dmabuf: fix use-after-free of dmabuf's file->f_inode

It is observed 'use-after-free' on the dmabuf's file->f_inode with the race between closing the dmabuf file and reading the dmabuf's debug info.

Consider the below scenario where P1 is closing the dma_buf file and P2 is reading the dma_buf's debug info in the system:

P1 P2
dma_buf_debug_show()
dma_buf_put()
__fput()

    file->f_op->release()
    dput()
    ....
      dentry_unlink_inode()
        iput(dentry->d_inode)
        (where the inode is freed)
					mutex_lock(&db_list.lock)
					read 'dma_buf->file->f_inode'
					(the same inode is freed by P1)
					mutex_unlock(&db_list.lock)
      dentry->d_op->d_release()-->
        dma_buf_release()
          .....
          mutex_lock(&db_list.lock)
          removes the dmabuf from the list
          mutex_unlock(&db_list.lock)

In the above scenario, when dma_buf_put() is called on a dma_buf, it first frees the dma_buf's file->f_inode(=dentry->d_inode) and then removes this dma_buf from the system db_list. In between P2 traversing the db_list tries to access this dma_buf's file->f_inode that was freed by P1 which is a use-after-free case.

Since, __fput() calls f_op->release first and then later calls the d_op->d_release, move the dma_buf's db_list removal from d_release() to f_op->release(). This ensures that dma_buf's file->f_inode is not accessed after it is released.

---
V2: Resending with stable tags and Acks

V1: https://lore.kernel.org/patchwork/patch/1360118/

 drivers/dma-buf/dma-buf.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

index 0eb80c1..a14dcbb 100644
@@ -76,10 +76,6 @@ static void dma_buf_release(struct dentry *dentry)
 
 	dmabuf->ops->release(dmabuf);
 
-	mutex_lock(&db_list.lock);
-	list_del(&dmabuf->list_node);
-	mutex_unlock(&db_list.lock);
-
 	if (dmabuf->resv == (struct dma_resv *)&dmabuf[1])
 		dma_resv_fini(dmabuf->resv);
 

@@ -88,6 +84,22 @@ static void dma_buf_release(struct dentry *dentry)
 	kfree(dmabuf);
 }
 
+static int dma_buf_file_release(struct inode *inode, struct file *file)
+{
+	struct dma_buf *dmabuf;
+
+	if (!is_dma_buf_file(file))
+		return -EINVAL;
+
+	dmabuf = file->private_data;
+
+	mutex_lock(&db_list.lock);
+	list_del(&dmabuf->list_node);
+	mutex_unlock(&db_list.lock);
+
+	return 0;
+}
+
 static const struct dentry_operations dma_buf_dentry_ops = {
 	.d_dname = dmabuffs_dname,
 	.d_release = dma_buf_release,

@@ -413,6 +425,7 @@ static void dma_buf_show_fdinfo(struct seq_file *m, struct file *file)
 }
 
 static const struct file_operations dma_buf_fops = {
+	.release	= dma_buf_file_release,
 	.mmap		= dma_buf_mmap_internal,
 	.llseek		= dma_buf_llseek,
 	.poll		= dma_buf_poll,

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