fs/jfs: Catch infinite recursion

It's possible with a fuzzed filesystem for JFS to keep getblk()-ing
the same data over and over again, leading to stack exhaustion.

Check if we'd be calling the function with exactly the same data as
was passed in, and if so abort.

I'm not sure what the performance impact of this is and am open to
better ideas.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/fs/jfs.c

@@ -304,7 +304,16 @@ getblk (struct grub_jfs_treehead *treehead,
 			   << (grub_le_to_cpu16 (data->sblock.log2_blksz)
 			       - GRUB_DISK_SECTOR_BITS), 0,
 			   sizeof (*tree), (char *) tree))
+	{
+	  if (grub_memcmp (&tree->treehead, treehead, sizeof (struct grub_jfs_treehead)) ||
+	      grub_memcmp (&tree->extents, extents, 254 * sizeof (struct grub_jfs_tree_extent)))
 	    ret = getblk (&tree->treehead, &tree->extents[0], 254, data, blk);
+	  else
+	    {
+	      grub_error (GRUB_ERR_BAD_FS, "jfs: infinite recursion detected");
+	      ret = -1;
+	    }
+	}
       grub_free (tree);
       return ret;
     }