CVE-2026-74439: iommu/vt-d: Clear Present bit before tearing down scalable-mode context entry

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

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

devicepasidtableteardown() zeroes the 128-bit scalable-mode context entry with contextclearentry() 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 intelpasidfreetable() 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 domaincontextclearone() and the copied-context path, but devicepasidtableteardown() 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.