linux-kernel - Re: [PATCH 1/2] zram: factor-out zram_decompress

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Date:	Mon, 29 Oct 2012 21:57:08 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] zram: factor-out zram_decompress_page() function (v2)

On (10/29/12 11:32), Nitin Gupta wrote:
> 
> Nothing should be printed (even a debug only message) for
> the ZRAM_ZERO case. This case can be quite common for certain
> kinds of data and would cause a huge log spew.  Also (!handle) case
> is not the same as zero-filled page case, so this message would
> be misleading.
> 
> So, we should either get rid of this warning entirely or only do
> pr_debug("Read before write ....") for (!handle) case and log nothing
> for ZRAM_ZERO case.
> 

I'd rather remove this message. Will resend.



By the way, about use after-free. I'm afraid you fix is not covering 100% of the
cases.

The problem is with this case:
[..]
334 
335         if (unlikely(clen > max_zpage_size)) {
336                 zram_stat_inc(&zram->stats.bad_compress);
337                 src = uncmem;
338                 clen = PAGE_SIZE;
339         }
340 
[..]

where uncmem could be:
-- kmap'ed page
-- kmalloc'ed page

both of which were unmap'ed/kfree'd before. you moved kfree to the end of the function,
while kunmap_atomic(user_mem) is still happening before src = uncmem/memcpy(cmem, src, clen)
pair.


	-ss
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