backlight: qcom-wled: Fix off-by-one maximum with default num_strings

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 enables more stringent checks on the maximum value when
qcom,enabled-strings is provided in the DT, by parsing num-strings after
enabled-strings to allow it to check against (and in a subsequent patch
override) the length of enabled-strings: it is invalid to set
num-strings higher than that.
The DT currently utilizes it to get around an incorrect fixed read of
four elements from that array (has been addressed in a prior patch) by
setting a lower num-strings where desired.

Fixes: 93c64f1ea1e8 ("leds: add Qualcomm PM8941 WLED driver")
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-By: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211115203459.1634079-5-marijn.suijten@somainline.org

diff --git a/drivers/video/backlight/qcom-wled.c b/drivers/video/backlight/qcom-wled.c
index 9d883e70213493b231f39f674931fb7af82eadb4..ab10910971e9f367d90d0f2d13db9f6750c26aeb 100644 (file)
--- a/drivers/video/backlight/qcom-wled.c
+++ b/drivers/video/backlight/qcom-wled.c
@@ -1255,21 +1255,6 @@ 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));
@@ -1343,11 +1328,6 @@ 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[] = {
@@ -1371,11 +1351,6 @@ 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[] = {
@@ -1399,11 +1374,6 @@ 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,
@@ -1522,8 +1492,6 @@ 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));
@@ -1554,6 +1522,22 @@ 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 && val > string_len) {
+                       dev_err(dev, "qcom,num-strings exceeds qcom,enabled-strings\n");
+                       return -EINVAL;
+               }
+
+               cfg->num_strings = val;
+       }
+
        return 0;
 }