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

From: Joerg Roedel
Date: Tue Dec 02 2008 - 15:24:32 EST

On Tue, Dec 02, 2008 at 08:27:59PM +0100, Ingo Molnar wrote:
> * Joerg Roedel <joerg.roedel@xxxxxxx> 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@xxxxxxx>
> > ---
> > 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.

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