Re: [PATCH] misc: fastrpc: remove mmap entry before remote unmap

[Ekansh Gupta]

On 24-06-2026 23:16, Yousef Alhouseen wrote:
> FASTRPC_IOCTL_MUNMAP looked up a buffer and dropped fl->lock.
> 
> It only deleted the list entry after the remote unmap succeeded.
> 
> Two threads can therefore find the same entry and unmap it twice.
> 
> That can lead to a double list deletion and double free.
> 
> Detach the entry before invoking the DSP.
> 
> Put it back if the remote unmap fails.
> 
There was a fix that got recently merged to fix the same problem[1].

Please check it and let me know if your change still holds.

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/drivers/misc/fastrpc.c?id=902b8ec9dd1098cc133cffa8be0b6378544f27ba

//ekansh> Signed-off-by: Yousef Alhouseen <alhouseenyousef@xxxxxxxxx>
> ---
>  drivers/misc/fastrpc.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index ba4ade874..6a5239b1c 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -1902,7 +1902,8 @@ static int fastrpc_req_munmap_impl(struct fastrpc_user *fl, struct fastrpc_buf *
>  	if (!err) {
>  		dev_dbg(dev, "unmmap\tpt 0x%09lx OK\n", buf->raddr);
>  		spin_lock(&fl->lock);
> -		list_del(&buf->node);
> +		if (!list_empty(&buf->node))
> +			list_del_init(&buf->node);
>  		spin_unlock(&fl->lock);
>  		fastrpc_buf_free(buf);
>  	} else {
> @@ -1917,6 +1918,7 @@ 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;
> @@ -1925,6 +1927,7 @@ static int fastrpc_req_munmap(struct fastrpc_user *fl, char __user *argp)
>  	list_for_each_entry_safe(iter, b, &fl->mmaps, node) {
>  		if ((iter->raddr == req.vaddrout) && (iter->size == req.size)) {
>  			buf = iter;
> +			list_del_init(&buf->node);
>  			break;
>  		}
>  	}
> @@ -1936,7 +1939,14 @@ 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);
> +	}
> +
> +	return err;
>  }
>  
>  static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)