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SubjectRe: [PATCH] x86: fix broken flushing in GART nofullflush path
u
* Joerg Roedel <joro@8bytes.org> wrote:

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