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

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Message-ID: <50B6BE34.2010109@vflare.org>
Date:	Wed, 28 Nov 2012 17:45:24 -0800
From:	Nitin Gupta <ngupta@...are.org>
To:	Minchan Kim <minchan@...nel.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

On 11/28/2012 05:33 PM, Minchan Kim wrote:
> On Wed, Nov 28, 2012 at 02:15:05PM +0900, Minchan Kim wrote:
>> 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.
>>>
> I got it. You shouldn't mention partial write for proper description
> because it was usecase for swap-over-zram so it can't make partial write.
> Without partial write mention, your description is right but the problem
> I mentioned above is another problem for partial write so we need a another
> patch. I will cook.

I will include explanation of both the cases: use of unmapped buffer
and freed buffer, when I resend these patches.

>
>>> 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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