binder: fix race between munmap() and direct reclaim

Todd Kjos · gregkh · commit 5cec2d2e5839 · 2019-03-21T06:51:32.000+01:00
An munmap() on a binder device causes binder_vma_close() to be called
which clears the alloc-&gt;vma pointer.

If direct reclaim causes binder_alloc_free_page() to be called, there
is a race where alloc-&gt;vma is read into a local vma pointer and then
used later after the mm-&gt;mmap_sem is acquired. This can result in
calling zap_page_range() with an invalid vma which manifests as a
use-after-free in zap_page_range().

The fix is to check alloc-&gt;vma after acquiring the mmap_sem (which we
were acquiring anyway) and skip zap_page_range() if it has changed
to NULL.

Signed-off-by: Todd Kjos &lt;tkjos@google.com&gt;
Reviewed-by: Joel Fernandes (Google) &lt;joel@joelfernandes.org&gt;
Cc: stable &lt;stable@vger.kernel.org&gt;
Signed-off-by: Greg Kroah-Hartman &lt;gregkh@linuxfoundation.org&gt;
diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
@@ -927,14 +927,13 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
 
 	index = page - alloc->pages;
 	page_addr = (uintptr_t)alloc->buffer + index * PAGE_SIZE;
+
+	mm = alloc->vma_vm_mm;
+	if (!mmget_not_zero(mm))
+		goto err_mmget;
+	if (!down_write_trylock(&mm->mmap_sem))
+		goto err_down_write_mmap_sem_failed;
 	vma = binder_alloc_get_vma(alloc);
-	if (vma) {
-		if (!mmget_not_zero(alloc->vma_vm_mm))
-			goto err_mmget;
-		mm = alloc->vma_vm_mm;
-		if (!down_read_trylock(&mm->mmap_sem))
-			goto err_down_write_mmap_sem_failed;
-	}
 
 	list_lru_isolate(lru, item);
 	spin_unlock(lock);
@@ -945,10 +944,9 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
 		zap_page_range(vma, page_addr, PAGE_SIZE);
 
 		trace_binder_unmap_user_end(alloc, index);
-
-		up_read(&mm->mmap_sem);
-		mmput(mm);
 	}
+	up_write(&mm->mmap_sem);
+	mmput(mm);
 
 	trace_binder_unmap_kernel_start(alloc, index);
 
