[PATCH v2 04/13] backlight: qcom-wled: Fix off-by-one maximum with default num_strings

[Marijn Suijten]

When not specifying num-strings in the DT the default is used, but +1 is
added to it which turns WLED3 into 4 and WLED4/5 into 5 strings instead
of 3 and 4 respectively, causing out-of-bounds reads and register
read/writes.  This +1 exists for a deficiency in the DT parsing code,
and is simply omitted entirely - solving this oob issue - by parsing the
property separately much like qcom,enabled-strings.

This also allows more stringent checks on the maximum value when
qcom,enabled-strings is provided in the DT.  Note that num-strings is
parsed after enabled-strings to give it final sign-off over the length,
which DT currently utilizes to get around an incorrect fixed read of
four elements from that array (has been addressed in a prior patch).

Fixes: 93c64f1ea1e8 ("leds: add Qualcomm PM8941 WLED driver")
Signed-off-by: Marijn Suijten <marijn.suijten@xxxxxxxxxxxxxx>
Reviewed-By: AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxxx>
---
 drivers/video/backlight/qcom-wled.c | 51 +++++++++++------------------
 1 file changed, 19 insertions(+), 32 deletions(-)

diff --git a/drivers/video/backlight/qcom-wled.c b/drivers/video/backlight/qcom-wled.c
index 977cd75827d7..c5232478a343 100644
--- a/drivers/video/backlight/qcom-wled.c
+++ b/drivers/video/backlight/qcom-wled.c
@@ -1253,21 +1253,6 @@ static const struct wled_var_cfg wled5_ovp_cfg = {
 	.size = 16,
 };
 
-static u32 wled3_num_strings_values_fn(u32 idx)
-{
-	return idx + 1;
-}
-
-static const struct wled_var_cfg wled3_num_strings_cfg = {
-	.fn = wled3_num_strings_values_fn,
-	.size = 3,
-};
-
-static const struct wled_var_cfg wled4_num_strings_cfg = {
-	.fn = wled3_num_strings_values_fn,
-	.size = 4,
-};
-
 static u32 wled3_switch_freq_values_fn(u32 idx)
 {
 	return 19200 / (2 * (1 + idx));
@@ -1341,11 +1326,6 @@ static int wled_configure(struct wled *wled)
 			.val_ptr = &cfg->switch_freq,
 			.cfg = &wled3_switch_freq_cfg,
 		},
-		{
-			.name = "qcom,num-strings",
-			.val_ptr = &cfg->num_strings,
-			.cfg = &wled3_num_strings_cfg,
-		},
 	};
 
 	const struct wled_u32_opts wled4_opts[] = {
@@ -1369,11 +1349,6 @@ static int wled_configure(struct wled *wled)
 			.val_ptr = &cfg->switch_freq,
 			.cfg = &wled3_switch_freq_cfg,
 		},
-		{
-			.name = "qcom,num-strings",
-			.val_ptr = &cfg->num_strings,
-			.cfg = &wled4_num_strings_cfg,
-		},
 	};
 
 	const struct wled_u32_opts wled5_opts[] = {
@@ -1397,11 +1372,6 @@ static int wled_configure(struct wled *wled)
 			.val_ptr = &cfg->switch_freq,
 			.cfg = &wled3_switch_freq_cfg,
 		},
-		{
-			.name = "qcom,num-strings",
-			.val_ptr = &cfg->num_strings,
-			.cfg = &wled4_num_strings_cfg,
-		},
 		{
 			.name = "qcom,modulator-sel",
 			.val_ptr = &cfg->mod_sel,
@@ -1520,8 +1490,6 @@ static int wled_configure(struct wled *wled)
 			*bool_opts[i].val_ptr = true;
 	}
 
-	cfg->num_strings = cfg->num_strings + 1;
-
 	string_len = of_property_count_elems_of_size(dev->of_node,
 						     "qcom,enabled-strings",
 						     sizeof(u32));
@@ -1552,6 +1520,25 @@ static int wled_configure(struct wled *wled)
 		}
 	}
 
+	rc = of_property_read_u32(dev->of_node, "qcom,num-strings", &val);
+	if (!rc) {
+		if (val < 1 || val > wled->max_string_count) {
+			dev_err(dev, "qcom,num-strings must be between 1 and %d\n",
+				wled->max_string_count);
+			return -EINVAL;
+		}
+
+		if (string_len > 0) {
+			dev_warn(dev, "qcom,num-strings and qcom,enabled-strings are ambiguous\n");
+			if (val > string_len) {
+				dev_err(dev, "qcom,num-strings exceeds qcom,enabled-strings\n");
+				return -EINVAL;
+			}
+		}
+
+		cfg->num_strings = val;
+	}
+
 	return 0;
 }
 
-- 
2.33.0