Re: [PATCH] nfsd: free nfsd_file by gc after adding it to lru list

Chuck Lever
2025-01-14T19:17:58Z

On 1/13/25 8:54 PM, Li Lingfeng wrote:
>
> 在 2025/1/13 22:07, Chuck Lever 写道:
>> Hello!
>>
>> On 1/12/25 9:59 PM, Li Lingfeng wrote:
>>> In nfsd_file_put, after inserting the nfsd_file into the nfsd_file_lru
>>> list, gc may be triggered in another thread and immediately release this
>>> nfsd_file, which will lead to a UAF when accessing this nfsd_file again.
>>
>> Do you happen to have a reproducer that can trigger this issue?
>>
> Hi, thanks for your reply.
>
> Here is the reproducer:
>
> Patch:
> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
>
> index 585163b4e11c..8a8245b0ce32 100644
> --- a/fs/nfsd/filecache.c
> +++ b/fs/nfsd/filecache.c
> @@ -356,6 +356,7 @@ nfsd_file_get(struct nfsd_file *nf)
>  void
>  nfsd_file_put(struct nfsd_file *nf)
>  {
> + static int count = 0;
>         might_sleep();
>         trace_nfsd_file_put(nf);
>
> @@ -370,6 +371,11 @@ nfsd_file_put(struct nfsd_file *nf)
>
>                 /* Try to add it to the LRU. If that fails, decrement. */
>                 if (nfsd_file_lru_add(nf)) {
> + if (count++ % 3 == 0) {
> + printk("%s %d sleep before test...\n", __func__, __LINE__);
> + msleep(5000);

This will certainly misbehave because garbage collection evicts files
after 2 seconds, and here the reproducer waits /5/ seconds.

But I can see how it is unsafe to continue dereferencing @nf after the
reference count is dropped. Jeff, any comments about this failure mode
or the proposed fix?

More below.

