linux-kernel - Re: [RFC PATCH 2/4] uaccess: Add non-pagefault user-space read functions

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Message-ID: <CAGXu5jL+8nBen8_BjTziF=bGQm7hijV7YMa_a53vXfFT4+mwzQ@mail.gmail.com>
Date:   Mon, 25 Feb 2019 09:06:55 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Changbin Du <changbin.du@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Nadav Amit <namit@...are.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 2/4] uaccess: Add non-pagefault user-space read functions

On Mon, Feb 25, 2019 at 6:06 AM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> +static __always_inline long strncpy_from_unsafe_common(char *dst,
> +                               const char __user *unsafe_addr, long count)
> +{
> +       const char __user *src = unsafe_addr;
> +       int ret;
> +
> +       pagefault_disable();
> +       do {
> +               ret = __get_user(*dst++, src++);
> +       } while (dst[-1] && ret == 0 && src - unsafe_addr < count);
> +       dst[-1] = '\0';
> +       pagefault_enable();
> +
> +       return ret ? -EFAULT : src - unsafe_addr;
> +}

I'm all for always NUL-truncating, but this isn't "strncpy" (which has
the buggy maybe-I-didn't-NUL-terminate behavior). Can we call this
strscpy_...() instead?

-- 
Kees Cook