vfs: Copy struct mount.mnt_id to userspace using put_user()

The mnt_id field can be copied with put_user(), so there is no need to
use copy_to_user(). In both cases, hardened usercopy is being bypassed
since the size is constant, and not open to runtime manipulation.

This patch is verbatim from Brad Spengler/PaX Team's PAX_USERCOPY
whitelisting code in the last public patch of grsecurity/PaX based on my
understanding of the code. Changes or omissions from the original code are
mine and don't reflect the original grsecurity/PaX code.

Signed-off-by: David Windsor <dave@nullcore.net>
[kees: adjust commit log]
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 0ace128..0ee7274 100644 (file)
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -69,8 +69,7 @@ static long do_sys_name_to_handle(struct path *path,
        } else
                retval = 0;
        /* copy the mount id */
-       if (copy_to_user(mnt_id, &real_mount(path->mnt)->mnt_id,
-                        sizeof(*mnt_id)) ||
+       if (put_user(real_mount(path->mnt)->mnt_id, mnt_id) ||
            copy_to_user(ufh, handle,
                         sizeof(struct file_handle) + handle_bytes))
                retval = -EFAULT;