[PATCH v2 2/2] fbdev: serialize mode sysfs access with lock_fb_info()

[Melbin K Mathew <mlbnkm1@gmail.com>]

show_mode(), show_modes(), and store_mode() access fb_info->modelist
and fb_info->mode without holding lock_fb_info(). store_modes() takes
lock_fb_info() while replacing the modelist and freeing the old one.

A concurrent reader or writer can load a pointer to an old modelist
entry before store_modes() frees it, then dereference freed memory or
store a stale freed pointer in fb_info->mode.

Take lock_fb_info() in show_mode(), show_modes(), and store_mode() to
serialize with store_modes(). In show_mode(), copy the mode to the
stack and format after dropping the lock. In store_mode(), split
activate() into a _locked variant to avoid double-locking, and hold
the locks for the modelist walk, mode conversion, activation, and
fb_info->mode assignment together.

Cc: stable@vger.kernel.org
Signed-off-by: Melbin K Mathew <mlbnkm1@gmail.com>
---
 drivers/video/fbdev/core/fbsysfs.c | 46 ++++++++++++++++++++++++------
 1 file changed, 38 insertions(+), 8 deletions(-)

diff --git a/drivers/video/fbdev/core/fbsysfs.c b/drivers/video/fbdev/core/fbsysfs.c
index af21dc5052..d3d60c555b 100644
--- a/drivers/video/fbdev/core/fbsysfs.c
+++ b/drivers/video/fbdev/core/fbsysfs.c
@@ -12,19 +12,24 @@
 #include "fb_internal.h"
 #include "fbcon.h"
 
+static int activate_locked(struct fb_info *fb_info,
+			    struct fb_var_screeninfo *var)
+{
+	var->activate |= FB_ACTIVATE_FORCE;
+	return fb_set_var_from_user(fb_info, var);
+}
+
 static int activate(struct fb_info *fb_info, struct fb_var_screeninfo *var)
 {
 	int err;
 
-	var->activate |= FB_ACTIVATE_FORCE;
 	console_lock();
 	lock_fb_info(fb_info);
-	err = fb_set_var_from_user(fb_info, var);
+	err = activate_locked(fb_info, var);
 	unlock_fb_info(fb_info);
 	console_unlock();
-	if (err)
-		return err;
-	return 0;
+
+	return err;
 }
 
 static int mode_string(char *buf, size_t size, unsigned int offset,
@@ -65,6 +70,9 @@ static ssize_t store_mode(struct device *device, struct device_attribute *attr,
 
 	memset(&var, 0, sizeof(var));
 
+	console_lock();
+	lock_fb_info(fb_info);
+
 	list_for_each_entry(modelist, &fb_info->modelist, list) {
 		mode = &modelist->mode;
 		i = mode_string(mstr, sizeof(mstr), 0, mode);
@@ -72,12 +80,22 @@ static ssize_t store_mode(struct device *device, struct device_attribute *attr,
 
 			var = fb_info->var;
 			fb_videomode_to_var(&var, mode);
-			if ((err = activate(fb_info, &var)))
+			err = activate_locked(fb_info, &var);
+			if (err) {
+				unlock_fb_info(fb_info);
+				console_unlock();
 				return err;
+			}
 			fb_info->mode = mode;
+			unlock_fb_info(fb_info);
+			console_unlock();
 			return count;
 		}
 	}
+
+	unlock_fb_info(fb_info);
+	console_unlock();
+
 	return -EINVAL;
 }
 
@@ -85,11 +103,20 @@ static ssize_t show_mode(struct device *device, struct device_attribute *attr,
 			 char *buf)
 {
 	struct fb_info *fb_info = dev_get_drvdata(device);
+	struct fb_videomode mode;
+	bool have_mode = false;
 
-	if (!fb_info->mode)
+	lock_fb_info(fb_info);
+	if (fb_info->mode) {
+		mode = *fb_info->mode;
+		have_mode = true;
+	}
+	unlock_fb_info(fb_info);
+
+	if (!have_mode)
 		return 0;
 
-	return mode_string(buf, PAGE_SIZE, 0, fb_info->mode);
+	return mode_string(buf, PAGE_SIZE, 0, &mode);
 }
 
 static ssize_t store_modes(struct device *device,
@@ -137,12 +164,15 @@ static ssize_t show_modes(struct device *device, struct device_attribute *attr,
 	const struct fb_videomode *mode;
 
 	i = 0;
+	lock_fb_info(fb_info);
 	list_for_each_entry(modelist, &fb_info->modelist, list) {
 		mode = &modelist->mode;
 		i += mode_string(buf, PAGE_SIZE, i, mode);
 		if (i >= PAGE_SIZE - 1)
 			break;
 	}
+	unlock_fb_info(fb_info);
+
 	return i;
 }
 
-- 
2.39.5