 [inline]
> >>>>>>> __kmalloc_node_track_caller_noprof+0x20f/0x440 mm/slub.c:4178
> >>>>>>> kmalloc_reserve+0xef/0x2c0 net/core/skbuff.c:609
> >>>>>>> __alloc_skb+0x164/0x380 net/core/skbuff.c:678
> >>>>>>> alloc_skb include/linux/skbuff.h:1322 [inline]
> >>>>>>> nsim_dev_trap_skb_build drivers/net/netdevsim/dev.c:748 [inline]
> >>>>>>> nsim_dev_trap_report drivers/net/netdevsim/dev.c:805 [inline]
> >>>>>>> nsim_dev_trap_report_work+0x2a4/0xc80 drivers/net/netdevsim/dev.c:850
> >>>>>>> process_one_work+0x958/0x1ad0 kernel/workqueue.c:3231
> >>>>>>> process_scheduled_works kernel/workqueue.c:3312 [inline]
> >>>>>>> worker_thread+0x6c8/0xf00 kernel/workqueue.c:3393
> >>>>>>> kthread+0x2c1/0x3a0 kernel/kthread.c:389
> >>>>>>> ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
> >>>>>>> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
> >>>>>>>
> >>>>>>> Freed by task 8716:
> >>>>>>> kasan_save_stack+0x33/0x60 mm/kasan/common.c:47
> >>>>>>> kasan_save_track+0x14/0x30 mm/kasan/common.c:68
> >>>>>>> kasan_save_free_info+0x3b/0x60 mm/kasan/generic.c:579
> >>>>>>> poison_slab_object+0xf7/0x160 mm/kasan/common.c:240
> >>>>>>> __kasan_slab_free+0x32/0x50 mm/kasan/common.c:256
> >>>>>>> kasan_slab_free include/linux/kasan.h:184 [inline]
> >>>>>>> slab_free_hook mm/slub.c:2250 [inline]
> >>>>>>> slab_free mm/slub.c:4474 [inline]
> >>>>>>> kfree+0x12a/0x3b0 mm/slub.c:4595
> >>>>>>> skb_kfree_head net/core/skbuff.c:1086 [inline]
> >>>>>>> skb_free_head+0x108/0x1d0 net/core/skbuff.c:1098
> >>>>>>> skb_release_data+0x75d/0x990 net/core/skbuff.c:1125
> >>>>>>> skb_release_all net/core/skbuff.c:1190 [inline]
> >>>>>>> __kfree_skb net/core/skbuff.c:1204 [inline]
> >>>>>>> consume_skb net/core/skbuff.c:1436 [inline]
> >>>>>>> consume_skb+0xbf/0x100 net/core/skbuff.c:1430
> >>>>>>> nsim_dev_trap_report drivers/net/netdevsim/dev.c:821 [inline]
> >>>>>>> nsim_dev_trap_report_work+0x878/0xc80 drivers/net/netdevsim/dev.c:850
> >>>>>>> process_one_work+0x958/0x1ad0 kernel/workqueue.c:3231
> >>>>>>> process_scheduled_works kernel/workqueue.c:3312 [inline]
> >>>>>>> worker_thread+0x6c8/0xf00 kernel/workqueue.c:3393
> >>>>>>> kthread+0x2c1/0x3a0 kernel/kthread.c:389
> >>>>>>> ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147
> >>>>>>> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
> >>>>>>>
> >>>>>>> The buggy address belongs to the object at ffff8880773fa000
> >>>>>>> which belongs to the cache kmalloc-4k of size 4096
> >>>>>>> The buggy address is located 16 bytes inside of
> >>>>>>> freed 4096-byte region [ffff8880773fa000, ffff8880773fb000)
> >>>>>>>
> >>>>>>> The buggy address belongs to the physical page:
> >>>>>>> page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x773f8
> >>>>>>> head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> >>>>>>> flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
> >>>>>>> page_type: 0xfdffffff(slab)
> >>>>>>> raw: 00fff00000000040 ffff88801ac42140 ffffea0001e63600 dead000000000002
> >>>>>>> raw: 0000000000000000 0000000000040004 00000001fdffffff 0000000000000000
> >>>>>>> head: 00fff00000000040 ffff88801ac42140 ffffea0001e63600 dead000000000002
> >>>>>>> head: 0000000000000000 0000000000040004 00000001fdffffff 0000000000000000
> >>>>>>> head: 00fff00000000003 ffffea0001dcfe01 ffffffffffffffff 0000000000000000
> >>>>>>> head: 0000000700000008 0000000000000000 00000000ffffffff 0000000000000000
> >>>>>>> page dumped because: kasan: bad access detected
> >>>>>>> page_owner tracks the page as allocated
> >>>>>>> page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd2040(__GFP_IO|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 5240, tgid 5240 (syz-executor), ts 74499202771, free_ts 74134964798
> >>>>>>> set_page_owner include/linux/page_owner.h:32 [inline]
