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Subject[PATCH 5.4 90/92] binder: Fix race between mmap() and binder_alloc_print_pages()
Date
From: Jann Horn <jannh@google.com>

commit 8eb52a1ee37aafd9b796713aa0b3ab9cbc455be3 upstream.

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>

---
drivers/android/binder_alloc.c | 22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)

--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -841,14 +841,20 @@ void binder_alloc_print_pages(struct seq
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);

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