Re: [PATCH 2/2] misc: fastrpc: fix init ordering race in rpmsg probe

[Ekansh Gupta <ekansh.gupta@oss.qualcomm.com>]

On 01-10-2026 16:31, Vinayak Katoch wrote:
> fastrpc_device_register() calls misc_register() which makes the device
> node visible to userspace. However, kref_init(), spin_lock_init(),
> INIT_LIST_HEAD(), dev_set_drvdata() and fastrpc_cb_devices_create()
> were all called after the switch/misc_register block, leaving a window
> where userspace can open the device and call fastrpc_device_open() on
> a partially initialised channel context.
> 
> On a concurrent open(), fastrpc_channel_ctx_get() calls kref_get() on
> a kzalloc-zeroed, never kref_init()'d refcount of 0, triggering:
> 
>   refcount_t: addition on 0; use-after-free.
>   Call trace:
>    refcount_warn_saturate+0x120/0x148 (P)
>    fastrpc_device_open+0x204/0x258 [fastrpc]
>    misc_open+0xd8/0x1a0
> 
> Additionally, fastrpc_cb_devices_create() not having run means
> sesscount is still 0, so fastrpc_session_alloc() returns NULL and
> fastrpc_device_open() hits the error path which calls
> fastrpc_channel_ctx_put() on the already-bogus refcount. This
> triggers the free callback, kfree()-ing the channel context. A
> subsequent IRQ-path fastrpc_rpmsg_callback() then dereferences the
> freed spinlock:
> 
>   Unable to handle kernel paging request at virtual address ffffcd5841510ca0
>   Internal error: Oops: 0000000096000047 [#1] SMP
>   Call trace:
>    queued_spin_lock_slowpath+0x414/0x5e0 (P)
>    _raw_spin_lock_irqsave+0x74/0x90
>    fastrpc_rpmsg_callback+0x4c/0xf0 [fastrpc]
>    qcom_glink_native_rx+0x6c4/0xf10
>    qcom_glink_smem_intr+0x1c/0x38 [qcom_glink_smem]
>   Kernel panic - not syncing: Oops: Fatal exception in interrupt
> 
> Fix by moving all channel context initialisation (kref_init,
> spin_lock_init, INIT_LIST_HEAD, idr_init, dev_set_drvdata) and
> fastrpc_cb_devices_create() before fastrpc_device_register(), ensuring
> the struct is fully ready before the device node is visible to
> userspace.
> 
> Add err_depopulate label in the error path since
> fastrpc_cb_devices_create() now runs before misc_register(); a
> fastrpc_device_register() failure must call fastrpc_cb_devices_destroy()
> to clean up the compute-cb children before freeing data.
> 
> Fixes: 3abe3ab3cdab ("misc: fastrpc: add secure domain support")
> Signed-off-by: Vinayak Katoch <vinayak.katoch@oss.qualcomm.com>
Cc: stable for both the fix patches.> ---
>  drivers/misc/fastrpc.c | 43 +++++++++++++++++++++++--------------------
>  1 file changed, 23 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 7ab1cbbf19bb..93d07d28a0f1 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -2614,6 +2614,23 @@ static int fastrpc_rpmsg_probe(struct rpmsg_device *rpdev)
>  	data->poll_mode_supported = soc_data->poll_mode_supported ||
>  		of_machine_get_match(fastrpc_poll_supported_machines);
>  
> +	kref_init(&data->refcount);
> +	atomic_set(&data->ctx_seq, 0);
> +
> +	rdev->dma_mask = &data->dma_mask;
> +	dma_set_mask_and_coherent(rdev, DMA_BIT_MASK(32));
> +	INIT_LIST_HEAD(&data->users);
> +	INIT_LIST_HEAD(&data->invoke_interrupted_mmaps);
> +	spin_lock_init(&data->lock);
> +	idr_init(&data->ctx_idr);
> +	data->domain_id = domain_id;
> +	data->rpdev = rpdev;
> +	dev_set_drvdata(&rpdev->dev, data);
> +
> +	err = fastrpc_cb_devices_create(rpdev);
> +	if (err)
> +		goto err_free_data;
> +
>  	switch (domain_id) {
>  	case ADSP_DOMAIN_ID:
>  	case MDSP_DOMAIN_ID:
> @@ -2622,7 +2639,7 @@ static int fastrpc_rpmsg_probe(struct rpmsg_device *rpdev)
>  		data->unsigned_support = false;
>  		err = fastrpc_device_register(rdev, data, secure_dsp, domain);
>  		if (err)
> -			goto err_free_data;
> +			goto err_depopulate;
>  		break;
>  	case CDSP_DOMAIN_ID:
>  	case GDSP_DOMAIN_ID:
> @@ -2630,7 +2647,7 @@ static int fastrpc_rpmsg_probe(struct rpmsg_device *rpdev)
>  		/* Create both device nodes so that we can allow both Signed and Unsigned PD */
>  		err = fastrpc_device_register(rdev, data, true, domain);
>  		if (err)
> -			goto err_free_data;
> +			goto err_depopulate;
>  
>  		err = fastrpc_device_register(rdev, data, false, domain);
>  		if (err)
> @@ -2638,26 +2655,9 @@ static int fastrpc_rpmsg_probe(struct rpmsg_device *rpdev)
>  		break;
>  	default:
>  		err = -EINVAL;
> -		goto err_free_data;
> +		goto err_depopulate;
>  	}
>  
> -	kref_init(&data->refcount);
> -	atomic_set(&data->ctx_seq, 0);
> -
> -	rdev->dma_mask = &data->dma_mask;
> -	dma_set_mask_and_coherent(rdev, DMA_BIT_MASK(32));
> -	INIT_LIST_HEAD(&data->users);
> -	INIT_LIST_HEAD(&data->invoke_interrupted_mmaps);
> -	spin_lock_init(&data->lock);
> -	idr_init(&data->ctx_idr);
> -	data->domain_id = domain_id;
> -	data->rpdev = rpdev;
> -	dev_set_drvdata(&rpdev->dev, data);
> -
> -	err = fastrpc_cb_devices_create(rpdev);
> -	if (err)
> -		goto err_deregister_fdev;
> -
>  	if (data->domain_id == ADSP_DOMAIN_ID && data->sesscount > 0) {
>  		struct fastrpc_session_ctx *last_sess;
>  		struct fastrpc_session_ctx *dup_sess;
> @@ -2682,6 +2682,9 @@ static int fastrpc_rpmsg_probe(struct rpmsg_device *rpdev)
>  	if (data->secure_fdevice)
>  		misc_deregister(&data->secure_fdevice->miscdev);
>  
> +err_depopulate:
> +	fastrpc_cb_devices_destroy(rpdev);
In case of CDSP/GDSP, if secure device register is successful and some
application opens device, that could result in cctx ref being taken and
direct kfree(data) might cause problems, can you check that path also?> +
>  err_free_data:
>  	kfree(data);
>  	return err;
>