On Fri, Sep 18, 2020 at 4:55 PM Al Viro <[email protected]> wrote: > > On Fri, Sep 18, 2020 at 04:31:33PM -0700, Andy Lutomirski wrote: > > > check_zeroed_user() looks buggy. It does: > > > > if (!user_access_begin(from, size)) > > return -EFAULT; > > > > unsafe_get_user(val, (unsigned long __user *) from, err_fault); > > > > This is wrong if size < sizeof(unsigned long) -- you read outside the > > area you verified using user_access_begin(). > > Read the code immediately prior to that. from will be word-aligned, > and size will be extended accordingly. If the area acceptable for > user_access_begin() ends *NOT* on a word boundary, you have a problem > and I would strongly recommend to seek professional help. > > All reads in that thing are word-aligned and word-sized. So I very > much doubt that your analysis is correct.
Maybe -ETOOTIRED, but I seriously question the math in here. Suppose from == (unsigned long *)1 and size == 1. Then align is 1, and we do: from -= align; size += align; So now from = 0 and size = 2. Now we do user_access_begin(0, 2) and then immediately read 4 or 8 bytes. No good. --Andy
