binder: add lockless binder_alloc_(set|get)_vma()

commit 0fa53349c3acba0239369ba4cd133740a408d246 upstream.

Bring back the original lockless design in binder_alloc to determine
whether the buffer setup has been completed by the ->mmap() handler.
However, this time use smp_load_acquire() and smp_store_release() to
wrap all the ordering in a single macro call.

Also, add comments to make it evident that binder uses alloc->vma to
determine when the binder_alloc has been fully initialized. In these
scenarios acquiring the mmap_lock is not required.

Fixes: a43cfc87caaf ("android: binder: stop saving a pointer to the VMA")
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20230502201220.1756319-3-cmllamas@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 eb082b33115b199d661abb08f37a774aff0f3c82..e7c9d466f8e8ea73f87fa273739ee72f9d10945d 100644 (file)
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -309,17 +309,18 @@ static int binder_update_page_range(struct binder_alloc *alloc, int allocate,
        return vma ? -ENOMEM : -ESRCH;
 }
 
+static inline void binder_alloc_set_vma(struct binder_alloc *alloc,
+               struct vm_area_struct *vma)
+{
+       /* pairs with smp_load_acquire in binder_alloc_get_vma() */
+       smp_store_release(&alloc->vma, vma);
+}
+
 static inline struct vm_area_struct *binder_alloc_get_vma(
                struct binder_alloc *alloc)
 {
-       struct vm_area_struct *vma = NULL;
-
-       if (alloc->vma) {
-               /* Look at description in binder_alloc_set_vma */
-               smp_rmb();
-               vma = alloc->vma;
-       }
-       return vma;
+       /* pairs with smp_store_release in binder_alloc_set_vma() */
+       return smp_load_acquire(&alloc->vma);
 }
 
 static bool debug_low_async_space_locked(struct binder_alloc *alloc, int pid)
@@ -382,6 +383,7 @@ static struct binder_buffer *binder_alloc_new_buf_locked(
        size_t size, data_offsets_size;
        int ret;
 
+       /* Check binder_alloc is fully initialized */
        if (!binder_alloc_get_vma(alloc)) {
                binder_alloc_debug(BINDER_DEBUG_USER_ERROR,
                                   "%d: binder_alloc_buf, no vma\n",
@@ -777,7 +779,9 @@ int binder_alloc_mmap_handler(struct binder_alloc *alloc,
        buffer->free = 1;
        binder_insert_free_buffer(alloc, buffer);
        alloc->free_async_space = alloc->buffer_size / 2;
-       alloc->vma = vma;
+
+       /* Signal binder_alloc is fully initialized */
+       binder_alloc_set_vma(alloc, vma);
 
        return 0;
 
@@ -959,7 +963,7 @@ int binder_alloc_get_allocated_count(struct binder_alloc *alloc)
  */
 void binder_alloc_vma_close(struct binder_alloc *alloc)
 {
-       alloc->vma = 0;
+       binder_alloc_set_vma(alloc, NULL);
 }
 
 /**