Fix some checks if framebuffer is active

As potential security vulnerability to shutdown the process, it is required to check if the framebuffer is really in use. Without the check, a Python user could shutdown the program by trying to execute a draw function on a not opened framebuffer without initialized offscreen buffer. The result would be a NULL pointer access to the offscreen buffer. Additional to closing this security vulnerability, adds a function to check if a framebuffer is opened only for internal purpose in the validation process before executing the draw operation.

native/framebuffers.c

@@ -26,6 +26,15 @@ void pyfb_init(void) {
     }
 }
 
+int __APISTATUS_internal pyfb_fbused(uint8_t fbnum) {
+    if(framebuffers[fbnum].fb_fd == -1) {
+        return 0;
+    }
+
+    // else
+    return 1;
+}
+
 void __APISTATUS_internal pyfb_fblock(uint8_t fbnum) {
     // first test if this device number is valid
     if(fbnum >= MAX_FRAMEBUFFERS) {
@@ -315,6 +324,13 @@ void pyfb_ssetPixel(uint8_t fbnum, unsigned long int x, unsigned long int y, con
     // Is valid, so lock it!
     lock(framebuffers[fbnum].fb_lock);
 
+    // next, test if the device is really in use
+    if(framebuffers[fbnum].fb_fd == -1) {
+        // this framebuffer is not in use, so ignore
+        unlock(framebuffers[fbnum].fb_lock);
+        return;
+    }
+
     // check if all values are valid
     unsigned int xres = framebuffers[fbnum].fb_info.vinfo.xres;
     unsigned int yres = framebuffers[fbnum].fb_info.vinfo.yres;
@@ -362,6 +378,13 @@ void pyfb_sdrawHorizontalLine(uint8_t fbnum,
     // Is valid, so lock it!
     lock(framebuffers[fbnum].fb_lock);
 
+    // next, test if the device is really in use
+    if(framebuffers[fbnum].fb_fd == -1) {
+        // this framebuffer is not in use, so ignore
+        unlock(framebuffers[fbnum].fb_lock);
+        return;
+    }
+
     // Now test if the ranges are okay
     unsigned long int xres = framebuffers[fbnum].fb_info.vinfo.xres;
     unsigned long int yres = framebuffers[fbnum].fb_info.vinfo.yres;
@@ -407,6 +430,13 @@ void pyfb_sdrawVerticalLine(uint8_t fbnum,
     // Is valid, so lock it!
     lock(framebuffers[fbnum].fb_lock);
 
+    // next, test if the device is really in use
+    if(framebuffers[fbnum].fb_fd == -1) {
+        // this framebuffer is not in use, so ignore
+        unlock(framebuffers[fbnum].fb_lock);
+        return;
+    }
+
     // Now test if the ranges are okay
     unsigned long int xres = framebuffers[fbnum].fb_info.vinfo.xres;
     unsigned long int yres = framebuffers[fbnum].fb_info.vinfo.yres;

native/pyframebuffer.h

@@ -250,6 +250,29 @@ extern void __APISTATUS_internal pyfb_fblock(uint8_t fbnum);
  */
 extern void __APISTATUS_internal pyfb_fbunlock(uint8_t fbnum);
 
+/**
+ * Checks if the framebuffer is really opened. This is only used internally as checking function before
+ * executing a draw operation to prevent NULL pointer access.
+ * 
+ * The access should work like this:
+ * \code{.c}
+ * if(!pyfb_fbused(fbnum)) {
+ *     // any error handling
+ *     return;
+ * }
+ * 
+ * // if get here, the framebuffer is in use
+ * \endcode
+ * 
+ * Please lock the framebuffer before invoking this function. This ensures that the status request is keept
+ * up to date or is not modified at the same time this function is testing it.
+ * 
+ * @param fbnum The framebuffer number to check
+ * 
+ * @return If is in use 1, else 0
+ */
+extern int __APISTATUS_internal pyfb_fbused(uint8_t fbnum);
+
 /**
  * Paints a exactly horizontal line. This function is secure because before painting,
  * it validates the arguments.