> >>>>>>> post_alloc_hook+0x2d1/0x350 mm/page_alloc.c:1500
> >>>>>>> prep_new_page mm/page_alloc.c:1508 [inline]
> >>>>>>> get_page_from_freelist+0x1351/0x2e50 mm/page_alloc.c:3446
> >>>>>>> __alloc_pages_noprof+0x22b/0x2460 mm/page_alloc.c:4702
> >>>>>>> __alloc_pages_node_noprof include/linux/gfp.h:269 [inline]
> >>>>>>> alloc_pages_node_noprof include/linux/gfp.h:296 [inline]
> >>>>>>> alloc_slab_page+0x4e/0xf0 mm/slub.c:2319
> >>>>>>> allocate_slab mm/slub.c:2482 [inline]
> >>>>>>> new_slab+0x84/0x260 mm/slub.c:2535
> >>>>>>> ___slab_alloc+0xdac/0x1870 mm/slub.c:3721
> >>>>>>> __slab_alloc.constprop.0+0x56/0xb0 mm/slub.c:3811
> >>>>>>> __slab_alloc_node mm/slub.c:3864 [inline]
> >>>>>>> slab_alloc_node mm/slub.c:4026 [inline]
> >>>>>>> __do_kmalloc_node mm/slub.c:4158 [inline]
> >>>>>>> __kmalloc_noprof+0x379/0x410 mm/slub.c:4171
> >>>>>>> kmalloc_noprof include/linux/slab.h:694 [inline]
> >>>>>>> tomoyo_realpath_from_path+0xbf/0x710 security/tomoyo/realpath.c:251
> >>>>>>> tomoyo_get_realpath security/tomoyo/file.c:151 [inline]
> >>>>>>> tomoyo_path_number_perm+0x245/0x5b0 security/tomoyo/file.c:723
> >>>>>>> security_file_ioctl+0x9b/0x240 security/security.c:2908
> >>>>>>> __do_sys_ioctl fs/ioctl.c:901 [inline]
> >>>>>>> __se_sys_ioctl fs/ioctl.c:893 [inline]
> >>>>>>> __x64_sys_ioctl+0xbb/0x210 fs/ioctl.c:893
> >>>>>>> do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> >>>>>>> do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
> >>>>>>> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >>>>>>> page last free pid 5233 tgid 5233 stack trace:
> >>>>>>> reset_page_owner include/linux/page_owner.h:25 [inline]
> >>>>>>> free_pages_prepare mm/page_alloc.c:1101 [inline]
> >>>>>>> free_unref_folios+0x9e9/0x1390 mm/page_alloc.c:2667
> >>>>>>> folios_put_refs+0x560/0x760 mm/swap.c:1039
> >>>>>>> free_pages_and_swap_cache+0x36d/0x510 mm/swap_state.c:332
> >>>>>>> __tlb_batch_free_encoded_pages+0xf9/0x290 mm/mmu_gather.c:136
> >>>>>>> tlb_batch_pages_flush mm/mmu_gather.c:149 [inline]
> >>>>>>> tlb_flush_mmu_free mm/mmu_gather.c:366 [inline]
> >>>>>>> tlb_flush_mmu mm/mmu_gather.c:373 [inline]
> >>>>>>> tlb_finish_mmu+0x168/0x7b0 mm/mmu_gather.c:465
> >>>>>>> unmap_region+0x342/0x420 mm/mmap.c:2441
> >>>>>>> do_vmi_align_munmap+0x1107/0x19c0 mm/mmap.c:2754
> >>>>>>> do_vmi_munmap+0x231/0x410 mm/mmap.c:2830
> >>>>>>> __vm_munmap+0x142/0x330 mm/mmap.c:3109
> >>>>>>> __do_sys_munmap mm/mmap.c:3126 [inline]
> >>>>>>> __se_sys_munmap mm/mmap.c:3123 [inline]
> >>>>>>> __x64_sys_munmap+0x61/0x90 mm/mmap.c:3123
> >>>>>>> do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> >>>>>>> do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
> >>>>>>> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >>>>>>>
> >>>>>>> Memory state around the buggy address:
> >>>>>>> ffff8880773f9f00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >>>>>>> ffff8880773f9f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> >>>>>>>> ffff8880773fa000: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >>>>>>>                       ^
> >>>>>>> ffff8880773fa080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >>>>>>> ffff8880773fa100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >>>>>>> ==================================================================
> >>>>>>>
> >>>>>>>
> >>>>>>> ---
> >>>>>>> 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@...glegroups.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
> >>>>>>
> >>>>>> So we're tearing down the server and cleaning out the nfsd_file hash,
> >>>>>> and we hit a UAF. That probably means that we freed a nfsd_file without
> >>>>>> removing it from the hash? Maybe we should add a WARN_ON() in
> >>>>>> nfsd_file_slab_free that checks whether the item is still hashed?