> + printk("%s %d sleep done\n", __func__, __LINE__);
> + }
>                         /* If it's still hashed, we're done */
>                         if (test_bit(NFSD_FILE_HASHED, &nf->nf_flags)) {
>                                 nfsd_file_schedule_laundrette();
>
> Steps:
> mkfs.ext4 -F /dev/sdb
> mount /dev/sdb /mnt/sdb
> echo "/mnt *(rw,no_root_squash,fsid=0)" > /etc/exports
> echo "/mnt/sdb *(rw,no_root_squash,fsid=1)" >> /etc/exports
> systemctl restart nfs-server
>
> for i in {1..10}; do filename="file$i.txt"; echo "123" > /mnt/
> sdb/"$filename"; done
> mount -t nfs -o rw,relatime,vers=3 127.0.0.1:/mnt/sdb /mnt/sdbb
> sh test.sh
>
> test.sh:
> for i in {1..10}; do
>     filename="file$i.txt"
>     cat /mnt/sdbb/"$filename" &
> done
>
> [  205.114996][ T2409]
> ==================================================================
>
> [  205.115006][ T2409] BUG: KASAN: slab-use-after-free in
> nfsd_file_put+0x190/0x3b0
> [  205.115055][ T2409] Read of size 8 at addr ffff88810798f3e8 by task
> nfsd/2409
> [  205.115062][ T2409]
> [  205.115068][ T2409] CPU: 1 UID: 0 PID: 2409 Comm: nfsd Not tainted
> 6.13.0-rc6-00038-g09a0fa92e5b4-dirty #83
> [  205.115078][ T2409] Hardware name: QEMU Standard PC (i440FX + PIIX,
> 1996), BIOS ?-20190727_073836-buildvm-
> ppc64le-16.ppc.fedoraproject.org-3.fc31 04/01/2014
> 123[  205.115085][ T2409] Call Trace:
> [  205.115095][ T2409]  <TASK>
>
> [  205.115101][ T2409]  dump_stack_lvl+0x68/0xa0
> [  205.115117][ T2409] print_address_description.constprop.0+0x2c/0x3d0
> [  205.115132][ T2409]  ? nfsd_file_put+0x190/0x3b0
> [  205.115140][ T2409]  print_report+0xb3/0x270
> 123[  205.115147][ T2409]  ? kasan_addr_to_slab+0xd/0xa0
> [  205.115160][ T2409]  kasan_report+0x93/0xc0
>
> [  205.115168][ T2409]  ? nfsd_file_put+0x190/0x3b0
> [  205.115196][ T2409]  kasan_check_range+0xf6/0x1b0
> [  205.115207][ T2409]  nfsd_file_put+0x190/0x3b0
> [  205.115217][ T2409]  nfsd_read+0x151/0x3b0
> [  205.115229][ T2409]  ? __pfx_nfsd_read+0x10/0x10
> [  205.115238][ T2409]  ? lock_acquire+0x15c/0x3f0
> [  205.115249][ T2409]  ? nfsd_init_request+0x6b/0x300
> [  205.115259][ T2409]  ? rcu_is_watching+0x20/0x50
> [  205.115272][ T2409]  nfsd3_proc_read+0x1c9/0x280
> [  205.115285][ T2409]  nfsd_dispatch+0x1b7/0x4c0
> [  205.115294][ T2409]  ? __pfx_nfsd_dispatch+0x10/0x10
> [  205.115305][ T2409]  ? __asan_memset+0x24/0x50
> [  205.115315][ T2409]  ? rcu_is_watching+0x20/0x50
> [  205.115325][ T2409]  svc_process_common+0x615/0xd20
> [  205.115346][ T2409]  ? __pfx_svc_process_common+0x10/0x10
> [  205.115359][ T2409]  ? __pfx_nfsd_dispatch+0x10/0x10
> [  205.115374][ T2409]  ? mark_held_locks+0x24/0x90
> [  205.115388][ T2409]  ? xdr_init_decode+0x11d/0x190
> [  205.115410][ T2409]  svc_process+0x2a8/0x430
> [  205.115435][ T2409]  svc_handle_xprt+0x71c/0xb40
> [  205.115458][ T2409]  svc_recv+0x5f1/0x9b0
> [  205.115477][ T2409]  ? svc_recv+0xab/0x9b0
> [  205.115501][ T2409]  nfsd+0x1e7/0x390
> [  205.115522][ T2409]  ? __pfx_nfsd+0x10/0x10
> [  205.115536][ T2409]  kthread+0x1a7/0x1f0
> [  205.115551][ T2409]  ? kthread+0xfb/0x1f0
> [  205.139288][ T2409]  ? __pfx_kthread+0x10/0x10
> [  205.139769][ T2409]  ret_from_fork+0x34/0x60
> [  205.140238][ T2409]  ? __pfx_kthread+0x10/0x10
> [  205.140701][ T2409]  ret_from_fork_asm+0x1a/0x30
> [  205.141226][ T2409]  </TASK>
> [  205.141548][ T2409]
> [  205.141809][ T2409] Allocated by task 2409:
> [  205.142254][ T2409]  kasan_save_stack+0x24/0x50
> [  205.142726][ T2409]  kasan_save_track+0x14/0x30
> [  205.143214][ T2409]  __kasan_slab_alloc+0x87/0x90
> [  205.143694][ T2409]  kmem_cache_alloc_noprof+0x162/0x4a0
