linux-kernel - Re: Coverity: zram

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Date:   Fri, 11 Nov 2022 09:37:10 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     coverity-bot <keescook@...omium.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Alexey Romanov <avromanov@...rdevices.ru>,
        linux-kernel@...r.kernel.org, Nick Terrell <terrelln@...com>,
        Minchan Kim <minchan@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Nitin Gupta <ngupta@...are.org>, Jens Axboe <axboe@...nel.dk>,
        Nhat Pham <nphamcs@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-block@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        linux-next@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: Coverity: zram_recompress(): OVERRUN

On (22/11/10 08:47), coverity-bot wrote:
[..]
> 1704     	class_index_old = zs_lookup_class_index(zram->mem_pool, comp_len_old);
> 1705     	/*
> 1706     	 * Iterate the secondary comp algorithms list (in order of priority)
> 1707     	 * and try to recompress the page.
> 1708     	 */
> 1709     	for (; prio < prio_max; prio++) {
> vvv     CID 1527270:    (OVERRUN)
> vvv     Overrunning array "zram->comps" of 4 8-byte elements at element index 4 (byte offset 39) using index "prio" (which evaluates to 4).
> 1710     		if (!zram->comps[prio])
> 1711     			continue;
> 1712
> 1713     		/*
> 1714     		 * Skip if the object is already re-compressed with a higher
> 1715     		 * priority algorithm (or same algorithm).

prio_max is always limited and max value it can have is 4 (ZRAM_MAX_COMPS).
Depending on use case we can limit prio_max even to lower values.

So we have

	for (; prio < 4; prio++) {
		foo = comps[prio];
	}

I don't see how prio can be 4 inside of this loop.