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

[Jay Buddhabhatti]

Hi Sudeep,

Thanks for the review. Please find my response inline.

On 10/1/2026 8:14 PM, Sudeep Holla wrote:
> 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.

I will add that check in new version. If the Clock rate control bit is 
set, Linux clears its local copy by setting rate_ctrl_forbidden. The 
clock remains registered, but clk-scmi does not install set_rate and 
scmi_clock_rate_set() returns -EACCES. This avoids exposing rate changes 
when no valid rates were described.

Regards,
Jay
>