Dell Dimension 8300 reboots when grub2 cbfs module is loaded

[Andrei Borzenkov <arvidjaar@gmail.com>]

I was debugging problem reported by user on Dell Dimension 8300 - it 
rebooted when doing "ls -l". It turned out, the problem was triggered by 
loading cbfs which probed for header. System has 2GB memory, and attempt 
to read from address 0xffffffff caused instant reboot. 0xffffffff was 
returned by read from non-existing address 0xfffffffc.

The proof of concept patch below avoids it, but I wonder what the proper 
fix is.

diff --git a/grub-core/fs/cbfs.c b/grub-core/fs/cbfs.c
index a34eb88..a5a2fde 100644
--- a/grub-core/fs/cbfs.c
+++ b/grub-core/fs/cbfs.c
@@ -344,8 +344,9 @@ init_cbfsdisk (void)

    ptr = *(grub_uint32_t *) 0xfffffffc;
    head = (struct cbfs_header *) (grub_addr_t) ptr;
+  grub_dprintf ("cbfs", "head=%p\n", head);

-  if (!validate_head (head))
+  if (0xffffffff - ptr < sizeof (*head) || !validate_head (head))
      return;

    cbfsdisk_size = ALIGN_UP (grub_be_to_cpu32 (head->romsize),