android: binder: fix lockdep check on clearing vma

When munmapping a vma, the mmap_lock can be degraded to a write before
calling close() on the file handle. The binder close() function calls
binder_alloc_set_vma() to clear the vma address, which now has a lock dep
check for writing on the mmap_lock. Change the lockdep check to ensure
the reading lock is held while clearing and keep the write check while
writing.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 472a68df605b ("android: binder: stop saving a pointer to the VMA")
Signed-off-by: Liam R. Howlett <[e-mail]>
Reported-by: [e-mail]
Acked-by: Todd Kjos <[e-mail]>
Cc: "Arve Hjønnevåg" <[e-mail]>
Cc: Christian Brauner (Microsoft) <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Hridya Valsaraju <[e-mail]>
Cc: Joel Fernandes <[e-mail]>
Cc: Martijn Coenen <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

drivers/android/binder_alloc.c

···
 {
 	unsigned long vm_start = 0;
 
+	/*
+	 * Allow clearing the vma with holding just the read lock to allow
+	 * munmapping downgrade of the write lock before freeing and closing the
+	 * file using binder_alloc_vma_close().
+	 */
 	if (vma) {
 		vm_start = vma->vm_start;
 		alloc->vma_vm_mm = vma->vm_mm;
+		mmap_assert_write_locked(alloc->vma_vm_mm);
+	} else {
+		mmap_assert_locked(alloc->vma_vm_mm);
 	}
 
-	mmap_assert_write_locked(alloc->vma_vm_mm);
 	alloc->vma_addr = vm_start;
 }