> >>>>>>
> >>>>>> It is strange though. struct nfsd_file is 112 bytes on my machine, but
> >>>>>> the warning is about a 4k allocation. I guess that just means that the
> >>>>>> page got recycled into a different slabcache.
> >>>>>
> >>>>> The code that is crashing hasn't come close to touching anything that is
> >>>>> thought to be an nfsd_file.
> >>>>> The error is detected in the list_add() in rhashtable_walk_enter() when
> >>>>> the new on-stack iterator is being attached to the bucket_table that is being
> >>>>> iterated.  So that bucket_table must (now) be an invalid address.
> >>>>>
> >>>>> The handling of NFSD_FILE_CACHE_UP is strange.  nfsd_file_cache_init()
> >>>>> sets it, but doesn't clear it on failure.  So if nfsd_file_cache_init()
> >>>>> fails for some reason, nfsd_file_cache_shutdown() would still try to
> >>>>> clean up if it was called.
> >>>>>
> >>>>> So suppose nfsd_startup_generic() is called.  It increments nfsd_users
> >>>>> from 0 so continues to nfsd_file_cache_init() which fails for some
> >>>>> reason after initialising nfsd_file_rhltable and then destroying it.
> >>>>> This will leave nfsd_file_rhltable.tbl as a pointer to a large
> >>>>> allocation which has been freed.  nfsd_startup_generic() will then
> >>>>> decrement nfsd_users back to zero, but NFSD_FILE_CACHE_UP will still be
> >>>>> set.
> >>>>>
> >>>>> When nfsd_startup_generic() is called again, nfsd_file_cache_init() will
> >>>>> skip initialisation because NFSD_FILE_CACHE_UP is set.  When
> >>>>> nfsd_file_cache_shutdown() is then called it will clean up an rhltable
> >>>>> that has already been destroyed.  We get exactly the reported symptom.
> >>>>>
> >>>>> I *think* nfsd_file_cache_init() can only fail with -ENOMEM and I would
> >>>>> expect to see a warning when that happened.  In any case
> >>>>> nfsd_file_cache_init() uses pr_err() for any failure except
> >>>>> rhltable_init(), and that only fails if the params are inconsistent.
> >>>>>
> >>>>> So I think there are problems with NFSD_FILE_CACHE_UP settings and I
> >>>>> think they could trigger this bug if a kmalloc failed, but I don't think
> >>>>> that a kmalloc failed and I think there must be some other explanation
> >>>>> here.
> >>>>
> >>>> Also, the FILE_CACHE_UP logic has been around for several releases.
> >>>> Why is this UAF showing up only now? The "unable to register"
> >>>> messages suggest a possible reason.
> >>>>
> >>>
> >>> Good point. I didn't notice those. 111 is ECONNREFUSED, so it sounds
> >>> like rpcbind isn't up in this situation. Maybe that's a hint toward a
> >>> reproducer?
> >>
> >> I've enabled KASAN, shut down NFSD, and disabled rpcbind.
> >> Using the /proc/fs/nfsd/threads interface, I can reproduce
> >> the "unable to register" messages, but not the KASAN splat.
> >>
> >> I think Neil's analysis above is correct or very close. I
> >> don't have nfsdctl set up here, but my guess is that the
> >> syzbot is abusing the new netlink command API, causing
> >> nfsd_file_cache_init() to fail somehow and the error
> >> flow is leaving the kernel vulnerable to a UAF.
