Re: [PATCH v2] f2fs: compress: fix UAF of f2fs_inode_info in f2fs_free_dic

[Chao Yu <chao@kernel.org>]

On 6/5/25 10:44, Zhiguo Niu wrote:
> Chao Yu <chao@kernel.org> 于2025年6月5日周四 10:23写道：
>>
>> On 6/4/25 19:37, Zhiguo Niu wrote:
>>> The decompress_io_ctx may be released asynchronously after
>>> I/O completion. If this file is deleted immediately after read,
>>> and the kworker of processing post_read_wq has not been executed yet
>>> due to high workloads, It is possible that the inode(f2fs_inode_info)
>>> is evicted and freed before it is used f2fs_free_dic.
>>>
>>>     The UAF case as below:
>>>     Thread A                                      Thread B
>>>     - f2fs_decompress_end_io
>>>      - f2fs_put_dic
>>>       - queue_work
>>>         add free_dic work to post_read_wq
>>>                                                    - do_unlink
>>>                                                     - iput
>>>                                                      - evict
>>>                                                       - call_rcu
>>>     This file is deleted after read.
>>>
>>>     Thread C                                 kworker to process post_read_wq
>>>     - rcu_do_batch
>>>      - f2fs_free_inode
>>>       - kmem_cache_free
>>>      inode is freed by rcu
>>>                                              - process_scheduled_works
>>>                                               - f2fs_late_free_dic
>>>                                                - f2fs_free_dic
>>>                                                 - f2fs_release_decomp_mem
>>>                                       read (dic->inode)->i_compress_algorithm
>>>
>>> This patch use __iget before f2fs_free_dic and iput after free the dic.
>>>
>>> Cc: Daeho Jeong <daehojeong@google.com>
>>> Fixes: bff139b49d9f ("f2fs: handle decompress only post processing in softirq")
>>> Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
>>> Signed-off-by: Baocong Liu <baocong.liu@unisoc.com>
>>> ---
>>> v2: use __iget/iput function
>>> ---
>>>  fs/f2fs/compress.c | 16 +++++++++++-----
>>>  1 file changed, 11 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/fs/f2fs/compress.c b/fs/f2fs/compress.c
>>> index b3c1df9..3f0c18d 100644
>>> --- a/fs/f2fs/compress.c
>>> +++ b/fs/f2fs/compress.c
>>> @@ -1687,7 +1687,7 @@ static void f2fs_release_decomp_mem(struct decompress_io_ctx *dic,
>>>  }
>>>
>>>  static void f2fs_free_dic(struct decompress_io_ctx *dic,
>>> -             bool bypass_destroy_callback);
>>> +             bool bypass_destroy_callback, bool late_free);
>>>
>>>  struct decompress_io_ctx *f2fs_alloc_dic(struct compress_ctx *cc)
>>>  {
>>> @@ -1743,12 +1743,12 @@ struct decompress_io_ctx *f2fs_alloc_dic(struct compress_ctx *cc)
>>>       return dic;
>>>
>>>  out_free:
>>> -     f2fs_free_dic(dic, true);
>>> +     f2fs_free_dic(dic, true, false);
>>>       return ERR_PTR(ret);
>>>  }
>>>
>>>  static void f2fs_free_dic(struct decompress_io_ctx *dic,
>>> -             bool bypass_destroy_callback)
>>> +             bool bypass_destroy_callback, bool late_free)
>>>  {
>>>       int i;
>>>
>>> @@ -1775,6 +1775,8 @@ static void f2fs_free_dic(struct decompress_io_ctx *dic,
>>>       }
>>>
>>>       page_array_free(dic->inode, dic->rpages, dic->nr_rpages);
>>> +     if (late_free)
>>> +             iput(dic->inode);
>>>       kmem_cache_free(dic_entry_slab, dic);
>>>  }
>>>
>>> @@ -1783,16 +1785,20 @@ static void f2fs_late_free_dic(struct work_struct *work)
>>>       struct decompress_io_ctx *dic =
>>>               container_of(work, struct decompress_io_ctx, free_work);
>>>
>>> -     f2fs_free_dic(dic, false);
>>> +     f2fs_free_dic(dic, false, true);
>>>  }
>>>
>>>  static void f2fs_put_dic(struct decompress_io_ctx *dic, bool in_task)
>>>  {
>>>       if (refcount_dec_and_test(&dic->refcnt)) {
>>>               if (in_task) {
>>> -                     f2fs_free_dic(dic, false);
>>> +                     f2fs_free_dic(dic, false, false);
>>>               } else {
>>>                       INIT_WORK(&dic->free_work, f2fs_late_free_dic);
>>> +                     /* to avoid inode is evicted simultaneously */
>>> +                     spin_lock(&dic->inode->i_lock);
>>> +                     __iget(dic->inode);
>>> +                     spin_unlock(&dic->inode->i_lock);
>>
>> Oh, how about using igrab() instead?
> Hi Chao,
> after I check igrab, I think it is ok,
> but here We don't need care about igarb's return value, right?

I guess "return NULL" should be impossible case, but still we can add
f2fs_bug_on() to detect any potential bug, right?

Thanks,

> Thanks!
> 
>>
>>>                       queue_work(F2FS_I_SB(dic->inode)->post_read_wq,
>>>                                       &dic->free_work);
>>>               }
>>