Re: [PATCH] zram: Prevent use of unmapped buffer

[Nitin Gupta]

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@xxxxxxxxxx>
> > > Reported-by: Mihail Kasadjikov <hamer.mk@xxxxxxxxx>
> > > Reported-by: Tomas M <tomas@xxxxxxxx>
> > 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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