linux-kernel - Re: [PATCH] erofs: fix a potential NULL dereference of alloc

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Date:   Tue, 22 Mar 2022 16:28:09 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     xkernel.wang@...mail.com
Cc:     xiang@...nel.org, chao@...nel.org, linux-erofs@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] erofs: fix a potential NULL dereference of alloc_pages()

On Tue, Mar 22, 2022 at 04:08:12PM +0800, xkernel.wang@...mail.com wrote:
> From: Xiaoke Wang <xkernel.wang@...mail.com>
> 
> alloc_pages() returns the page on success or NULL if allocation fails,
> while set_page_private() will dereference `newpage`. So it is better to
> catch the memory error in case other errors happen.
> 
> Signed-off-by: Xiaoke Wang <xkernel.wang@...mail.com>
> ---
>  fs/erofs/zdata.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
> index 11c7a1a..36a5421 100644
> --- a/fs/erofs/zdata.c
> +++ b/fs/erofs/zdata.c
> @@ -735,11 +735,15 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
>  		struct page *const newpage =
>  				alloc_page(GFP_NOFS | __GFP_NOFAIL);
>  

It's really a nofail allocation, am I missing something?

Thanks,
Gao Xiang

> -		set_page_private(newpage, Z_EROFS_SHORTLIVED_PAGE);
> -		err = z_erofs_attach_page(clt, newpage,
> -					  Z_EROFS_PAGE_TYPE_EXCLUSIVE);
> -		if (!err)
> -			goto retry;
> +		if (!newpage) {
> +			err = -ENOMEM;
> +		} else {
> +			set_page_private(newpage, Z_EROFS_SHORTLIVED_PAGE);
> +			err = z_erofs_attach_page(clt, newpage,
> +						Z_EROFS_PAGE_TYPE_EXCLUSIVE);
> +			if (!err)
> +				goto retry;
> +		}
>  	}
>  
>  	if (err)
> --