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

On 2021-11-12 12:08:39, Daniel Thompson wrote:

On Fri, Nov 12, 2021 at 01:26:57AM +0100, Marijn Suijten wrote:

quoted

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@somainline.org>
Reviewed-By: AngeloGioacchino Del Regno <redacted>
---
 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

@@ -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");

The warning should also be below the error message on the next if statement.

Agreed.

This warning occurs even when there is no ambiguity.

This could be:

	if (string_len > 0 && val != string_len)

Combined these changes allows us to give a much more helpful and assertive
warning message:

qcom,num-strings mis-matches and will partially override
qcom,enabled-strings (remove qcom,num-strings?)

I want to let the user know it's set regardless of whether they're
equivalent; no need to set both.

How about:

    Only one of qcom,num-strings or qcom,enabled-strings should be set

That should be more descriptive?  Otherwise, let me know if you really
want to allow users to (unnecessarily) set both - or if it can / should
be caught in DT validation instead.

- Marijn

quoted

+			if (val > string_len) {
+				dev_err(dev, "qcom,num-strings exceeds qcom,enabled-strings\n");
+				return -EINVAL;
+			}
+		}

Daniel.