Re: [PATCH 2/2] NFSD: fix race between nfsd registration and exports_proc

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2025-03-06 at 14:50 +0530, Maninder Singh wrote:
> As of now nfsd calls create_proc_exports_entry() at start of init_nfsd
> and cleanup by remove_proc_entry() at last of exit_nfsd.
> 
> Which causes kernel OOPs if there is race between below 2 operations:
> (i) exportfs -r
> (ii) mount -t nfsd none /proc/fs/nfsd
> 
> for 5.4 kernel ARM64:
> 
> CPU 1:
> el1_irq+0xbc/0x180
> arch_counter_get_cntvct+0x14/0x18
> running_clock+0xc/0x18
> preempt_count_add+0x88/0x110
> prep_new_page+0xb0/0x220
> get_page_from_freelist+0x2d8/0x1778
> __alloc_pages_nodemask+0x15c/0xef0
> __vmalloc_node_range+0x28c/0x478
> __vmalloc_node_flags_caller+0x8c/0xb0
> kvmalloc_node+0x88/0xe0
> nfsd_init_net+0x6c/0x108 [nfsd]
> ops_init+0x44/0x170
> register_pernet_operations+0x114/0x270
> register_pernet_subsys+0x34/0x50
> init_nfsd+0xa8/0x718 [nfsd]
> do_one_initcall+0x54/0x2e0
> 
> CPU 2 :
> Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
> 
> PC is at : exports_net_open+0x50/0x68 [nfsd]
> 
> Call trace:
> exports_net_open+0x50/0x68 [nfsd]
> exports_proc_open+0x2c/0x38 [nfsd]
> proc_reg_open+0xb8/0x198
> do_dentry_open+0x1c4/0x418
> vfs_open+0x38/0x48
> path_openat+0x28c/0xf18
> do_filp_open+0x70/0xe8
> do_sys_open+0x154/0x248
> 
> Sometimes it crashes at exports_net_open() and sometimes cache_seq_next_rcu().
> 
> and same is happening on latest 6.14 kernel as well:
> 
> [    0.000000] Linux version 6.14.0-rc5-next-20250304-dirty
> ...
> [  285.455918] Unable to handle kernel paging request at virtual address 00001f4800001f48
> ...
> [  285.464902] pc : cache_seq_next_rcu+0x78/0xa4
> ...
> [  285.469695] Call trace:
> [  285.470083]  cache_seq_next_rcu+0x78/0xa4 (P)
> [  285.470488]  seq_read+0xe0/0x11c
> [  285.470675]  proc_reg_read+0x9c/0xf0
> [  285.470874]  vfs_read+0xc4/0x2fc
> [  285.471057]  ksys_read+0x6c/0xf4
> [  285.471231]  __arm64_sys_read+0x1c/0x28
> [  285.471428]  invoke_syscall+0x44/0x100
> [  285.471633]  el0_svc_common.constprop.0+0x40/0xe0
> [  285.471870]  do_el0_svc_compat+0x1c/0x34
> [  285.472073]  el0_svc_compat+0x2c/0x80
> [  285.472265]  el0t_32_sync_handler+0x90/0x140
> [  285.472473]  el0t_32_sync+0x19c/0x1a0
> [  285.472887] Code: f9400885 93407c23 937d7c27 11000421 (f86378a3)
> [  285.473422] ---[ end trace 0000000000000000 ]---
> 
> It reproduced simply with below script:
> while [ 1 ]
> do
> /exportfs -r
> done &
> 
> while [ 1 ]
> do
> insmod /nfsd.ko
> mount -t nfsd none /proc/fs/nfsd
> umount /proc/fs/nfsd
> rmmod nfsd
> done &
> 
> So exporting interfaces to user space shall be done at last and
> cleanup at first place.
> 
> With change there is no Kernel OOPs.
> 
> Co-developed-by: Shubham Rana <s9.rana@samsung.com>
> Signed-off-by: Shubham Rana <s9.rana@samsung.com>
> Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
> ---
>  fs/nfsd/nfsctl.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
> index d773481bcf10..f9763ced743d 100644
> --- a/fs/nfsd/nfsctl.c
> +++ b/fs/nfsd/nfsctl.c
> @@ -2291,12 +2291,9 @@ static int __init init_nfsd(void)
>  	if (retval)
>  		goto out_free_pnfs;
>  	nfsd_lockd_init();	/* lockd->nfsd callbacks */
> -	retval = create_proc_exports_entry();
> -	if (retval)
> -		goto out_free_lockd;
>  	retval = register_pernet_subsys(&nfsd_net_ops);
>  	if (retval < 0)
> -		goto out_free_exports;
> +		goto out_free_lockd;
>  	retval = register_cld_notifier();
>  	if (retval)
>  		goto out_free_subsys;
> @@ -2307,12 +2304,17 @@ static int __init init_nfsd(void)
>  	if (retval)
>  		goto out_free_nfsd4;
>  	retval = genl_register_family(&nfsd_nl_family);
> +	if (retval)
> +		goto out_free_filesystem;
> +	retval = create_proc_exports_entry();
>  	if (retval)
>  		goto out_free_all;
>  	nfsd_localio_ops_init();
>  
>  	return 0;
>  out_free_all:
> +	genl_unregister_family(&nfsd_nl_family);
> +out_free_filesystem:
>  	unregister_filesystem(&nfsd_fs_type);
>  out_free_nfsd4:
>  	nfsd4_destroy_laundry_wq();
> @@ -2320,9 +2322,6 @@ static int __init init_nfsd(void)
>  	unregister_cld_notifier();
>  out_free_subsys:
>  	unregister_pernet_subsys(&nfsd_net_ops);
> -out_free_exports:
> -	remove_proc_entry("fs/nfs/exports", NULL);
> -	remove_proc_entry("fs/nfs", NULL);
>  out_free_lockd:
>  	nfsd_lockd_shutdown();
>  	nfsd_drc_slab_free();
> @@ -2335,14 +2334,14 @@ static int __init init_nfsd(void)
>  
>  static void __exit exit_nfsd(void)
>  {
> +	remove_proc_entry("fs/nfs/exports", NULL);
> +	remove_proc_entry("fs/nfs", NULL);
>  	genl_unregister_family(&nfsd_nl_family);
>  	unregister_filesystem(&nfsd_fs_type);
>  	nfsd4_destroy_laundry_wq();
>  	unregister_cld_notifier();
>  	unregister_pernet_subsys(&nfsd_net_ops);
>  	nfsd_drc_slab_free();
> -	remove_proc_entry("fs/nfs/exports", NULL);
> -	remove_proc_entry("fs/nfs", NULL);
>  	nfsd_lockd_shutdown();
>  	nfsd4_free_slabs();
>  	nfsd4_exit_pnfs();


To make sure I understand, the race is that sometimes the exports
interface gets created before the net namespace is set up, and then
that causes GPFs when exports_net_open tries to access the nfsd_net?

Thanks,
-- 
Jeff Layton <jlayton@kernel.org>