> >
> >
> > Based on the fact that it happened 8 times and no reproducer so far:
> > 1. Likely to be a true bug in nsfd
> > 2. LIkely to be a hard to trigger race condition
> > 3. The fact that alloc/free frequently don't match access tack
> > suggests that the access happens long after the free (pointer left in
> > some global structure)
> >
> > I also found one with matching stacks here:
> > https://syzkaller.appspot.com/text?tag=CrashLog&x=128bd39f980000
> >
> > It looks like the object is freed during creation:
> >
> > [ 3239.085992][ T8199] __kmem_cache_create_args(nfsd_file) failed with error -22
> > [ 3239.117916][ T8199] CPU: 1 UID: 0 PID: 8199 Comm: syz.1.6492 Not
> > tainted 6.12.0-rc1-syzkaller-00031-ge32cde8d2bd7 #0
> > [ 3239.129138][ T8199] Hardware name: Google Google Compute
> > Engine/Google Compute Engine, BIOS Google 09/13/2024
> > [ 3239.139255][ T8199] Call Trace:
> > [ 3239.142756][ T8199]  <TASK>
> > [ 3239.145753][ T8199]  dump_stack_lvl+0x16c/0x1f0
> > [ 3239.150510][ T8199]  __kmem_cache_create_args+0x130/0x3c0
> > [ 3239.156126][ T8199]  nfsd_file_cache_init+0x126/0x4d0
> > [ 3239.161408][ T8199]  ? net_generic+0xf4/0x2a0
> > [ 3239.165992][ T8199]  nfsd_svc+0x5db/0x940
> > [ 3239.170309][ T8199]  nfsd_nl_threads_set_doit+0x535/0xbe0
> > [ 3239.175933][ T8199]  genl_family_rcv_msg_doit+0x202/0x2f0
> > [ 3239.181560][ T8199]  ? __pfx_genl_family_rcv_msg_doit+0x10/0x10
> > [ 3239.187720][ T8199]  ? bpf_lsm_capable+0x9/0x10
> > [ 3239.192470][ T8199]  ? security_capable+0x7e/0x260
> > [ 3239.197482][ T8199]  genl_rcv_msg+0x565/0x800
> > [ 3239.202063][ T8199]  ? __pfx_genl_rcv_msg+0x10/0x10
> > [ 3239.207201][ T8199]  ? __pfx_nfsd_nl_threads_set_doit+0x10/0x10
> > [ 3239.213336][ T8199]  netlink_rcv_skb+0x165/0x410
> > [ 3239.218144][ T8199]  ? __pfx_genl_rcv_msg+0x10/0x10
> > [ 3239.223301][ T8199]  ? __pfx_netlink_rcv_skb+0x10/0x10
> > [ 3239.228636][ T8199]  ? down_read+0xc9/0x330
> > [ 3239.233012][ T8199]  ? __pfx_down_read+0x10/0x10
> > [ 3239.237833][ T8199]  ? netlink_deliver_tap+0x1ae/0xcf0
> > [ 3239.243429][ T8199]  genl_rcv+0x28/0x40
> > [ 3239.247447][ T8199]  netlink_unicast+0x53c/0x7f0
> > [ 3239.252257][ T8199]  ? __pfx_netlink_unicast+0x10/0x10
> > [ 3239.257579][ T8199]  ? __phys_addr_symbol+0x30/0x80
> > [ 3239.262641][ T8199]  ? __check_object_size+0x488/0x710
> > [ 3239.267972][ T8199]  netlink_sendmsg+0x8b8/0xd70
> > [ 3239.272776][ T8199]  ? __pfx_netlink_sendmsg+0x10/0x10
> > [ 3239.278115][ T8199]  ____sys_sendmsg+0x9ae/0xb40
> > [ 3239.282921][ T8199]  ? copy_msghdr_from_user+0x10b/0x160
> > [ 3239.288434][ T8199]  ? __pfx_____sys_sendmsg+0x10/0x10
> > [ 3239.293765][ T8199]  ? netlink_recvmsg+0x81c/0xf30
> > [ 3239.298736][ T8199]  ? __pfx___lock_acquire+0x10/0x10
> > [ 3239.303989][ T8199]  ___sys_sendmsg+0x135/0x1e0
> > [ 3239.308715][ T8199]  ? __pfx____sys_sendmsg+0x10/0x10
> > [ 3239.313976][ T8199]  ? lock_acquire+0x2f/0xb0
> > [ 3239.318521][ T8199]  ? __fget_files+0x40/0x3f0
> > [ 3239.323155][ T8199]  ? fdget+0x176/0x210
> > [ 3239.327348][ T8199]  __sys_sendmsg+0x117/0x1f0
> > [ 3239.331985][ T8199]  ? __pfx___sys_sendmsg+0x10/0x10
> > [ 3239.337153][ T8199]  ? __x64_sys_futex+0x1e1/0x4c0
> > [ 3239.342236][ T8199]  do_syscall_64+0xcd/0x250
> > [ 3239.346787][ T8199]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > [ 3239.352716][ T8199] RIP: 0033:0x7fd3c577dff9
> > [ 3239.357157][ T8199] Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f
> > 1f 40 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 a8 ff ff ff f7 d8
> > 64 89 01 48
> > [ 3239.376798][ T8199] RSP: 002b:00007fd3c65a7038 EFLAGS: 00000246
> > ORIG_RAX: 000000000000002e
> > [ 3239.385282][ T8199] RAX: ffffffffffffffda RBX: 00007fd3c5935f80
> > RCX: 00007fd3c577dff9
> > [ 3239.393282][ T8199] RDX: 0000000000008004 RSI: 0000000020000140
> > RDI: 0000000000000006
> > [ 3239.401287][ T8199] RBP: 00007fd3c57f0296 R08: 0000000000000000
> > R09: 0000000000000000
> > [ 3239.409291][ T8199] R10: 0000000000000000 R11: 0000000000000246
> > R12: 0000000000000000
> > [ 3239.417296][ T8199] R13: 0000000000000000 R14: 00007fd3c5935f80
> > R15: 00007ffca7d46f08
> > [ 3239.425314][ T8199]  </TASK>
> > [ 3239.605040][ T8199] nfsd: unable to create nfsd_file_slab
> > [ 3242.331528][ T8252] netlink: zone id is out of range
> > [ 3242.353608][ T8252] netlink: zone id is out of range
> > [ 3242.375964][ T8252] netlink: zone id is out of range
> > [ 3242.381340][ T8252] netlink: zone id is out of range
> > [ 3242.432868][ T8252] netlink: zone id is out of range
> > [ 3242.506182][ T8252] netlink: zone id is out of range
> > [ 3242.529087][ T8252] netlink: zone id is out of range
> > [ 3242.574253][ T8252] netlink: zone id is out of range
> > [ 3245.016756][ T8292] sock: sock_set_timeout: `syz.3.6511' (pid 8292)
> > tries to set negative timeout
> > [ 3245.419913][ T8295] svc: failed to register nfsdv3 RPC service (errno 111).
