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KVM: x86: fix L1TF's MMIO GFN calculation
One defense against L1TF in KVM is to always set the upper five bits of the *legal* physical address in the SPTEs for non-present and reserved SPTEs, e.g. MMIO SPTEs. In the MMIO case, the GFN of the MMIO SPTE may overlap with the upper five bits that are being usurped to defend against L1TF. To preserve the GFN, the bits of the GFN that overlap with the repurposed bits are shifted left into the reserved bits, i.e. the GFN in the SPTE will be split into high and low parts. When retrieving the GFN from the MMIO SPTE, e.g. to check for an MMIO access, get_mmio_spte_gfn() unshifts the affected bits and restores the original GFN for comparison. Unfortunately, get_mmio_spte_gfn() neglects to mask off the reserved bits in the SPTE that were used to store the upper chunk of the GFN. As a result, KVM fails to detect MMIO accesses whose GPA overlaps the repurprosed bits, which in turn causes guest panics and hangs. Fix the bug by generating a mask that covers the lower chunk of the GFN, i.e. the bits that aren't shifted by the L1TF mitigation. The alternative approach would be to explicitly zero the five reserved bits that are used to store the upper chunk of the GFN, but that requires additional run-time computation and makes an already-ugly bit of code even more inscrutable. I considered adding a WARN_ON_ONCE(low_phys_bits-1 <= PAGE_SHIFT) to warn if GENMASK_ULL() generated a nonsensical value, but that seemed silly since that would mean a system that supports VMX has less than 18 bits of physical address space... Reported-by:Sakari Ailus <sakari.ailus@iki.fi> Fixes: d9b47449c1a1 ("kvm: x86: Set highest physical address bits in non-present/reserved SPTEs") Cc: Junaid Shahid <junaids@google.com> Cc: Jim Mattson <jmattson@google.com> Cc: stable@vger.kernel.org Reviewed-by: