Re: [PATCH 3/4] f2fs: fix NULL pointer dereference in f2fs_verity_work()

[Eric Biggers]

On Thu, Mar 19, 2020 at 07:57:59PM +0800, Chao Yu wrote:
> If both compression and fsverity feature is on, generic/572 will
> report below NULL pointer dereference bug.
> 
>  BUG: kernel NULL pointer dereference, address: 0000000000000018
>  RIP: 0010:f2fs_verity_work+0x60/0x90 [f2fs]
>  #PF: supervisor read access in kernel mode
>  Workqueue: fsverity_read_queue f2fs_verity_work [f2fs]
>  RIP: 0010:f2fs_verity_work+0x60/0x90 [f2fs]
>  Call Trace:
>   process_one_work+0x16c/0x3f0
>   worker_thread+0x4c/0x440
>   ? rescuer_thread+0x350/0x350
>   kthread+0xf8/0x130
>   ? kthread_unpark+0x70/0x70
>   ret_from_fork+0x35/0x40
> 
> There are two issue in f2fs_verity_work():
> - it needs to traverse and verify all pages in bio.
> - if pages in bio belong to non-compressed cluster, accessing
> decompress IO context stored in page private will cause NULL
> pointer dereference.
> 
> Fix them.
> 
> Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
> ---
>  fs/f2fs/data.c | 35 ++++++++++++++++++++++++++++++-----
>  1 file changed, 30 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 5c5db09324b7..66e49fc1056e 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -187,12 +187,37 @@ static void f2fs_verify_pages(struct page **rpages, unsigned int cluster_size)
>  
>  static void f2fs_verify_bio(struct bio *bio)
>  {
> -	struct page *page = bio_first_page_all(bio);
> -	struct decompress_io_ctx *dic =
> -			(struct decompress_io_ctx *)page_private(page);
> +	struct bio_vec *bv;
> +	struct bvec_iter_all iter_all;
> +	struct decompress_io_ctx *dic, *pdic = NULL;
> +
> +	bio_for_each_segment_all(bv, bio, iter_all) {
> +		struct page *page = bv->bv_page;
> +
> +		dic = (struct decompress_io_ctx *)page_private(page);
> +
> +		if (dic) {
> +			if (dic != pdic) {
> +				f2fs_verify_pages(dic->rpages,
> +							dic->cluster_size);
> +				f2fs_free_dic(dic);
> +				pdic = dic;
> +			}
> +			continue;
> +		}
> +		pdic = dic;
>  
> -	f2fs_verify_pages(dic->rpages, dic->cluster_size);
> -	f2fs_free_dic(dic);
> +		if (bio->bi_status || PageError(page)) {
> +			ClearPageUptodate(page);
> +			ClearPageError(page);
> +		} else {
> +			if (fsverity_verify_page(page))
> +				SetPageUptodate(page);
> +			else
> +				SetPageError(page);
> +		}
> +		unlock_page(page);
> +	}

I'm a bit confused why you added SetPageError() before unlocking the page.
The other error paths actually clear the Error flag, not set it.  I thought
there's a reason for that?

- Eric