[PATCH 2/6] iommu/vt-d: Clear Present bit before tearing down scalable-mode context entry

[Lu Baolu]

From: Michael Bommarito <michael.bommarito@xxxxxxxxx>

device_pasid_table_teardown() zeroes the 128-bit scalable-mode context
entry with context_clear_entry() while the Present bit is still set. This
creates a window where the hardware can fetch a torn entry, with some
fields already zeroed while Present is still set, leading to unpredictable
behavior or spurious faults. The context-cache invalidation is issued only
after the entry has been zeroed, and intel_pasid_free_table() then frees
the PASID directory pages, so the IOMMU can keep walking a stale Present=1
entry that points at freed memory.

While x86 provides strong write ordering, the compiler may reorder the two
64-bit writes to the entry, and the hardware fetch is not guaranteed to be
atomic with respect to multiple CPU writes.

Commit c1e4f1dccbe9d ("iommu/vt-d: Clear Present bit before tearing down
context entry") fixed this exact pattern in domain_context_clear_one() and
the copied-context path, but device_pasid_table_teardown() was not
converted.

Align it with the "Guidance to Software for Invalidations" in the VT-d
spec, Section 6.5.3.3, using the same ownership handshake as the sibling
fix: clear only the Present bit, flush it to the IOMMU, perform the
context-cache invalidation, and only then zero the rest of the entry.

Fixes: 81e921fd32161 ("iommu/vt-d: Fix NULL domain on device release")
Signed-off-by: Michael Bommarito <michael.bommarito@xxxxxxxxx>
Assisted-by: Claude:claude-opus-4-7
Link: https://lore.kernel.org/r/20260528025557.3209367-1-michael.bommarito@xxxxxxxxx
Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
---
 drivers/iommu/intel/pasid.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/intel/pasid.c b/drivers/iommu/intel/pasid.c
index 89541b74ab8c..40910dc7363b 100644
--- a/drivers/iommu/intel/pasid.c
+++ b/drivers/iommu/intel/pasid.c
@@ -748,10 +748,12 @@ static void device_pasid_table_teardown(struct device *dev, u8 bus, u8 devfn)
 	}
 
 	did = context_domain_id(context);
-	context_clear_entry(context);
+	context_clear_present(context);
 	__iommu_flush_cache(iommu, context, sizeof(*context));
 	spin_unlock(&iommu->lock);
 	intel_context_flush_no_pasid(info, context, did);
+	context_clear_entry(context);
+	__iommu_flush_cache(iommu, context, sizeof(*context));
 }
 
 static int pci_pasid_table_teardown(struct pci_dev *pdev, u16 alias, void *data)
-- 
2.43.0