In the Linux kernel, the following vulnerability has been resolved: iommu/vt-d: Clear Present bit before tearing down copied context entry

In the Linux kernel, the following vulnerability has been resolved:

iommu/vt-d: Clear Present bit before tearing down copied context entry

copied_context_tear_down() zeroes the 128-bit context entry with
context_clear_entry() while the Present bit is still set, and only then
issues the context-cache and IOTLB invalidations. This leaves a window
in which hardware can fetch a torn entry, with some fields already zeroed
while Present is still set, leading to unpredictable behaviour or
spurious faults. While x86 provides strong write ordering, the compiler
may reorder the writes to the two 64-bit halves of the entry, and the
hardware fetch is not guaranteed to be atomic with respect to multiple
CPU writes.

There is no cacheline flush before the invalidation either, so on an
IOMMU without coherent access to the context table the zeroed entry may
not be visible to hardware at the point the invalidation is submitted.

Apply the same ownership handshake described in the VT-d spec, Section
6.5.3.3 ("Guidance to Software for Invalidations"): clear only the Present
bit, flush it out to the IOMMU, perform the invalidations, and only then
zero the remainder of the entry.