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

[Jay Buddhabhatti <jay.buddhabhatti@amd.com>]

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@amd.com>
---
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;
+
 	clkd->info.min_rate = clkd->r.rates[RATE_MIN];
 	if (!clkd->r.rate_discrete) {
 		clkd->info.max_rate = clkd->r.rates[RATE_MAX];
-- 
2.34.1