[PATCH v2] KVM: guest_memfd: take the invalidate lock when unbinding a dying file

[Shivank Garg]

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@xxxxxxxxxxxxxxx
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/20260728092027.225CF1F000E9@xxxxxxxxxxxxxxx
Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
---
Tested on a 7.3-rc1, AMD EPYC 7713.

Changes in V2:
- Treat a dying file as the normal case: always use slot->gmem.file and
  take invalidate lock, drop use of gmem_get_file for unbind and fold
  __kvm_gmem_unbind() into caller. (Sean) 
- Drop NUMA selftests changes from this series, and will be sent separately. (Sean)
- Link to V1: https://lore.kernel.org/kvm/20260823-shivank-gmem-fix-split-v1-0-512a29fb8e86@xxxxxxx
---
 virt/kvm/guest_memfd.c | 44 ++++++++++++++++++--------------------------
 1 file changed, 18 insertions(+), 26 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 625e62e1a031..21bb3710edfe 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -668,48 +668,40 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
 	return r;
 }
 
-static void __kvm_gmem_unbind(struct kvm_memory_slot *slot, struct gmem_file *f)
+void kvm_gmem_unbind(struct kvm_memory_slot *slot)
 {
+	struct file *file = slot->gmem.file;
 	unsigned long start = slot->gmem.pgoff;
 	unsigned long end = start + slot->npages;
+	struct gmem_file *f;
 
-	xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL);
-
-	/*
-	 * synchronize_srcu(&kvm->srcu) ensured that kvm_gmem_get_pfn()
-	 * cannot see this memslot.
-	 */
-	WRITE_ONCE(slot->gmem.file, NULL);
-}
-
-void kvm_gmem_unbind(struct kvm_memory_slot *slot)
-{
 	/*
 	 * Nothing to do if the underlying file was _already_ closed, as
 	 * kvm_gmem_release() invalidates and nullifies all bindings.
 	 */
-	if (!slot->gmem.file)
+	if (!file)
 		return;
 
-	CLASS(gmem_get_file, file)(slot);
-
 	/*
 	 * However, if the file is _being_ closed, then the bindings need to be
 	 * removed as kvm_gmem_release() might not run until after the memslot
-	 * is freed.  Note, modifying the bindings is safe even though the file
-	 * is dying as kvm_gmem_release() nullifies slot->gmem.file under
-	 * slots_lock, and only puts its reference to KVM after destroying all
-	 * bindings.  I.e. reaching this point means kvm_gmem_release() hasn't
-	 * yet destroyed the bindings or freed the gmem_file, and can't do so
-	 * until the caller drops slots_lock.
+	 * is freed.  Modifying the bindings is safe even if the file is dying
+	 * as kvm_gmem_release() nullifies slot->gmem.file under slots_lock,
+	 * and only puts its reference to KVM after destroying all bindings.
+	 * I.e. reaching this point means kvm_gmem_release() hasn't destroyed
+	 * the bindings or freed the gmem_file and can't do so until the caller
+	 * drops slots_lock, so there's no need to verify the file is live.
 	 */
-	if (!file) {
-		__kvm_gmem_unbind(slot, slot->gmem.file->private_data);
-		return;
-	}
+	f = file->private_data;
 
 	filemap_invalidate_lock(file->f_mapping);
-	__kvm_gmem_unbind(slot, file->private_data);
+	xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL);
+
+	/*
+	 * synchronize_srcu(&kvm->srcu) ensured that kvm_gmem_get_pfn()
+	 * cannot see this memslot.
+	 */
+	WRITE_ONCE(slot->gmem.file, NULL);
 	filemap_invalidate_unlock(file->f_mapping);
 }
 

---
base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
change-id: 20260901-gmem-unbind-fix-9f912bb3d3b8

Best regards,
-- 
Shivank Garg <shivankg@xxxxxxx>