Re: [RFC PATCH] firmware: arm_scmi: skip empty CLOCK_DESCRIBE_RATES replies

[Sudeep Holla]

On Thu, Oct 01, 2026 at 04:45:38AM -0700, Jay Buddhabhatti wrote:
> Some platforms advertise reserved or uninstantiated clock IDs that still
> succeed CLOCK_DESCRIBE_RATES with zero rates. After dynamic rate
> allocation, kcalloc(0) returns ZERO_SIZE_PTR and protocol init then
> dereferences rates[0], which panics.
> 
> Do not allocate or index the rate array when the firmware reports an
> empty list, so unused IDs are skipped instead of taking down the SCMI
> clock provider.
> 
> Fixes: 62ba967595e0 ("firmware: arm_scmi: Make clock rates allocation dynamic")
> Signed-off-by: Jay Buddhabhatti <jay.buddhabhatti@xxxxxxx>
> ---
> The SCMI server is the source of this zero rate and successful response
> and it should be fixed in SCMI server. This defensive check in Linux is
> still useful because firmware responses must be validated before
> de-referencing dynamically allocated data, The panic is a Linux
> regression introduced by dynamic rate allocation; previous fixed array
> tolerated the same response and other SCMI implementations could return
> the same unexpected response.
> ---
>  drivers/firmware/arm_scmi/clock.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/drivers/firmware/arm_scmi/clock.c b/drivers/firmware/arm_scmi/clock.c
> index 0278705d809e..8934a95527e2 100644
> --- a/drivers/firmware/arm_scmi/clock.c
> +++ b/drivers/firmware/arm_scmi/clock.c
> @@ -8,6 +8,7 @@
>  #include <linux/math64.h>
>  #include <linux/module.h>
>  #include <linux/limits.h>
> +#include <linux/slab.h>
>  #include <linux/sort.h>
>  
>  #include "protocols.h"
> @@ -484,6 +485,13 @@ iter_clk_describe_update_state(struct scmi_iterator_state *st,
>  	if (!st->max_resources) {
>  		unsigned int tot_rates = st->num_returned + st->num_remaining;
>  
> +		/*
> +		 * Unused/reserved clock IDs return 0 rates. kmalloc(0)
> +		 * returns ZERO_SIZE_PTR and must not be dereferenced.
> +		 */
> +		if (!tot_rates)
> +			return 0;
> +
>  		p->clkd->r.rates = devm_kcalloc(p->dev, tot_rates,
>  						sizeof(*p->clkd->r.rates), GFP_KERNEL);
>  		if (!p->clkd->r.rates)
> @@ -505,6 +513,9 @@ iter_clk_describe_process_response(const struct scmi_protocol_handle *ph,
>  	struct scmi_clk_ipriv *p = priv;
>  	const struct scmi_msg_resp_clock_describe_rates *r = response;
>  
> +	if (ZERO_OR_NULL_PTR(p->clkd->r.rates))
> +		return -EPROTO;
> +
>  	p->clkd->r.rates[p->clkd->r.num_rates] = RATE_TO_U64(r->rate[st->loop_idx]);
>  
>  	/* Count only effectively discovered rates */
> @@ -622,6 +633,13 @@ scmi_clock_describe_rates_get(const struct scmi_protocol_handle *ph,
>  	if (ret)
>  		return ret;
>  
> +	/*
> +	 * Some platforms expose reserved clock IDs with an empty
> +	 * CLOCK_DESCRIBE_RATES reply. Do not dereference rates[].
> +	 */
> +	if (!clkd->r.num_rates || ZERO_OR_NULL_PTR(clkd->r.rates))
> +		return 0;
> +

I expect the Clock rate control bit to be unset in the permissions for
these clock, else it may be dangerous to do this. Please add that check.

-- 
Regards,
Sudeep