Re: [bug report] out of bounds read parsing vmode commandline option

Hi Dan,

On Wed, Oct 4, 2017 at 2:50 PM, Dan Carpenter [off-list ref] wrote:

quoted

This bug predates git but it looks like it might be simple to fix if the
right person looked at the code.

        drivers/video/fbdev/controlfb.c:560 control_setup()
        error: buffer overflow 'control_mac_modes' 20 <= 21

drivers/video/fbdev/controlfb.c
   549  static void __init control_setup(char *options)
   550  {
   551          char *this_opt;
   552
   553          if (!options || !*options)
   554                  return;
   555
   556          while ((this_opt = strsep(&options, ",")) != NULL) {
   557                  if (!strncmp(this_opt, "vmode:", 6)) {
   558                          int vmode = simple_strtoul(this_opt+6, NULL, 0);
                                    ^^^^^
We get vmode from the command line.

   559                          if (vmode > 0 && vmode <= VMODE_MAX &&
                                                          ^^^^^^^^^
We check that it's <= 22.

   560                              control_mac_modes[vmode - 1].m[1] >= 0)
                                    ^^^^^^^^^^^^^^^^^
But the problem is that control_mac_modes[] only has 20 elements so the
highest valid index is 19.  vmode - 1 can be up to 21.

Nice catch!

The bug was introduced in v2.4.5.6, when 2 new modes were added to
macmodes.h, but control_mac_modes[] wasn't updated:

https://kernel.opensuse.org/cgit/kernel/diff/include/video/macmodes.h?h=v2.5.2&id=29f279c764808560eaceb88fef36cbc35c529aad

A simple fix is to check against ARRAY_SIZE(control_mac_modes) instead.

A better fix is to add the missing entries to control_mac_modes[], cfr. the
(gmail-whitespace-damaged) patch below:

--- a/drivers/video/fbdev/controlfb.h
+++ b/drivers/video/fbdev/controlfb.h

@@ -141,5 +141,7 @@ static struct max_cmodes control_mac_modes[] = {
        {{ 1, 2}},      /* 1152x870, 75Hz */
        {{ 0, 1}},      /* 1280x960, 75Hz */
        {{ 0, 1}},      /* 1280x1024, 75Hz */
+       {{ 1, 2}},      /* 1152x768, 60Hz */
+       {{ 0, 1}},      /* 1600x1024, 60Hz */
 };

(this array lists the maximum color mode (8, 16, or 32 bpp) for each
 video mode given RAM restrictions (2 or 4 MiB)).

The 1152x768 mode is probably OK. Given the 1600x1024 mode has a lower
dotclock (112 MHz) than the supported 1280x960 mode, it's probably OK, too.