Re: [Linaro-mm-sig] [PATCH] dmabuf: fix use-after-free of dmabuf's file->f_inode

[Christian König]

Am 04.01.21 um 12:36 schrieb Charan Teja Reddy:
> 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.
>
> Fixes: 4ab59c3c638c ("dma-buf: Move dma_buf_release() from fops to dentry_ops")
> Signed-off-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx>

Not an expert on the debugfs stuff in DMA-buf, but the explanation 
sounds perfectly correct to me.

Acked-by: Christian König <christian.koenig@xxxxxxx>

> ---
>   drivers/dma-buf/dma-buf.c | 21 +++++++++++++++++----
>   1 file changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 0eb80c1..a14dcbb 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -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,