> > [ 3245.467727][ T8295] svc: failed to register nfsaclv3 RPC service (errno 111).
> > [ 3245.693668][ T8295]
> > ==================================================================
> > [ 3245.701818][ T8295] BUG: KASAN: slab-use-after-free in
> > rhashtable_walk_enter+0x333/0x370
> > [ 3245.710150][ T8295] Read of size 8 at addr ffff8880301ac010 by task
> > syz.1.6512/8295
> > [ 3245.718011][ T8295]
> > [ 3245.720376][ T8295] CPU: 1 UID: 0 PID: 8295 Comm: syz.1.6512 Not
> > tainted 6.12.0-rc1-syzkaller-00031-ge32cde8d2bd7 #0
> > [ 3245.731118][ T8295] Hardware name: Google Google Compute
> > Engine/Google Compute Engine, BIOS Google 09/13/2024
> > [ 3245.741404][ T8295] Call Trace:
> > [ 3245.744728][ T8295]  <TASK>
> > [ 3245.747702][ T8295]  dump_stack_lvl+0x116/0x1f0
> > [ 3245.752629][ T8295]  print_report+0xc3/0x620
> > [ 3245.757152][ T8295]  ? __virt_addr_valid+0x5e/0x590
> > [ 3245.762261][ T8295]  ? __phys_addr+0xc6/0x150
> > [ 3245.766862][ T8295]  kasan_report+0xd9/0x110
> > [ 3245.771374][ T8295]  ? rhashtable_walk_enter+0x333/0x370
> > [ 3245.776919][ T8295]  ? rhashtable_walk_enter+0x333/0x370
> > [ 3245.782467][ T8295]  rhashtable_walk_enter+0x333/0x370
> > [ 3245.787843][ T8295]  __nfsd_file_cache_purge+0xad/0x470
> > [ 3245.793299][ T8295]  ? __pfx___nfsd_file_cache_purge+0x10/0x10
> > [ 3245.799445][ T8295]  ? lockdep_hardirqs_on+0x7c/0x110
> > [ 3245.804721][ T8295]  ? enable_work+0x246/0x340
> > [ 3245.809637][ T8295]  ? __pfx_enable_work+0x10/0x10
> > [ 3245.814651][ T8295]  nfsd_file_cache_shutdown+0xcf/0x4e0
> > [ 3245.820203][ T8295]  nfsd_svc+0x6a9/0x940
> > [ 3245.824435][ T8295]  nfsd_nl_threads_set_doit+0x535/0xbe0
> > [ 3245.830066][ T8295]  genl_family_rcv_msg_doit+0x202/0x2f0
> > [ 3245.835692][ T8295]  ? __pfx_genl_family_rcv_msg_doit+0x10/0x10
> > [ 3245.841845][ T8295]  ? bpf_lsm_capable+0x9/0x10
> > [ 3245.846733][ T8295]  ? security_capable+0x7e/0x260
> > [ 3245.851836][ T8295]  genl_rcv_msg+0x565/0x800
> > [ 3245.856427][ T8295]  ? __pfx_genl_rcv_msg+0x10/0x10
> > [ 3245.861530][ T8295]  ? __pfx_nfsd_nl_threads_set_doit+0x10/0x10
> > [ 3245.867684][ T8295]  netlink_rcv_skb+0x165/0x410
> > [ 3245.872559][ T8295]  ? __pfx_genl_rcv_msg+0x10/0x10
> > [ 3245.877679][ T8295]  ? __pfx_netlink_rcv_skb+0x10/0x10
> > [ 3245.883041][ T8295]  ? down_read+0xc9/0x330
> > [ 3245.887446][ T8295]  ? __pfx_down_read+0x10/0x10
> > [ 3245.892295][ T8295]  ? netlink_deliver_tap+0x1ae/0xcf0
> > [ 3245.897649][ T8295]  genl_rcv+0x28/0x40
> > [ 3245.901698][ T8295]  netlink_unicast+0x53c/0x7f0
> > [ 3245.906584][ T8295]  ? __pfx_netlink_unicast+0x10/0x10
> > [ 3245.912041][ T8295]  ? __phys_addr_symbol+0x30/0x80
> > [ 3245.917161][ T8295]  ? __check_object_size+0x488/0x710
> > [ 3245.922542][ T8295]  netlink_sendmsg+0x8b8/0xd70
> > [ 3245.927393][ T8295]  ? __pfx_netlink_sendmsg+0x10/0x10
> > [ 3245.932758][ T8295]  ____sys_sendmsg+0x9ae/0xb40
> > [ 3245.937616][ T8295]  ? copy_msghdr_from_user+0x10b/0x160
> > [ 3245.943164][ T8295]  ? __pfx_____sys_sendmsg+0x10/0x10
