[PATCH v2] PM / devfreq: Fix buffer overflow in trans_stat_show

[Christian Marangi]

Fix buffer overflow in trans_stat_show().

Convert simple snprintf to the more secure sysfs_emit.

Add condition checking if we are exceeding PAGE_SIZE and exit early from
loop. Also add at the end a warning that we exceeded PAGE_SIZE and that
stats is disabled.

Return -EFBIG in the case where we don't have enough space to write the
full transition table.

Also document in the ABI that this function can return -EFBIG error.

Cc: stable@xxxxxxxxxxxxxxx
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218041
Fixes: e552bbaf5b98 ("PM / devfreq: Add sysfs node for representing frequency transition information.")
Signed-off-by: Christian Marangi <ansuelsmth@xxxxxxxxx>
---
Changes v2:
- Squash and use directly sysfs_emit
- Fix typo for df->freq_table[2]
- Reduce check of PAGE_SIZE to only at the start of for loops

 Documentation/ABI/testing/sysfs-class-devfreq |  3 ++
 drivers/devfreq/devfreq.c                     | 47 ++++++++++++-------
 2 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-class-devfreq b/Documentation/ABI/testing/sysfs-class-devfreq
index 5e6b74f30406..1e7e0bb4c14e 100644
--- a/Documentation/ABI/testing/sysfs-class-devfreq
+++ b/Documentation/ABI/testing/sysfs-class-devfreq
@@ -52,6 +52,9 @@ Description:
 
 			echo 0 > /sys/class/devfreq/.../trans_stat
 
+		If the transition table is bigger than PAGE_SIZE, reading
+		this will return an -EFBIG error.
+
 What:		/sys/class/devfreq/.../available_frequencies
 Date:		October 2012
 Contact:	Nishanth Menon <nm@xxxxxx>
diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
index 63347a5ae599..3b8019ab523d 100644
--- a/drivers/devfreq/devfreq.c
+++ b/drivers/devfreq/devfreq.c
@@ -1688,7 +1688,7 @@ static ssize_t trans_stat_show(struct device *dev,
 			       struct device_attribute *attr, char *buf)
 {
 	struct devfreq *df = to_devfreq(dev);
-	ssize_t len;
+	ssize_t len = 0;
 	int i, j;
 	unsigned int max_state;
 
@@ -1697,7 +1697,7 @@ static ssize_t trans_stat_show(struct device *dev,
 	max_state = df->max_state;
 
 	if (max_state == 0)
-		return sprintf(buf, "Not Supported.\n");
+		return sysfs_emit(buf, "Not Supported.\n");
 
 	mutex_lock(&df->lock);
 	if (!df->stop_polling &&
@@ -1707,31 +1707,44 @@ static ssize_t trans_stat_show(struct device *dev,
 	}
 	mutex_unlock(&df->lock);
 
-	len = sprintf(buf, "     From  :   To\n");
-	len += sprintf(buf + len, "           :");
-	for (i = 0; i < max_state; i++)
-		len += sprintf(buf + len, "%10lu",
-				df->freq_table[i]);
+	len += sysfs_emit_at(buf, len, "     From  :   To\n");
+	len += sysfs_emit_at(buf, len, "           :");
+	for (i = 0; i < max_state; i++) {
+		if (len >= PAGE_SIZE - 1)
+			break;
+		len += sysfs_emit_at(buf, len, "%10lu",
+				     df->freq_table[i]);
+	}
 
-	len += sprintf(buf + len, "   time(ms)\n");
+	len += sysfs_emit_at(buf, len, "   time(ms)\n");
 
 	for (i = 0; i < max_state; i++) {
+		if (len >= PAGE_SIZE - 1)
+			break;
 		if (df->freq_table[i] == df->previous_freq)
-			len += sprintf(buf + len, "*");
+			len += sysfs_emit_at(buf, len, "*");
 		else
-			len += sprintf(buf + len, " ");
+			len += sysfs_emit_at(buf, len, " ");
 
-		len += sprintf(buf + len, "%10lu:", df->freq_table[i]);
-		for (j = 0; j < max_state; j++)
-			len += sprintf(buf + len, "%10u",
+		len += sysfs_emit_at(buf, len, "%10lu:", df->freq_table[i]);
+		for (j = 0; j < max_state; j++) {
+			if (len >= PAGE_SIZE - 1)
+				break;
+			len += sysfs_emit_at(buf, len, "%10u",
 				df->stats.trans_table[(i * max_state) + j]);
+		}
+
+		len += sysfs_emit_at(buf, len, "%10llu\n", (u64)
+				     jiffies64_to_msecs(df->stats.time_in_state[i]));
+	}
 
-		len += sprintf(buf + len, "%10llu\n", (u64)
-			jiffies64_to_msecs(df->stats.time_in_state[i]));
+	len += sysfs_emit_at(buf, len, "Total transition : %u\n",
+			     df->stats.total_trans);
+	if (len >= PAGE_SIZE - 1) {
+		pr_warn_once("devfreq transition table exceeds PAGE_SIZE. Disabling\n");
+		return -EFBIG;
 	}
 
-	len += sprintf(buf + len, "Total transition : %u\n",
-					df->stats.total_trans);
 	return len;
 }
 
-- 
2.43.0