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

Overview

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

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

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.

Affected

• Linux >= 81e921fd321614c2ad8ac333b041aae1da7a1c6d < e9e83bcfe37dc719182500dd823c03ab57d934f0
• Linux >= 81e921fd321614c2ad8ac333b041aae1da7a1c6d < 588718101e8449605f1c7e858fecb7cfa701cdab
• Linux >= 81e921fd321614c2ad8ac333b041aae1da7a1c6d < 7fd4077dc92b91b1b844333c0a06bb9e286db10a
• Linux >= 81e921fd321614c2ad8ac333b041aae1da7a1c6d < f46452c3df7a8d8a5addc0926e76ef19ea7da0a0
• Linux 333fe86968482ca701c609af590003bcea450e8f
• Linux >= 6.8.2 < 6.9
• Linux 6.9

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.