linux-kernel - Re: [PATCH] zram: Prevent use of unmapped buffer

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Message-ID: <20121128051505.GD23136@blaptop>
Date:	Wed, 28 Nov 2012 14:15:05 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Nitin Gupta <ngupta@...are.org>
Cc:	Greg KH <greg@...ah.com>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	Sam Hansen <solid.se7en@...il.com>, Tomas M <tomas@...x.org>,
	Mihail Kasadjikov <hamer.mk@...il.com>,
	Linux Driver Project <devel@...uxdriverproject.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] zram: Prevent use of unmapped buffer

Hi Nitin,

On Mon, Nov 26, 2012 at 11:26:07PM -0800, Nitin Gupta wrote:
> The commit c8f2f0db1 ("zram: Fix handling of incompressible pages")
> introduced a bug which caused a kunmap()'ed buffer to be used in case

I got confused by the descripton. :(
The descripton is not right.
The problem is to access freed memory, not accessing to kunmaped buffer.

partial I/O write.

1. uncmem = kmalloc
2. zram_decompress_page(uncmem)
3. memcpy(uncmem, user_mem)
4. lzo1x_1_compress(uncmem)
5. kfree(uncmem)
6. src = uncmem
7. memcpy(cmem, src, clen) <----- HIT

> of partial writes where the data was found to be incompressible.
> 
> This fixes bug 50081:
> https://bugzilla.kernel.org/show_bug.cgi?id=50081
> 
> Signed-off-by: Nitin Gupta <ngupta@...are.org>
> Reported-by: Mihail Kasadjikov <hamer.mk@...il.com>
> Reported-by: Tomas M <tomas@...x.org>

Good catch! Shame on me. I should have reivewed more carefully. :(
Please resend it with revised descrption and title.
I will rebase mh patchset just sent on top of this bug fix patch.

P.S) Sigh, Now code isn't clean due to partial read/write path handling.
     IMHO, sooner or later, we need refactoring.

-- 
Kind regards,
Minchan Kim
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