[PATCH v2] staging: nvec: validate battery response length before memcpy

[Sebastian Josue Alba Vives <sebasjosue84@gmail.com>]

From: Sebastián Alba Vives <sebasjosue84@gmail.com>

In nvec_power_notifier(), the response length from the embedded
controller is used directly as the size argument to memcpy() when
copying battery manufacturer, model, and type strings. The
destination buffers (bat_manu, bat_model, bat_type) are fixed at
30 bytes, but res->length is a u8 that can be up to 255, allowing
a heap buffer overflow.

Additionally, if res->length is less than 2, the subtraction
res->length - 2 wraps around as an unsigned value, resulting in a
large copy that corrupts kernel heap memory.

Introduce NVEC_BAT_STRING_SIZE to replace the hardcoded buffer
size, store res->length - 2 in a local copy_len variable for
clarity, and add bounds checks before each memcpy to ensure the
copy length does not exceed the destination buffer and that
res->length is at least 2 to prevent unsigned integer underflow.

Tested-by: Marc Dietrich <marvin24@gmx.de>
Cc: stable@vger.kernel.org
Signed-off-by: Sebastián Alba Vives <sebasjosue84@gmail.com>
---
v2:
  - Introduce NVEC_BAT_STRING_SIZE constant to replace hardcoded
    buffer size (Marc Dietrich)
  - Store res->length - 2 in local copy_len variable for clarity
    (Marc Dietrich)
  - Use NVEC_BAT_STRING_SIZE in strncmp call for consistency
 drivers/staging/nvec/nvec_power.c | 41 +++++++++++++++++++++----------
 1 file changed, 28 insertions(+), 13 deletions(-)

diff --git a/drivers/staging/nvec/nvec_power.c b/drivers/staging/nvec/nvec_power.c
index 2faab9fde..7b7980127 100644
--- a/drivers/staging/nvec/nvec_power.c
+++ b/drivers/staging/nvec/nvec_power.c
@@ -19,6 +19,7 @@
 #include "nvec.h"
 
 #define GET_SYSTEM_STATUS 0x00
+#define NVEC_BAT_STRING_SIZE 30
 
 struct nvec_power {
 	struct notifier_block notifier;
@@ -38,9 +39,9 @@ struct nvec_power {
 	int bat_temperature;
 	int bat_cap;
 	int bat_type_enum;
-	char bat_manu[30];
-	char bat_model[30];
-	char bat_type[30];
+	char bat_manu[NVEC_BAT_STRING_SIZE];
+	char bat_model[NVEC_BAT_STRING_SIZE];
+	char bat_type[NVEC_BAT_STRING_SIZE];
 };
 
 enum {
@@ -192,22 +193,36 @@ static int nvec_power_bat_notifier(struct notifier_block *nb,
 	case TEMPERATURE:
 		power->bat_temperature = res->plu - 2732;
 		break;
-	case MANUFACTURER:
-		memcpy(power->bat_manu, &res->plc, res->length - 2);
-		power->bat_manu[res->length - 2] = '\0';
+	case MANUFACTURER: {
+		size_t copy_len = res->length - 2;
+
+		if (res->length < 2 || copy_len > NVEC_BAT_STRING_SIZE - 1)
+			break;
+		memcpy(power->bat_manu, &res->plc, copy_len);
+		power->bat_manu[copy_len] = '\0';
 		break;
-	case MODEL:
-		memcpy(power->bat_model, &res->plc, res->length - 2);
-		power->bat_model[res->length - 2] = '\0';
+	}
+	case MODEL: {
+		size_t copy_len = res->length - 2;
+
+		if (res->length < 2 || copy_len > NVEC_BAT_STRING_SIZE - 1)
+			break;
+		memcpy(power->bat_model, &res->plc, copy_len);
+		power->bat_model[copy_len] = '\0';
 		break;
-	case TYPE:
-		memcpy(power->bat_type, &res->plc, res->length - 2);
-		power->bat_type[res->length - 2] = '\0';
+	}
+	case TYPE: {
+		size_t copy_len = res->length - 2;
+
+		if (res->length < 2 || copy_len > NVEC_BAT_STRING_SIZE - 1)
+			break;
+		memcpy(power->bat_type, &res->plc, copy_len);
+		power->bat_type[copy_len] = '\0';
 		/*
 		 * This differs a little from the spec fill in more if you find
 		 * some.
 		 */
-		if (!strncmp(power->bat_type, "Li", 30))
+		if (!strncmp(power->bat_type, "Li", NVEC_BAT_STRING_SIZE))
 			power->bat_type_enum = POWER_SUPPLY_TECHNOLOGY_LION;
 		else
 			power->bat_type_enum = POWER_SUPPLY_TECHNOLOGY_UNKNOWN;
-- 
2.43.0