BACKPORT: binder: fix race between munmap() and direct reclaim

commit 5cec2d2e5839f9c0fec319c523a911e0a7fd299f upstream.

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.

Bug: 120025196
Change-Id: I2f3284d294326ec7736303374769640a1e028783
Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Todd Kjos <tkjos@google.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: stable <stable@vger.kernel.org> # 4.14
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 9 insertions, 11 deletions

diff --git a/drivers/android/binder_alloc.c b/drivers/android/binder_alloc.c
index baecb4bf9d9b..c8614f3c70c4 100644
--- a/drivers/android/binder_alloc.c
+++ b/drivers/android/binder_alloc.c
@@ -924,15 +924,14 @@ 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;
+	/* Same as mmget_not_zero() in later kernel versions */
+	if (!atomic_inc_not_zero(&alloc->vma_vm_mm->mm_users))
+		goto err_mmget;
+	if (!down_write_trylock(&mm->mmap_sem))
+		goto err_down_write_mmap_sem_failed;
 	vma = alloc->vma;
-	if (vma) {
-		/* Same as mmget_not_zero() in later kernel versions */
-		if (!atomic_inc_not_zero(&alloc->vma_vm_mm->mm_users))
-			goto err_mmget;
-		mm = alloc->vma_vm_mm;
-		if (!down_write_trylock(&mm->mmap_sem))
-			goto err_down_write_mmap_sem_failed;
-	}
 
 	list_lru_isolate(lru, item);
 	spin_unlock(lock);
@@ -946,10 +945,9 @@ enum lru_status binder_alloc_free_page(struct list_head *item,
 			       PAGE_SIZE, NULL);
 
 		trace_binder_unmap_user_end(alloc, index);
-
-		up_write(&mm->mmap_sem);
-		mmput(mm);
 	}
+	up_write(&mm->mmap_sem);
+	mmput(mm);
 
 	trace_binder_unmap_kernel_start(alloc, index);