Re: [PATCH v5 2/5] misc: fastrpc: Remove buffer from list prior to unmap operation

[Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>]

On Fri, May 15, 2026 at 08:42:14PM +0800, Jianping Li wrote:
> From: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
> 
> fastrpc_req_munmap_impl() is called to unmap any buffer. The buffer is
> getting removed from the list after it is unmapped from DSP. This can
> create potential race conditions if any other thread removes the entry
> from list while unmap operation is ongoing. Remove the entry before

How can it remove the entry from the list?

> calling unmap operation.
> 
> Fixes: 2419e55e532de ("misc: fastrpc: add mmap/unmap support")
> Cc: stable@kernel.org
> Co-developed-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
> Signed-off-by: Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>
> Signed-off-by: Jianping Li <jianping.li@oss.qualcomm.com>
> ---
>  drivers/misc/fastrpc.c | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 8b21f85cd9f4..3c7c3b410d7d 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -1863,9 +1863,6 @@ static int fastrpc_req_munmap_impl(struct fastrpc_user *fl, struct fastrpc_buf *
>  				      &args[0]);
>  	if (!err) {
>  		dev_dbg(dev, "unmmap\tpt 0x%09lx OK\n", buf->raddr);
> -		spin_lock(&fl->lock);
> -		list_del(&buf->node);
> -		spin_unlock(&fl->lock);
>  		fastrpc_buf_free(buf);
>  	} else {
>  		dev_err(dev, "unmmap\tpt 0x%09lx ERROR\n", buf->raddr);
> @@ -1879,6 +1876,7 @@ static int fastrpc_req_munmap(struct fastrpc_user *fl, char __user *argp)
>  	struct fastrpc_buf *buf = NULL, *iter, *b;
>  	struct fastrpc_req_munmap req;
>  	struct device *dev = fl->sctx->dev;
> +	int err;
>  
>  	if (copy_from_user(&req, argp, sizeof(req)))
>  		return -EFAULT;
> @@ -1886,6 +1884,7 @@ static int fastrpc_req_munmap(struct fastrpc_user *fl, char __user *argp)
>  	spin_lock(&fl->lock);
>  	list_for_each_entry_safe(iter, b, &fl->mmaps, node) {
>  		if ((iter->raddr == req.vaddrout) && (iter->size == req.size)) {
> +			list_del(&iter->node);
>  			buf = iter;
>  			break;
>  		}
> @@ -1898,7 +1897,14 @@ static int fastrpc_req_munmap(struct fastrpc_user *fl, char __user *argp)
>  		return -EINVAL;
>  	}
>  
> -	return fastrpc_req_munmap_impl(fl, buf);
> +	err = fastrpc_req_munmap_impl(fl, buf);
> +	if (err) {
> +		spin_lock(&fl->lock);
> +		list_add_tail(&buf->node, &fl->mmaps);
> +		spin_unlock(&fl->lock);
> +	}

Is it expected that userspace tries to unmap it again? Or why is it
being added to the list?

> +
> +	return err;
>  }
>  
>  static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
> @@ -1989,14 +1995,17 @@ static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
>  
>  	if (copy_to_user((void __user *)argp, &req, sizeof(req))) {
>  		err = -EFAULT;
> -		goto err_assign;
> +		goto err_copy;
>  	}
>  
>  	dev_dbg(dev, "mmap\t\tpt 0x%09lx OK [len 0x%08llx]\n",
>  		buf->raddr, buf->size);
>  
>  	return 0;
> -
> +err_copy:
> +	spin_lock(&fl->lock);
> +	list_del(&buf->node);
> +	spin_unlock(&fl->lock);

This is a separate fix.

>  err_assign:
>  	fastrpc_req_munmap_impl(fl, buf);
>  
> -- 
> 2.43.0
> 

-- 
With best wishes
Dmitry