> [  205.144268][ T2409]  nfsd_file_do_acquire+0x3a2/0x1420
> [  205.144812][ T2409]  nfsd_file_acquire_gc+0x5a/0x80
> [  205.145338][ T2409]  nfsd_read+0xc6/0x3b0
> [  205.145766][ T2409]  nfsd3_proc_read+0x1c9/0x280
> [  205.146226][ T2409]  nfsd_dispatch+0x1b7/0x4c0
> [  205.146682][ T2409]  svc_process_common+0x615/0xd20
> [  205.147212][ T2409]  svc_process+0x2a8/0x430
> [  205.147652][ T2409]  svc_handle_xprt+0x71c/0xb40
> [  205.148140][ T2409]  svc_recv+0x5f1/0x9b0
> [  205.148569][ T2409]  nfsd+0x1e7/0x390
> [  205.148966][ T2409]  kthread+0x1a7/0x1f0
> [  205.149388][ T2409]  ret_from_fork+0x34/0x60
> [  205.149838][ T2409]  ret_from_fork_asm+0x1a/0x30
> [  205.150330][ T2409]
> [  205.150646][ T2409] Freed by task 0:
> [  205.151120][ T2409]  kasan_save_stack+0x24/0x50
> [  205.151778][ T2409]  kasan_save_track+0x14/0x30
> [  205.152409][ T2409]  kasan_save_free_info+0x3a/0x60
> [  205.153073][ T2409]  __kasan_slab_free+0x54/0x70
> [  205.153677][ T2409]  kmem_cache_free+0x159/0x5f0
> [  205.154169][ T2409]  rcu_do_batch+0x311/0x900
> [  205.154643][ T2409]  rcu_core+0x58c/0x6f0
> [  205.155065][ T2409]  handle_softirqs+0xf8/0x570
> [  205.155526][ T2409]  irq_exit_rcu+0x141/0x160
> [  205.156004][ T2409]  sysvec_apic_timer_interrupt+0x76/0x90
> [  205.156575][ T2409]  asm_sysvec_apic_timer_interrupt+0x1a/0x20
> [  205.157196][ T2409]
> [  205.157416][ T2409] Last potentially related work creation:
> [  205.157986][ T2409]  kasan_save_stack+0x24/0x50
> [  205.158404][ T2409]  __kasan_record_aux_stack+0xa6/0xc0
> [  205.158893][ T2409]  __call_rcu_common.constprop.0+0xa5/0x8f0
> [  205.159515][ T2409]  nfsd_file_dispose_list+0x93/0xc0
> [  205.160067][ T2409]  nfsd_file_net_dispose+0x1ad/0x1f0
> [  205.160613][ T2409]  nfsd+0x1ef/0x390
> [  205.161021][ T2409]  kthread+0x1a7/0x1f0
> [  205.161451][ T2409]  ret_from_fork+0x34/0x60
> [  205.161908][ T2409]  ret_from_fork_asm+0x1a/0x30
> [  205.162407][ T2409]
> [  205.162663][ T2409] The buggy address belongs to the object at
> ffff88810798f3b8
> [  205.162663][ T2409]  which belongs to the cache nfsd_file of size 128
> [  205.164061][ T2409] The buggy address is located 48 bytes inside of
> [  205.164061][ T2409]  freed 128-byte region [ffff88810798f3b8,
> ffff88810798f438)
> [  205.165446][ T2409]
> [  205.165678][ T2409] The buggy address belongs to the physical page:
> [  205.166340][ T2409] page: refcount:1 mapcount:0
> mapping:0000000000000000 index:0xffff88810798f4a8 pfn:0x10798e
> [  205.167356][ T2409] head: order:1 mapcount:0 entire_mapcount:0
> nr_pages_mapped:0 pincount:0
> [  205.168204][ T2409] flags: 0x17ffffc0000240(workingset|head|node=0|
> zone=2|lastcpupid=0x1fffff)
> [  205.169071][ T2409] page_type: f5(slab)
> [  205.169483][ T2409] raw: 0017ffffc0000240 ffff888443a06500
> ffff8881214b4bc8 ffff8881214b4bc8
> [  205.170431][ T2409] raw: ffff88810798f4a8 0000000000220005
> 00000001f5000000 0000000000000000
> [  205.171554][ T2409] head: 0017ffffc0000240 ffff888443a06500
> ffff8881214b4bc8 ffff8881214b4bc8
> [  205.172630][ T2409] head: ffff88810798f4a8 0000000000220005
> 00000001f5000000 0000000000000000
> [  205.173509][ T2409] head: 0017ffffc0000001 ffffea00041e6381
> ffffffffffffffff 0000000000000000
> [  205.174587][ T2409] head: 0000000000000002 0000000000000000
> 00000000ffffffff 0000000000000000
> [  205.175538][ T2409] page dumped because: kasan: bad access detected
> [  205.176193][ T2409]
> [  205.176427][ T2409] Memory state around the buggy address:
> [  205.176997][ T2409]  ffff88810798f280: fc fc fc fc fc fc fc fc fc fc
> fc fc fc fc fc fc
