[PATCH AUTOSEL 5.15 12/44] PM / devfreq: governor: Replace sscanf() with kstrtoul() in set_freq_store()

[Sasha Levin <sashal@kernel.org>]

From: Lifeng Zheng <zhenglifeng1@huawei.com>

[ Upstream commit 914cc799b28f17d369d5b4db3b941957d18157e8 ]

Replace sscanf() with kstrtoul() in set_freq_store() and check the result
to avoid invalid input.

Signed-off-by: Lifeng Zheng <zhenglifeng1@huawei.com>
Link: https://lore.kernel.org/lkml/20250421030020.3108405-2-zhenglifeng1@huawei.com/
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

## Backport Status: YES

This commit should be backported to stable kernel trees for the
following reasons:

1. **Security Issue - Input Validation Bug**: The original code uses
   `sscanf(buf, "%lu", &wanted)` at line 45 without checking the return
   value. This is a security vulnerability because:
   - `sscanf()` returns the number of successfully parsed items
   - If the input is malformed or contains invalid characters,
     `sscanf()` may return 0 but leave `wanted` uninitialized
   - The code then proceeds to use this potentially uninitialized value
     in `data->user_frequency = wanted`
   - This could lead to setting arbitrary frequency values from
     uninitialized stack data

2. **Real User Impact**: The devfreq userspace governor allows users to
   manually set device frequencies through sysfs. Invalid input handling
   could cause:
   - Setting incorrect frequencies based on uninitialized memory
   - Potential system instability if invalid frequencies are applied to
     hardware
   - Security implications as uninitialized stack data could be used

3. **Small and Contained Fix**: The change is minimal and low-risk:
   - Replaces `sscanf()` with `kstrtoul()` which has proper error
     checking
   - Adds explicit error handling that returns early on invalid input
   - The change is confined to a single function (`set_freq_store()`)
   - No architectural changes or new features added

4. **Follows Kernel Best Practices**: The kernel has been systematically
   replacing `sscanf()` with `kstrto*()` functions for better input
   validation. This is evident from similar commits found in the git log
   (e.g., commit a5556fa1107d for asus-wmi).

5. **Critical Subsystem**: While devfreq might not be as critical as
   core memory management, it controls device frequency scaling which
   can affect:
   - Power management
   - System performance
   - Hardware stability

6. **No Negative Side Effects**: The change only adds proper validation
   and doesn't modify the core functionality. Valid inputs will continue
   to work exactly as before.

The commit fixes a clear bug (missing input validation) that could lead
to undefined behavior and potential security issues, making it an
excellent candidate for stable backporting according to stable tree
rules.

 drivers/devfreq/governor_userspace.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/devfreq/governor_userspace.c b/drivers/devfreq/governor_userspace.c
index d69672ccacc4..8d057cea09d5 100644
--- a/drivers/devfreq/governor_userspace.c
+++ b/drivers/devfreq/governor_userspace.c
@@ -9,6 +9,7 @@
 #include <linux/slab.h>
 #include <linux/device.h>
 #include <linux/devfreq.h>
+#include <linux/kstrtox.h>
 #include <linux/pm.h>
 #include <linux/mutex.h>
 #include <linux/module.h>
@@ -39,10 +40,13 @@ static ssize_t set_freq_store(struct device *dev, struct device_attribute *attr,
 	unsigned long wanted;
 	int err = 0;
 
+	err = kstrtoul(buf, 0, &wanted);
+	if (err)
+		return err;
+
 	mutex_lock(&devfreq->lock);
 	data = devfreq->governor_data;
 
-	sscanf(buf, "%lu", &wanted);
 	data->user_frequency = wanted;
 	data->valid = true;
 	err = update_devfreq(devfreq);
-- 
2.39.5