linux-kernel - Re: [PATCH] x86: fix broken flushing in GART nofullflush path

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Message-ID: <20081203090218.GE5668@elte.hu>
Date:	Wed, 3 Dec 2008 10:02:18 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Joerg Roedel <joerg.roedel@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] x86: fix broken flushing in GART nofullflush path

u
* Joerg Roedel <joro@...tes.org> wrote:

> On Tue, Dec 02, 2008 at 08:27:59PM +0100, Ingo Molnar wrote:
> > 
> > * Joerg Roedel <joerg.roedel@....com> wrote:
> > 
> > > In the non-default nofullflush case the GART is only flushed when
> > > next_bit wraps around. But it can happen that an unmap operation unmaps
> > > memory which is behind the current next_bit location. If these addresses
> > > are reused it may result in stale GART IO/TLB entries. Fix this by
> > > setting the GART next_bit always behind an unmapped location.
> > > 
> > > Signed-off-by: Joerg Roedel <joerg.roedel@....com>
> > > ---
> > >  arch/x86/kernel/pci-gart_64.c |    2 ++
> > >  1 files changed, 2 insertions(+), 0 deletions(-)
> > 
> > applied to tip/x86/iommu, thanks Joerg!
> > 
> > a stale iotlb should not cause any problems in this particular GART case, 
> > right? It might be a security leak in a security-domain enforcing iotlb 
> > case, but the GART is a DMA bouncing helper in essence. Can you see any 
> > failure mode of this bug?
> 
> It can cause data corruption because the GART redirects the IO to a 
> wrong address because of a stale entry in its TLB. I found this bug 
> after I fixed the same issue in the AMD IOMMU code (commit 80be308d).
> 
> The reason this was not found earlier is that lazy flushing is not the 
> default in GART. But I didn't try to trigger the bug.

ah, indeed. The stale entries themselves are not an issue - the problem 
is reuse: if a stale entry is reused but then we do not flush, we can get 
data corruption. I've cherry-picked it over into x86/urgent as well.

	Ingo
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