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

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Date:   Thu, 4 Nov 2021 15:16:06 +0800
From:   Chao Yu <chao@...nel.org>
To:     Dongliang Mu <mudongliangabcd@...il.com>,
        Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: f2fs: fix UAF in f2fs_available_free_memory

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?

Thanks,

>   	f2fs_destroy_post_read_wq(sbi);
>   stop_ckpt_thread:
>   	f2fs_stop_ckpt_thread(sbi);
>