linux-kernel - [PATCH 1/3] cpufreq: fix broken buffer overflow detection in trans

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Date:   Tue, 24 Oct 2023 20:30:14 +0200
From:   Christian Marangi <ansuelsmth@...il.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Christian Marangi <ansuelsmth@...il.com>,
        Takashi Iwai <tiwai@...e.de>,
        Jonghwa Lee <jonghwa3.lee@...sung.com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Cc:     stable@...r.kernel.org
Subject: [PATCH 1/3] cpufreq: fix broken buffer overflow detection in trans_stats

Commit 3c0897c180c6 ("cpufreq: Use scnprintf() for avoiding potential
buffer overflow") switched from snprintf to the more secure scnprintf
but never updated the exit condition for PAGE_SIZE.

As the commit say and as scnprintf document, what scnprintf returns what
is actually written not counting the '\0' end char. This results in the
case of len exceeding the size, len set to PAGE_SIZE - 1, as it can be
written at max PAGESIZE - 1 (as '\0' is not counted)

Because of len is never set to PAGE_SIZE, the function never break early,
never print the warning and never return -EFBIG.

Fix this by fixing the condition to PAGE_SIZE -1 to correctly trigger
the error condition.

Cc: stable@...r.kernel.org
Fixes: 3c0897c180c6 ("cpufreq: Use scnprintf() for avoiding potential buffer overflow")
Signed-off-by: Christian Marangi <ansuelsmth@...il.com>
---
 drivers/cpufreq/cpufreq_stats.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
index a33df3c66c88..40a9ff18da06 100644
--- a/drivers/cpufreq/cpufreq_stats.c
+++ b/drivers/cpufreq/cpufreq_stats.c
@@ -131,23 +131,23 @@ static ssize_t show_trans_table(struct cpufreq_policy *policy, char *buf)
 	len += sysfs_emit_at(buf, len, "   From  :    To\n");
 	len += sysfs_emit_at(buf, len, "         : ");
 	for (i = 0; i < stats->state_num; i++) {
-		if (len >= PAGE_SIZE)
+		if (len >= PAGE_SIZE - 1)
 			break;
 		len += sysfs_emit_at(buf, len, "%9u ", stats->freq_table[i]);
 	}
-	if (len >= PAGE_SIZE)
-		return PAGE_SIZE;
+	if (len >= PAGE_SIZE - 1)
+		return PAGE_SIZE - 1;
 
 	len += sysfs_emit_at(buf, len, "\n");
 
 	for (i = 0; i < stats->state_num; i++) {
-		if (len >= PAGE_SIZE)
+		if (len >= PAGE_SIZE - 1)
 			break;
 
 		len += sysfs_emit_at(buf, len, "%9u: ", stats->freq_table[i]);
 
 		for (j = 0; j < stats->state_num; j++) {
-			if (len >= PAGE_SIZE)
+			if (len >= PAGE_SIZE - 1)
 				break;
 
 			if (pending)
@@ -157,12 +157,12 @@ static ssize_t show_trans_table(struct cpufreq_policy *policy, char *buf)
 
 			len += sysfs_emit_at(buf, len, "%9u ", count);
 		}
-		if (len >= PAGE_SIZE)
+		if (len >= PAGE_SIZE - 1)
 			break;
 		len += sysfs_emit_at(buf, len, "\n");
 	}
 
-	if (len >= PAGE_SIZE) {
+	if (len >= PAGE_SIZE - 1) {
 		pr_warn_once("cpufreq transition table exceeds PAGE_SIZE. Disabling\n");
 		return -EFBIG;
 	}
-- 
2.40.1