binder: fix race between munmap() and direct reclaim

An munmap() on a binder device causes binder_vma_close() to be called
which clears the alloc->vma pointer.

If direct reclaim causes binder_alloc_free_page() to be called, there
is a race where alloc->vma is read into a local vma pointer and then
used later after the mm->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->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 <tkjos@google.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 8 insertions, 10 deletions

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index 6389467670a0..195f120c4e8c 100644
--- 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);