> > [ 3245.948531][ T8295]  ? netlink_recvmsg+0x81c/0xf30
> > [ 3245.953535][ T8295]  ? __pfx___lock_acquire+0x10/0x10
> > [ 3245.958906][ T8295]  ___sys_sendmsg+0x135/0x1e0
> > [ 3245.963667][ T8295]  ? __pfx____sys_sendmsg+0x10/0x10
> > [ 3245.968960][ T8295]  ? lock_acquire+0x2f/0xb0
> > [ 3245.973562][ T8295]  ? __fget_files+0x40/0x3f0
> > [ 3245.978240][ T8295]  ? fdget+0x176/0x210
> > [ 3245.982379][ T8295]  __sys_sendmsg+0x117/0x1f0
> > [ 3245.987050][ T8295]  ? __pfx___sys_sendmsg+0x10/0x10
> > [ 3245.992252][ T8295]  ? __x64_sys_futex+0x1e1/0x4c0
> > [ 3245.997285][ T8295]  do_syscall_64+0xcd/0x250
> > [ 3246.001884][ T8295]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > [ 3246.007856][ T8295] RIP: 0033:0x7fd3c577dff9
> > [ 3246.012333][ T8295] Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f
> > 1f 40 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 a8 ff ff ff f7 d8
> > 64 89 01 48
> > [ 3246.032094][ T8295] RSP: 002b:00007fd3c65a7038 EFLAGS: 00000246
> > ORIG_RAX: 000000000000002e
> > [ 3246.040597][ T8295] RAX: ffffffffffffffda RBX: 00007fd3c5935f80
> > RCX: 00007fd3c577dff9
> > [ 3246.048626][ T8295] RDX: 0000000000008004 RSI: 0000000020000140
> > RDI: 0000000000000003
> > [ 3246.056739][ T8295] RBP: 00007fd3c57f0296 R08: 0000000000000000
> > R09: 0000000000000000
> > [ 3246.064770][ T8295] R10: 0000000000000000 R11: 0000000000000246
> > R12: 0000000000000000
> > [ 3246.072801][ T8295] R13: 0000000000000000 R14: 00007fd3c5935f80
> > R15: 00007ffca7d46f08
> > [ 3246.080840][ T8295]  </TASK>
> > [ 3246.083901][ T8295]
> > [ 3246.086354][ T8295] Allocated by task 8199:
> > [ 3246.090716][ T8295]  kasan_save_stack+0x33/0x60
> > [ 3246.095449][ T8295]  kasan_save_track+0x14/0x30
> > [ 3246.100192][ T8295]  __kasan_kmalloc+0xaa/0xb0
> > [ 3246.104861][ T8295]  __kmalloc_node_noprof+0x211/0x440
> > [ 3246.110263][ T8295]  __kvmalloc_node_noprof+0xad/0x1a0
> > [ 3246.115630][ T8295]  bucket_table_alloc.isra.0+0x86/0x460
> > [ 3246.121336][ T8295]  rhashtable_init_noprof+0x43b/0x7d0
> > [ 3246.126797][ T8295]  rhltable_init_noprof+0x20/0x60
> > [ 3246.131899][ T8295]  nfsd_file_cache_init+0xd2/0x4d0
> > [ 3246.137089][ T8295]  nfsd_svc+0x5db/0x940
> > [ 3246.141306][ T8295]  nfsd_nl_threads_set_doit+0x535/0xbe0
> > [ 3246.147008][ T8295]  genl_family_rcv_msg_doit+0x202/0x2f0
> > [ 3246.152628][ T8295]  genl_rcv_msg+0x565/0x800
> > [ 3246.157209][ T8295]  netlink_rcv_skb+0x165/0x410
> > [ 3246.162039][ T8295]  genl_rcv+0x28/0x40
> > [ 3246.166089][ T8295]  netlink_unicast+0x53c/0x7f0
> > [ 3246.170946][ T8295]  netlink_sendmsg+0x8b8/0xd70
> > [ 3246.175774][ T8295]  ____sys_sendmsg+0x9ae/0xb40
> > [ 3246.180605][ T8295]  ___sys_sendmsg+0x135/0x1e0
> > [ 3246.185447][ T8295]  __sys_sendmsg+0x117/0x1f0
> > [ 3246.190110][ T8295]  do_syscall_64+0xcd/0x250
> > [ 3246.194685][ T8295]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > [ 3246.200643][ T8295]
> > [ 3246.203001][ T8295] Freed by task 8199:
