Use an atomic XCHG to write TDP MMU SPTEs that have volatile bits, even
if mmu_lock is held for write, as volatile SPTEs can be written by other
tasks/vCPUs outside of mmu_lock.  If a vCPU uses the to-be-modified SPTE
to write a page, the CPU can cache the translation as WRITABLE in the TLB
despite it being seen by KVM as !WRITABLE, and/or KVM can clobber the
Accessed/Dirty bits and not properly tag the backing page.

Exempt non-leaf SPTEs from atomic updates as KVM itself doesn't modify
non-leaf SPTEs without holding mmu_lock, they do not have Dirty bits, and
KVM doesn't consume the Accessed bit of non-leaf SPTEs.

Dropping the Dirty and/or Writable bits is most problematic for dirty
logging, as doing so can result in a missed TLB flush and eventually a
missed dirty page.  In the unlikely event that the only dirty page(s) is
a clobbered SPTE, clear_dirty_gfn_range() will see the SPTE as not dirty
(based on the Dirty or Writable bit depending on the method) and so not
update the SPTE and ultimately not flush.  If the SPTE is cached in the
TLB as writable before it is clobbered, the guest can continue writing
the associated page without ever taking a write-protect fault.

For most (all?) file back memory, dropping the Dirty bit is a non-issue.
The primary MMU write-protects its PTEs on writeback, i.e. KVM's dirty
bit is effectively ignored because the primary MMU will mark that page
dirty when the write-protection is lifted, e.g. when KVM faults the page
back in for write.

The Accessed bit is a complete non-issue.  Aside from being unused for
non-leaf SPTEs, KVM doesn't do a TLB flush when aging SPTEs, i.e. the
Accessed bit may be dropped anyways.

Lastly, the Writable bit is also problematic as an extension of the Dirty
bit, as KVM (correctly) treats the Dirty bit as volatile iff the SPTE is
!DIRTY && WRITABLE.  If KVM fixes an MMU-writable, but !WRITABLE, SPTE
out of mmu_lock, then it can allow the CPU to set the Dirty bit despite
the SPTE being !WRITABLE when it is checked by KVM.  But that all depends
on the Dirty bit being problematic in the first place.

Fixes: 2f2fad08 ("kvm: x86/mmu: Add functions to handle changed TDP SPTEs")
Cc: stable@vger.kernel.org
Cc: Ben Gardon <bgardon@google.com>
Cc: David Matlack <dmatlack@google.com>
Cc: Venkatesh Srinivas <venkateshs@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220423034752.1161007-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>