[PATCH 6.12.y] KVM: guest_memfd: take the invalidate lock when unbinding a dying file

[Sasha Levin <sashal@kernel.org>]

From: Shivank Garg <shivankg@amd.com>

[ Upstream commit 0ee2c883b62deb74d8c43007e8049dc006d3a2fb ]

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>
[ sashal: adapted unbinding to the existing kvm_gmem structure and explicit kvm_gmem_get_file()/fput() reference handling ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 virt/kvm/guest_memfd.c | 48 ++++++++++++++++--------------------------
 1 file changed, 18 insertions(+), 30 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 71afb0ca0cc94..3c90413d92755 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -534,53 +534,41 @@ 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 kvm_gmem *gmem)
+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;
-
-	xa_store_range(&gmem->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)
-{
-	struct file *file;
+	struct kvm_gmem *gmem;
 
 	/*
 	 * 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;
 
-	file = kvm_gmem_get_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;
-	}
+	gmem = file->private_data;
 
 	filemap_invalidate_lock(file->f_mapping);
-	__kvm_gmem_unbind(slot, file->private_data);
-	filemap_invalidate_unlock(file->f_mapping);
+	xa_store_range(&gmem->bindings, start, end - 1, NULL, GFP_KERNEL);
 
-	fput(file);
+	/*
+	 * 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);
 }
 
 /* Returns a locked folio on success.  */
-- 
2.53.0