Re: WARNING in ex_handler_uaccess

[Al Viro]

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.