Some legacy fbdev drivers may incorrectly set info->screen_size to a
value larger than the actual mapped framebuffer size (info->fix.smem_len)
during mode switches. This could allow out-of-bounds I/O and system
memory accesses in fb_io_read(), fb_io_write(), fb_sys_read(), and
fb_sys_write().

Prevent this by clamping total_size to smem_len when smem_len is non-zero.
Virtual framebuffers (smem_len == 0) are unaffected.

This is a hardening measure; no specific crash is fixed by this patch.

Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn>
---
v2:
 - Apply the clamp to fb_sys_read() and fb_sys_write() as well to prevent
   similar OOB accesses in system memory framebuffers (Reported by Sashiko).
 - Update commit message to clearly state this is a hardening measure.
---
 drivers/video/fbdev/core/fb_io_fops.c  | 16 ++++++++++++++++
 drivers/video/fbdev/core/fb_sys_fops.c | 16 ++++++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/drivers/video/fbdev/core/fb_io_fops.c b/drivers/video/fbdev/core/fb_io_fops.c
index 6ab60fcd0050..335f16d2cc23 100644
--- a/drivers/video/fbdev/core/fb_io_fops.c
+++ b/drivers/video/fbdev/core/fb_io_fops.c
@@ -24,6 +24,14 @@ ssize_t fb_io_read(struct fb_info *info, char __user *buf, size_t count, loff_t
 	if (total_size == 0)
 		total_size = info->fix.smem_len;
 
+	/*
+	 * Security Hardening: Defend against buggy legacy drivers that may
+	 * calculate a malformed screen_size. Clamp total_size to the actual
+	 * hardware mapped memory limit (smem_len) to prevent OOB access.
+	 */
+	if (info->fix.smem_len && total_size > info->fix.smem_len)
+		total_size = info->fix.smem_len;
+
 	if (p >= total_size)
 		return 0;
 
@@ -88,6 +96,14 @@ ssize_t fb_io_write(struct fb_info *info, const char __user *buf, size_t count,
 	if (total_size == 0)
 		total_size = info->fix.smem_len;
 
+	/*
+	 * Security Hardening: Defend against buggy legacy drivers that may
+	 * calculate a malformed screen_size. Clamp total_size to the actual
+	 * hardware mapped memory limit (smem_len) to prevent OOB access.
+	 */
+	if (info->fix.smem_len && total_size > info->fix.smem_len)
+		total_size = info->fix.smem_len;
+
 	if (p > total_size)
 		return -EFBIG;
 
diff --git a/drivers/video/fbdev/core/fb_sys_fops.c b/drivers/video/fbdev/core/fb_sys_fops.c
index be96b3b3942e..e97cf02f7c70 100644
--- a/drivers/video/fbdev/core/fb_sys_fops.c
+++ b/drivers/video/fbdev/core/fb_sys_fops.c
@@ -35,6 +35,14 @@ ssize_t fb_sys_read(struct fb_info *info, char __user *buf, size_t count,
 	if (total_size == 0)
 		total_size = info->fix.smem_len;
 
+	/*
+	 * Security Hardening: Defend against buggy legacy drivers that may
+	 * calculate a malformed screen_size. Clamp total_size to the actual
+	 * hardware mapped memory limit (smem_len) to prevent OOB access.
+	 */
+	if (info->fix.smem_len && total_size > info->fix.smem_len)
+		total_size = info->fix.smem_len;
+
 	if (p >= total_size)
 		return 0;
 
@@ -80,6 +88,14 @@ ssize_t fb_sys_write(struct fb_info *info, const char __user *buf,
 	if (total_size == 0)
 		total_size = info->fix.smem_len;
 
+	/*
+	 * Security Hardening: Defend against buggy legacy drivers that may
+	 * calculate a malformed screen_size. Clamp total_size to the actual
+	 * hardware mapped memory limit (smem_len) to prevent OOB access.
+	 */
+	if (info->fix.smem_len && total_size > info->fix.smem_len)
+		total_size = info->fix.smem_len;
+
 	if (p > total_size)
 		return -EFBIG;
 
-- 
2.34.1