KVM: guest_memfd: take the invalidate lock when unbinding a dying file

kvm_gmem_unbind() takes mapping->invalidate_lock only if the
get_file_active() succeeds. When the guest_memfd file is dying, its
reference count is already zero so get_file_active fails and the binding
is removed without holding the invalidate lock.

kvm_gmem_invalidate_{start,end}() checks f->bindings independently to
decide whether to begin or end KVM MMU invalidation. So, the bindings
must therefore remain stable between the two calls. Otherwise, unbind
can remove a binding after start increments mmu_invalidate_in_progress,
but before end finds the binding and decrements it.
Example, unbind race with memory failure:

  CPU 0: memory failure               CPU 1: memslot delete
  ----------------------------------  ---------------------------
                                      (guest_memfd file is dying)
  kvm_gmem_error_folio()
    kvm_gmem_invalidate_start()
      finds binding
      mmu_invalidate_in_progress++
                                      kvm_gmem_unbind()
                                        get_file_active() fails
                                        removes binding
    kvm_gmem_invalidate_end()
      no binding found
      counter stays elevated

mmu_invalidate_retry() then returns 1 forever, so guest page faults
retry without ever installing a mapping and the guest hangs.

Remove the distinction between live and dying files in the unbind path.
Always use slot->gmem.file and take the invalidate lock when it's non-NULL.
Normal unbind callers hold slots_lock, which prevents release from clearing
the pointer or freeing the file state until unbind completes. Final VM
teardown can only see a NULL pointer because guest_memfd pins the KVM until
release has cleared all bindings.

Fixes: ae431059e75d ("KVM: guest_memfd: Remove bindings on memslot deletion when gmem is dying")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260728092027.225CF1F000E9@smtp.kernel.org
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Shivank Garg <shivankg@amd.com>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Link: https://patch.msgid.link/20260901-gmem-unbind-fix-v2-1-12febe1d2216@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

0 files changed, 0 insertions, 0 deletions