> > [ 3246.207011][ T8295]  kasan_save_stack+0x33/0x60
> > [ 3246.211840][ T8295]  kasan_save_track+0x14/0x30
> > [ 3246.216587][ T8295]  kasan_save_free_info+0x3b/0x60
> > [ 3246.221674][ T8295]  __kasan_slab_free+0x51/0x70
> > [ 3246.226594][ T8295]  kfree+0x14f/0x4b0
> > [ 3246.230547][ T8295]  kvfree+0x47/0x50
> > [ 3246.234426][ T8295]  rhashtable_free_and_destroy+0x16c/0x990
> > [ 3246.240316][ T8295]  nfsd_file_cache_init+0x3e0/0x4d0
> > [ 3246.245587][ T8295]  nfsd_svc+0x5db/0x940
> > [ 3246.249802][ T8295]  nfsd_nl_threads_set_doit+0x535/0xbe0
> > [ 3246.255425][ T8295]  genl_family_rcv_msg_doit+0x202/0x2f0
> > [ 3246.261048][ T8295]  genl_rcv_msg+0x565/0x800
> > [ 3246.265634][ T8295]  netlink_rcv_skb+0x165/0x410
> > [ 3246.270463][ T8295]  genl_rcv+0x28/0x40
> > [ 3246.274539][ T8295]  netlink_unicast+0x53c/0x7f0
> > [ 3246.279369][ T8295]  netlink_sendmsg+0x8b8/0xd70
> > [ 3246.284200][ T8295]  ____sys_sendmsg+0x9ae/0xb40
> > [ 3246.289023][ T8295]  ___sys_sendmsg+0x135/0x1e0
> > [ 3246.293760][ T8295]  __sys_sendmsg+0x117/0x1f0
> > [ 3246.298430][ T8295]  do_syscall_64+0xcd/0x250
> > [ 3246.303005][ T8295]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > [ 3246.308966][ T8295]
> > [ 3246.311330][ T8295] The buggy address belongs to the object at
> > ffff8880301ac000
> > [ 3246.311330][ T8295]  which belongs to the cache kmalloc-4k of size 4096
> > [ 3246.325434][ T8295] The buggy address is located 16 bytes inside of
> > [ 3246.325434][ T8295]  freed 4096-byte region [ffff8880301ac000,
> > ffff8880301ad000)
> > [ 3246.339289][ T8295]
> > [ 3246.341654][ T8295] The buggy address belongs to the physical page:
> > [ 3246.348116][ T8295] page: refcount:1 mapcount:0
> > mapping:0000000000000000 index:0x0 pfn:0x301a8
> > [ 3246.356987][ T8295] head: order:3 mapcount:0 entire_mapcount:0
> > nr_pages_mapped:0 pincount:0
> > [ 3246.365630][ T8295] flags:
> > 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
> > [ 3246.373252][ T8295] page_type: f5(slab)
> > [ 3246.377291][ T8295] raw: 00fff00000000040 ffff88801ac42140
> > ffffea0000db9800 dead000000000002
> > [ 3246.385938][ T8295] raw: 0000000000000000 0000000000040004
> > 00000001f5000000 0000000000000000
> > [ 3246.394589][ T8295] head: 00fff00000000040 ffff88801ac42140
> > ffffea0000db9800 dead000000000002
> > [ 3246.403325][ T8295] head: 0000000000000000 0000000000040004
> > 00000001f5000000 0000000000000000
> > [ 3246.412047][ T8295] head: 00fff00000000003 ffffea0000c06a01
> > ffffffffffffffff 0000000000000000
> > [ 3246.420777][ T8295] head: 0000000000000008 0000000000000000
> > 00000000ffffffff 0000000000000000
> > [ 3246.429501][ T8295] page dumped because: kasan: bad access detected
> > [ 3246.435995][ T8295] page_owner tracks the page as allocated
> > [ 3246.441746][ T8295] page last allocated via order 3, migratetype
> > Unmovable, gfp_mask
> > 0xd2040(__GFP_IO|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC),
> > pid 21493, tgid 21493 (syz.0.3187), ts 1292101106947, free_ts
