Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:2 as permitted sender) client-ip=2620:137:e000::3:2;
Date:   Tue, 7 Nov 2023 16:39:11 +0900
From:   Sergey Senozhatsky <senozhatsky@chromium.org>
To:     Minchan Kim <minchan@kernel.org>
Cc:     Sergey Senozhatsky <senozhatsky@chromium.org>,
        linux-kernel@vger.kernel.org, linux-block@vger.kernel.org,
        zhouxianrong <zhouxianrong@huawei.com>,
        Vasily Averin <vasily.averin@linux.dev>
Subject: Re: [PATCH] zram: unsafe zram_get_element call in zram_read_page()
Message-ID: <20231107073911.GB11577@google.com>
References: <d10cdf1d-4a67-48df-b389-3a51f60e9431@linux.dev>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <d10cdf1d-4a67-48df-b389-3a51f60e9431@linux.dev>
Precedence: bulk

On (23/11/06 22:54), Vasily Averin wrote:
> @@ -1362,14 +1362,14 @@ static int zram_read_page(struct zram *zram, struct page *page, u32 index,
>  		ret = zram_read_from_zspool(zram, page, index);
>  		zram_slot_unlock(zram, index);
>  	} else {
> +		unsigned long entry = zram_get_element(zram, index);
>  		/*
>  		 * The slot should be unlocked before reading from the backing
>  		 * device.
>  		 */
>  		zram_slot_unlock(zram, index);
>  
> -		ret = read_from_bdev(zram, page, zram_get_element(zram, index),
> -				     parent);
> +		ret = read_from_bdev(zram, page, entry, parent);

Hmmm,
We may want to do more here. Basically, we probably need to re-confirm
after read_from_bdev() that the entry at index still has ZRAM_WB set
and, if so, that it points to the same blk_idx. IOW, check that it has
not been free-ed and re-used under us.

Minchan, what do you think? Is that scenario possible?