> [  205.177810][ T2409]  ffff88810798f300: fc fc fc fc fc fc fc fc fc fc
> fc fc fc fc fc fc
> [  205.178601][ T2409] >ffff88810798f380: fc fc fc fc fc fc fc fa fb fb
> fb fb fb fb fb fb
> [  205.179420]
> [ T2409]                                                           ^
> [  205.180190][ T2409]  ffff88810798f400: fb fb fb fb fb fb fb fc fc fc
> fc fc fc fc fc fc
> [  205.181000][ T2409]  ffff88810798f480: fc fc fc fc fc fc fc fc fc fc
> fc fc fc fc fc fc
> [  205.181844][ T2409]
> ==================================================================
> [  205.182677][ T2409] Disabling lock debugging due to kernel taint
>
>>
>>> All the places where unhash is done will also perform lru_remove, so
>>> there
>>> is no need to do lru_remove separately here. After inserting the
>>> nfsd_file
>>> into the nfsd_file_lru list, it can be released by relying on gc.
>>>
>>> Fixes: 4a0e73e635e3 ("NFSD: Leave open files out of the filecache LRU")
>>
>> The code that is being replaced below was introduced in ac3a2585f018
>> ("nfsd: rework refcounting in filecache"). This fix won't apply to
>> kernels that have only 4a0e73e635e3 but not ac3a2585f018, for instance.
>>
>> At the very least we need to add "Cc: stable@vger.kernel.org # v6.2" but
>> I'm not convinced "Fixes: 4a0e73e635e3" is correct.
> The replaced code is indeed introduced by ac3a2585f018 ("nfsd: rework
> refcounting in filecache").
> However, commit 4a0e73e635e3 ("NFSD: Leave open files out of the filecache
> LRU") added the nfsd_file_lru_add operation in nfsd_file_put, which
> enables concurrent removal of the current nfsd_file by gc, potentially
> triggering UAF when accessing nfsd_file below.
> Therefore, I use 4a0e73e635e3 as the fix tag.
>>
>>
>>> Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
>>> ---
>>>   fs/nfsd/filecache.c | 12 ++----------
>>>   1 file changed, 2 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
>>> index a1cdba42c4fa..37b65cb1579a 100644
>>> --- a/fs/nfsd/filecache.c
>>> +++ b/fs/nfsd/filecache.c
>>> @@ -372,18 +372,10 @@ nfsd_file_put(struct nfsd_file *nf)
>>>           /* Try to add it to the LRU.  If that fails, decrement. */
>>>           if (nfsd_file_lru_add(nf)) {
>>>               /* If it's still hashed, we're done */
>>> -            if (test_bit(NFSD_FILE_HASHED, &nf->nf_flags)) {
>>> +            if (list_lru_count(&nfsd_file_lru))
>>>                   nfsd_file_schedule_laundrette();
>>> -                return;
>>> -            }
>>
>> The above change does not seem to be related to the fix described
>> in the patch description. Can you help me understand why this is
>> needed?
> Firstly, I removed the access operations on nfsd_file, including checking
> nf->nf_flags and nfsd_file_lru_remove, to prevent triggering UAF after
> concurrent release of nfsd_file by gc.

Understood and agreed.

> Secondly, I kept nfsd_file_schedule_laundrette and used list_lru_count to
> determine whether to trigger gc, with the aim of initiating gc and
> reclaiming nfsd_file as soon as possible even if no other process triggers
> gc.

The list_lru_count() remains non-zero for extended periods which might
cause the proposed code to trigger the laundrette more often than is
optimal.

>>> -            /*
>>> -             * We're racing with unhashing, so try to remove it from
>>> -             * the LRU. If removal fails, then someone else already
>>> -             * has our reference.
>>> -             */
>>> -            if (!nfsd_file_lru_remove(nf))
>>> -                return;
>>> +            return;
>>>           }
>>>       }
>>>       if (refcount_dec_and_test(&nf->nf_ref))
>>
>>

--
Chuck Lever