vcsa: clamp header values when they don't fit

The /dev/vcsa* devices have a fixed char-sized header that stores the
screen geometry and cursor location. Let's make sure it doesn't contain
random garbage when those values exceed 255. If ever it becomes necessary
to convey larger screen info to user space then a larger header in the
not-yet-implemented /dev/vcsua* devices should be considered.
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/vc_screen.c

@@ -335,8 +335,9 @@ vcs_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
 			if (p < HEADER_SIZE) {
 				size_t tmp_count;
 
-				con_buf0[0] = (char)vc->vc_rows;
-				con_buf0[1] = (char)vc->vc_cols;
+				/* clamp header values if they don't fit */
+				con_buf0[0] = min(vc->vc_rows, 0xFFu);
+				con_buf0[1] = min(vc->vc_cols, 0xFFu);
 				getconsxy(vc, con_buf0 + 2);
 
 				con_buf_start += p;

drivers/tty/vt/vt.c

@@ -4607,8 +4607,9 @@ EXPORT_SYMBOL_GPL(screen_pos);
 
 void getconsxy(struct vc_data *vc, unsigned char *p)
 {
-	p[0] = vc->vc_x;
-	p[1] = vc->vc_y;
+	/* clamp values if they don't fit */
+	p[0] = min(vc->vc_x, 0xFFu);
+	p[1] = min(vc->vc_y, 0xFFu);
 }
 
 void putconsxy(struct vc_data *vc, unsigned char *p)