linux-kernel - Re: [PATCH] fs: f2fs: fix UAF in f2fs_available_free

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Date:   Thu, 4 Nov 2021 15:42:29 +0800
From:   Chao Yu <chao@...nel.org>
To:     Dongliang Mu <mudongliangabcd@...il.com>
Cc:     Jaegeuk Kim <jaegeuk@...nel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs: f2fs: fix UAF in f2fs_available_free_memory

On 2021/11/4 15:35, Dongliang Mu wrote:
> On Thu, Nov 4, 2021 at 3:16 PM Chao Yu <chao@...nel.org> wrote:
>>
>> On 2021/11/3 22:22, Dongliang Mu wrote:
>>> f2fs_fill_super
>>> -> f2fs_build_segment_manager
>>>      -> create_discard_cmd_control
>>>         -> f2fs_start_discard_thread
>>>
>>> It invokes kthread_run to create a thread and run issue_discard_thread.
>>>
>>> However, if f2fs_build_node_manager fails, the control flow goes to
>>> free_nm and calls f2fs_destroy_node_manager. This function will free
>>> sbi->nm_info. However, if issue_discard_thread accesses sbi->nm_info
>>> after the deallocation, but before the f2fs_stop_discard_thread, it will
>>> cause UAF(Use-after-free).
>>>
>>> -> f2fs_destroy_segment_manager
>>>      -> destroy_discard_cmd_control
>>>         -> f2fs_stop_discard_thread
>>>
>>> Fix this by switching the order of f2fs_build_segment_manager
>>> and f2fs_build_node_manager.
>>>
>>> Signed-off-by: Dongliang Mu <mudongliangabcd@...il.com>
>>> ---
>>>    fs/f2fs/super.c | 16 ++++++++--------
>>>    1 file changed, 8 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>>> index 78ebc306ee2b..1a23b64cfb74 100644
>>> --- a/fs/f2fs/super.c
>>> +++ b/fs/f2fs/super.c
>>> @@ -4135,18 +4135,18 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
>>>        }
>>>
>>>        /* setup f2fs internal modules */
>>> -     err = f2fs_build_segment_manager(sbi);
>>> -     if (err) {
>>> -             f2fs_err(sbi, "Failed to initialize F2FS segment manager (%d)",
>>> -                      err);
>>> -             goto free_sm;
>>> -     }
>>>        err = f2fs_build_node_manager(sbi);
>>>        if (err) {
>>>                f2fs_err(sbi, "Failed to initialize F2FS node manager (%d)",
>>>                         err);
>>>                goto free_nm;
>>>        }
>>> +     err = f2fs_build_segment_manager(sbi);
>>> +     if (err) {
>>> +             f2fs_err(sbi, "Failed to initialize F2FS segment manager (%d)",
>>> +                      err);
>>> +             goto free_sm;
>>> +     }
>>>
>>>        /* For write statistics */
>>>        sbi->sectors_written_start = f2fs_get_sectors_written(sbi);
>>> @@ -4351,10 +4351,10 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
>>>        sbi->node_inode = NULL;
>>>    free_stats:
>>>        f2fs_destroy_stats(sbi);
>>> -free_nm:
>>> -     f2fs_destroy_node_manager(sbi);
>>>    free_sm:
>>>        f2fs_destroy_segment_manager(sbi);
>>> +free_nm:
>>> +     f2fs_destroy_node_manager(sbi);
>>
>> IIRC, above two functions shouldn't not be called reversely due to some
>> resource dependency, Jaegeuk, please help to confirm this.
>>
>> So I suggest to call destroy_discard_cmd_control() before
>> f2fs_destroy_node_manager(), is it fine to you?
> 
> Maybe f2fs_stop_discard_thread is better than
> destroy_discard_cmd_control. It only stops the kthread, leading to
> fewer side effects.

Fine to me. :)

Thanks,

> 
> How do you think?
> 
>>
>> Thanks,
>>
>>>        f2fs_destroy_post_read_wq(sbi);
>>>    stop_ckpt_thread:
>>>        f2fs_stop_ckpt_thread(sbi);
>>>