linux-kernel - Re: [PATCH] zram: fix null dereference of handle

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Date:   Tue, 19 Sep 2017 14:39:35 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, kernel-team <kernel-team@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] zram: fix null dereference of handle

On (09/19/17 11:34), Minchan Kim wrote:
[..]
>  static void zram_meta_free(struct zram *zram, u64 disksize)
>  {
>  	size_t num_pages = disksize >> PAGE_SHIFT;
> @@ -876,11 +855,18 @@ static int __zram_bvec_read(struct zram *zram, struct page *page, u32 index,
>  		zram_slot_unlock(zram, index);
>  	}
>  
> -	if (zram_same_page_read(zram, index, page, 0, PAGE_SIZE))
> -		return 0;
> -
>  	zram_slot_lock(zram, index);
>  	handle = zram_get_handle(zram, index);
> +	if (unlikely(!handle || zram_test_flag(zram, index, ZRAM_SAME))) {
> +		void *mem;


is this branch really unlikely()? ZRAM_SAME ratio really depends,
on some setups it can be quite likely, I suspect.


another question, "!handle  ==  value & ZRAM_SAME"? if so, then why not
just check for `flags & ZRAM_SAME'? if not then:

-  for `value & ZRAM_SAME' you fill the page with zram_get_element(zram, index)
   and return 0. ok.

-  for !handle.... you also fill the page with zram_get_element(zram, index)
   and return 0. is this ok? shouldn't !handle return error in this case?


I really suspect that there are some paths that can lead to !handle
entry, that will not be ZRAM_SAME. e.g. error return from compression
path.

	-ss

> +		mem = kmap_atomic(page);
> +		zram_fill_page(mem, PAGE_SIZE, zram_get_element(zram, index));
> +		kunmap_atomic(mem);
> +		zram_slot_unlock(zram, index);
> +		return 0;
> +	}
> +
>  	size = zram_get_obj_size(zram, index);
>  
>  	src = zs_map_object(zram->mem_pool, handle, ZS_MM_RO);
> -- 
> 2.7.4
>