> > 1292053895765
> > [ 3246.462737][ T8295]  post_alloc_hook+0x2d1/0x350
> > [ 3246.467575][ T8295]  get_page_from_freelist+0x101e/0x3070
> > [ 3246.473199][ T8295]  __alloc_pages_noprof+0x223/0x25c0
> > [ 3246.478567][ T8295]  alloc_pages_mpol_noprof+0x2c9/0x610
> > [ 3246.484105][ T8295]  new_slab+0x2ba/0x3f0
> > [ 3246.488335][ T8295]  ___slab_alloc+0xd1d/0x16f0
> > [ 3246.493128][ T8295]  __slab_alloc.constprop.0+0x56/0xb0
> > [ 3246.498579][ T8295]  __kmalloc_noprof+0x379/0x410
> > [ 3246.503511][ T8295]  tomoyo_realpath_from_path+0xbf/0x710
> > [ 3246.509162][ T8295]  tomoyo_check_open_permission+0x2a7/0x3b0
> > [ 3246.515145][ T8295]  tomoyo_file_open+0xcf/0x100
> > [ 3246.519975][ T8295]  security_file_open+0x64c/0x9d0
> > [ 3246.525092][ T8295]  do_dentry_open+0x57c/0x1530
> > [ 3246.530014][ T8295]  vfs_open+0x82/0x3f0
> > [ 3246.534177][ T8295]  path_openat+0x1e6a/0x2d60
> > [ 3246.538831][ T8295]  do_filp_open+0x1dc/0x430
> > [ 3246.543412][ T8295] page last free pid 12198 tgid 12198 stack trace:
> > [ 3246.549952][ T8295]  free_unref_page+0x5f4/0xdc0
> > [ 3246.554884][ T8295]  __put_partials+0x14c/0x170
> > [ 3246.559637][ T8295]  qlist_free_all+0x4e/0x120
> > [ 3246.564302][ T8295]  kasan_quarantine_reduce+0x192/0x1e0
> > [ 3246.569924][ T8295]  __kasan_slab_alloc+0x69/0x90
> > [ 3246.574848][ T8295]  kmem_cache_alloc_node_noprof+0x153/0x310
> > [ 3246.580821][ T8295]  __alloc_skb+0x2b3/0x380
> > [ 3246.585351][ T8295]  alloc_skb_with_frags+0xe4/0x850
> > [ 3246.590526][ T8295]  sock_alloc_send_pskb+0x7f1/0x980
> > [ 3246.595806][ T8295]  mld_newpack.isra.0+0x1d4/0x7e0
> > [ 3246.600904][ T8295]  add_grhead+0x299/0x340
> > [ 3246.605299][ T8295]  add_grec+0x111e/0x1670
> > [ 3246.609696][ T8295]  mld_ifc_work+0x41f/0xca0
> > [ 3246.614273][ T8295]  process_one_work+0x958/0x1b30
> > [ 3246.619294][ T8295]  worker_thread+0x6c8/0xf00
> > [ 3246.623964][ T8295]  kthread+0x2c1/0x3a0
> > [ 3246.628104][ T8295]
> > [ 3246.630461][ T8295] Memory state around the buggy address:
> > [ 3246.636132][ T8295]  ffff8880301abf00: fc fc fc fc fc fc fc fc fc
> > fc fc fc fc fc fc fc
> > [ 3246.644246][ T8295]  ffff8880301abf80: fc fc fc fc fc fc fc fc fc
> > fc fc fc fc fc fc fc
> > [ 3246.652375][ T8295] >ffff8880301ac000: fa fb fb fb fb fb fb fb fb
> > fb fb fb fb fb fb fb
> > [ 3246.660488][ T8295]                          ^
> > [ 3246.665151][ T8295]  ffff8880301ac080: fb fb fb fb fb fb fb fb fb
> > fb fb fb fb fb fb fb
> > [ 3246.673265][ T8295]  ffff8880301ac100: fb fb fb fb fb fb fb fb fb
> > fb fb fb fb fb fb fb
> > [ 3246.681390][ T8295]
> > ==================================================================
>
> This:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/cel/linux.git/commit/?h=nfsd-fixes&id=dc0d0f885aa422f621bc1c2124133eff566b0bc8
>
> is in the v6.12-rc pile. We think it addresses this crasher.

Let's tell syzbot about the fix:

#syz fix: NFSD: Mark filecache "down" if init fails

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