bootconfig: Extend the magic check range to the preceding 3 bytes

Since Grub may align the size of initrd to 4 if user pass
initrd from cpio, we have to check the preceding 3 bytes as well.

Link: https://lkml.kernel.org/r/160520205132.303174.4876760192433315429.stgit@devnote2
Cc: stable@vger.kernel.org
Fixes: 85c46b78da58 ("bootconfig: Add bootconfig magic word for indicating bootconfig explicitly")
Reported-by: Chen Yu <yu.chen.surf@gmail.com>
Tested-by: Chen Yu <yu.chen.surf@gmail.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/init/main.c b/init/main.c
index 130376e..20baced 100644 (file)
--- a/init/main.c
+++ b/init/main.c
@@ -269,14 +269,24 @@ static void * __init get_boot_config_from_initrd(u32 *_size, u32 *_csum)
        u32 size, csum;
        char *data;
        u32 *hdr;
+       int i;
 
        if (!initrd_end)
                return NULL;
 
        data = (char *)initrd_end - BOOTCONFIG_MAGIC_LEN;
-       if (memcmp(data, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN))
-               return NULL;
+       /*
+        * Since Grub may align the size of initrd to 4, we must
+        * check the preceding 3 bytes as well.
+        */
+       for (i = 0; i < 4; i++) {
+               if (!memcmp(data, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN))
+                       goto found;
+               data--;
+       }
+       return NULL;
 
+found:
        hdr = (u32 *)(data - 8);
        size = hdr[0];
        csum = hdr[1];