binder: Fix race between mmap() and binder_alloc_print_pages()

binder_alloc_print_pages() iterates over
alloc->pages[0..alloc->buffer_size-1] under alloc->mutex.
binder_alloc_mmap_handler() writes alloc->pages and alloc->buffer_size
without holding that lock, and even writes them before the last bailout
point.

Unfortunately we can't take the alloc->mutex in the ->mmap() handler
because mmap_sem can be taken while alloc->mutex is held.
So instead, we have to locklessly check whether the binder_alloc has been
fully initialized with binder_alloc_get_vma(), like in
binder_alloc_new_buf_locked().

Fixes: 8ef4665aa129 ("android: binder: Add page usage in binder stats")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191018205631.248274-1-jannh@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 1cbe378..d718ed0 100644 (file)
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -840,14 +840,20 @@ void binder_alloc_print_pages(struct seq_file *m,
        int free = 0;
 
        mutex_lock(&alloc->mutex);
-       for (i = 0; i < alloc->buffer_size / PAGE_SIZE; i++) {
-               page = &alloc->pages[i];
-               if (!page->page_ptr)
-                       free++;
-               else if (list_empty(&page->lru))
-                       active++;
-               else
-                       lru++;
+       /*
+        * Make sure the binder_alloc is fully initialized, otherwise we might
+        * read inconsistent state.
+        */
+       if (binder_alloc_get_vma(alloc) != NULL) {
+               for (i = 0; i < alloc->buffer_size / PAGE_SIZE; i++) {
+                       page = &alloc->pages[i];
+                       if (!page->page_ptr)
+                               free++;
+                       else if (list_empty(&page->lru))
+                               active++;
+                       else
+                               lru++;
+               }
        }
        mutex_unlock(&alloc->mutex);
        seq_printf(m, "  pages: %d:%d:%d\n", active, lru, free);