memory: brcmstb_dpfe: fix array index out of bounds

We would overrun the error_text array if we hit a TIMEOUT condition,
because we were using the error code "ETIMEDOUT" (which is 110) as an
array index.

We fix the problem by correcting the array index and by providing a
function to retrieve error messages rather than accessing the array
directly. The function includes a bounds check that prevents the array
from being overrun.

Link: https://lore.kernel.org/linux-arm-kernel/38d00022-730c-948a-917c-d86382df8cb9@canonical.com/
Link: https://lore.kernel.org/r/20200822205000.15841-1-mmayer@broadcom.com
Fixes: 2f330caf ("memory: brcmstb: Add driver for DPFE")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>

drivers/memory/brcmstb_dpfe.c

@@ -188,11 +188,6 @@ struct brcmstb_dpfe_priv {
 	struct mutex lock;
 };
 
-static const char * const error_text[] = {
-	"Success", "Header code incorrect", "Unknown command or argument",
-	"Incorrect checksum", "Malformed command", "Timed out",
-};
-
 /*
  * Forward declaration of our sysfs attribute functions, so we can declare the
  * attribute data structures early.
@@ -307,6 +302,20 @@ static const struct dpfe_api dpfe_api_v3 = {
 	},
 };
 
+static const char *get_error_text(unsigned int i)
+{
+	static const char * const error_text[] = {
+		"Success", "Header code incorrect",
+		"Unknown command or argument", "Incorrect checksum",
+		"Malformed command", "Timed out", "Unknown error",
+	};
+
+	if (unlikely(i >= ARRAY_SIZE(error_text)))
+		i = ARRAY_SIZE(error_text) - 1;
+
+	return error_text[i];
+}
+
 static bool is_dcpu_enabled(struct brcmstb_dpfe_priv *priv)
 {
 	u32 val;
@@ -445,7 +454,7 @@ static int __send_command(struct brcmstb_dpfe_priv *priv, unsigned int cmd,
 	}
 	if (resp != 0) {
 		mutex_unlock(&priv->lock);
-		return -ETIMEDOUT;
+		return -ffs(DCPU_RET_ERR_TIMEDOUT);
 	}
 
 	/* Compute checksum over the message */
@@ -695,7 +704,7 @@ static ssize_t generic_show(unsigned int command, u32 response[],
 
 	ret = __send_command(priv, command, response);
 	if (ret < 0)
-		return sprintf(buf, "ERROR: %s\n", error_text[-ret]);
+		return sprintf(buf, "ERROR: %s\n", get_error_text(-ret));
 
 	return 0;
 }