[PATCH v4 11/15] firmware: arm_scmi: Fix bound iterators returning too many items

[Cristian Marussi <cristian.marussi@arm.com>]

From: Geert Uytterhoeven <geert+renesas@glider.be>

When using a bound-iterator with an upper bound, commands are sent, and
responses are received, until the upper bound is reached.  However, it
is up to the SCMI provider implementation to decide how many rates are
returned in response to a single CLOCK_DESCRIBE_RATES command.  If the
last response contains rates beyond the specified upper bound, they are
still passed up for further processing.  This may lead to buffer
overflows in unprepared callsites.

While the imprecise bound handling may have been intentional (it was
mentioned in the commit message introducing the code), it is still
confusing for users, and may cause hard to debug crashes.  Fix this by
strictly enforcing the upper bound.

Note that this may cause an increase in the number of
CLOCK_DESCRIBE_RATES commands issued, as retrieving the last rate may no
longer be done inadvertentently, but require its own command.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
 drivers/firmware/arm_scmi/driver.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
index cb4865fd8af2..fd031a8d40df 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c
@@ -1820,6 +1820,7 @@ static int __scmi_iterator_run(void *iter, unsigned int *start, unsigned int *en
 	const struct scmi_protocol_handle *ph;
 	struct scmi_iterator_state *st;
 	struct scmi_iterator *i;
+	unsigned int n;
 
 	if (!iter)
 		return -EINVAL;
@@ -1852,13 +1853,17 @@ static int __scmi_iterator_run(void *iter, unsigned int *start, unsigned int *en
 			return -EINVAL;
 		}
 
-		for (st->loop_idx = 0; st->loop_idx < st->num_returned; st->loop_idx++) {
+		if (end)
+			n = min(st->num_returned, *end - st->desc_index + 1);
+		else
+			n = st->num_returned;
+		for (st->loop_idx = 0; st->loop_idx < n; st->loop_idx++) {
 			ret = iops->process_response(ph, i->resp, st, i->priv);
 			if (ret)
 				return ret;
 		}
 
-		st->desc_index += st->num_returned;
+		st->desc_index += n;
 		ph->xops->reset_rx_to_maxsz(ph, i->t);
 		/*
 		 * check for both returned and remaining to avoid infinite
-- 
2.53.0