Re: [PATCH v2 1/2] misc: fastrpc: Fix NULL pointer dereference in rpmsg callback

[Bjorn Andersson <andersson@kernel.org>]

On Mon, May 04, 2026 at 10:47:00PM +0530, Mukesh Ojha wrote:
> A NULL pointer dereference was observed on Hawi at boot when the DSP
> sends a glink message before fastrpc_rpmsg_probe() has completed
> initialization:
> 
>   Unable to handle kernel NULL pointer dereference at virtual address 0000000000000178
>   pc : _raw_spin_lock_irqsave+0x34/0x8c
>   lr : fastrpc_rpmsg_callback+0x3c/0xcc [fastrpc]
>   ...
>   Call trace:
>    _raw_spin_lock_irqsave+0x34/0x8c (P)
>    fastrpc_rpmsg_callback+0x3c/0xcc [fastrpc]
>    qcom_glink_native_rx+0x538/0x6a4
>    qcom_glink_smem_intr+0x14/0x24 [qcom_glink_smem]
> 
> The faulting address 0x178 corresponds to the lock variable inside
> struct fastrpc_channel_ctx, confirming that cctx is NULL when
> fastrpc_rpmsg_callback() attempts to take the spinlock.
> 
> There are two issues here. First, dev_set_drvdata() is called before
> spin_lock_init() and idr_init(), leaving a window where the callback
> can retrieve a valid cctx pointer but operate on an uninitialized
> spinlock. Second, the rpmsg channel becomes live as soon as the driver
> is bound, so fastrpc_rpmsg_callback() can fire before dev_set_drvdata()
> is called at all, resulting in dev_get_drvdata() returning NULL.
> 
> Fix both issues by moving all cctx initialization ahead of
> dev_set_drvdata() so the structure is fully initialized before it
> becomes visible to the callback, and add a NULL check in
> fastrpc_rpmsg_callback() as a guard against any remaining window.
> 
> Fixes: f6f9279f2bf0 ("misc: fastrpc: Add Qualcomm fastrpc basic driver model")
> Cc: stable@vger.kernel.org
> Signed-off-by: Mukesh Ojha <mukesh.ojha@oss.qualcomm.com>

The fix looks good to me.

Reviewed-by: Bjorn Andersson <andersson@kernel.org>


But I can't help wonder, what's in that message? Should we make sure to
handle it, longer term?

Regards,
Bjorn

> ---
> Changes in v2: https://lore.kernel.org/lkml/20260417200146.184425-1-mukesh.ojha@oss.qualcomm.com/
>  - Added stable mailing list and fixes tag.
> 
>  drivers/misc/fastrpc.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 1080f9acf70a..a1a54453bb7e 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -2431,7 +2431,6 @@ static int fastrpc_rpmsg_probe(struct rpmsg_device *rpdev)
>  
>  	kref_init(&data->refcount);
>  
> -	dev_set_drvdata(&rpdev->dev, data);
>  	rdev->dma_mask = &data->dma_mask;
>  	dma_set_mask_and_coherent(rdev, DMA_BIT_MASK(32));
>  	INIT_LIST_HEAD(&data->users);
> @@ -2440,6 +2439,7 @@ static int fastrpc_rpmsg_probe(struct rpmsg_device *rpdev)
>  	idr_init(&data->ctx_idr);
>  	data->domain_id = domain_id;
>  	data->rpdev = rpdev;
> +	dev_set_drvdata(&rpdev->dev, data);
>  
>  	err = of_platform_populate(rdev->of_node, NULL, NULL, rdev);
>  	if (err)
> @@ -2513,6 +2513,9 @@ static int fastrpc_rpmsg_callback(struct rpmsg_device *rpdev, void *data,
>  	if (len < sizeof(*rsp))
>  		return -EINVAL;
>  
> +	if (!cctx)
> +		return -ENODEV;
> +
>  	ctxid = ((rsp->ctx & FASTRPC_CTXID_MASK) >> 4);
>  
>  	spin_lock_irqsave(&cctx->lock, flags);
> -- 
> 2.53.0
> 
>