linux-kernel - Re: [PATCH v2 3/4] misc: fastrpc: Remove buffer from list prior to unmap operation

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Message-ID: <3p4quidza7rwxng74fxcfoflo62tgakl7hummwsqmisaqmkwwp@nae55u2ehza4>
Date: Thu, 15 Jan 2026 22:47:45 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Jianping Li <jianping.li@....qualcomm.com>
Cc: srini@...nel.org, amahesh@....qualcomm.com, arnd@...db.de,
        gregkh@...uxfoundation.org, linux-arm-msm@...r.kernel.org,
        Ekansh Gupta <ekansh.gupta@....qualcomm.com>,
        thierry.escande@...aro.org, abelvesa@...nel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        quic_chennak@...cinc.com, stable@...nel.org
Subject: Re: [PATCH v2 3/4] misc: fastrpc: Remove buffer from list prior to
 unmap operation

On Thu, Jan 15, 2026 at 04:28:50PM +0800, Jianping Li wrote:
> From: Ekansh Gupta <ekansh.gupta@....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
> calling unmap operation.
> 
> Fixes: 2419e55e532de ("misc: fastrpc: add mmap/unmap support")
> Cc: stable@...nel.org
> Co-developed-by: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
> Signed-off-by: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
> Signed-off-by: Jianping Li <jianping.li@....qualcomm.com>
> ---
>  drivers/misc/fastrpc.c | 28 ++++++++++++++++++++--------
>  1 file changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 4f12fa5a05aa..833c265add5e 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -202,6 +202,8 @@ struct fastrpc_buf {
>  	/* mmap support */
>  	struct list_head node; /* list of user requested mmaps */
>  	uintptr_t raddr;
> +	/* Lock for buf->node */
> +	spinlock_t *list_lock;

Why do you need to lock this? Isn't fl->lock enough?

>  };
>  
>  struct fastrpc_dma_buf_attachment {
> @@ -441,6 +443,7 @@ static int __fastrpc_buf_alloc(struct fastrpc_user *fl, struct device *dev,
>  	buf->size = size;
>  	buf->dev = dev;
>  	buf->raddr = 0;
> +	buf->list_lock = &fl->lock;
>  
>  	buf->virt = dma_alloc_coherent(dev, buf->size, &buf->dma_addr,
>  				       GFP_KERNEL);
> @@ -1865,9 +1868,6 @@ 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);
> @@ -1881,6 +1881,7 @@ 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;
> @@ -1888,6 +1889,7 @@ 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;
>  		}
> @@ -1900,7 +1902,14 @@ 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(buf->list_lock);
> +		list_add_tail(&buf->node, &fl->mmaps);
> +		spin_unlock(buf->list_lock);
> +	}
> +
> +	return err;
>  }
>  
>  static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
> @@ -1985,20 +1994,23 @@ static int fastrpc_req_mmap(struct fastrpc_user *fl, char __user *argp)
>  		}
>  	}
>  
> -	spin_lock(&fl->lock);
> +	spin_lock(buf->list_lock);
>  	list_add_tail(&buf->node, &fl->mmaps);
> -	spin_unlock(&fl->lock);
> +	spin_unlock(buf->list_lock);
>  
>  	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(buf->list_lock);
> +	list_del(&buf->node);
> +	spin_unlock(buf->list_lock);
>  err_assign:
>  	fastrpc_req_munmap_impl(fl, buf);
>  
> -- 
> 2.43.0
> 

-- 
With best wishes
Dmitry