linux-hardening - Re: [PATCH] vt: Fix potential read overflow of kernel memory

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Message-ID: <CADmuW3Wbgb7s+jRm8F0hcjzreWysVdzNvv778yUbGCOxAJHwjQ@mail.gmail.com>
Date:   Wed, 30 Aug 2023 15:25:54 -0400
From:   Azeem Shaikh <azeemshaikh38@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jirislaby@...nel.org>, linux-hardening@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] vt: Fix potential read overflow of kernel memory

On Wed, Aug 30, 2023 at 1:57 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Wed, Aug 30, 2023 at 04:04:10PM +0000, Azeem Shaikh wrote:
> > strlcpy() reads the entire source buffer first.
> > This read may exceed the destination size limit if
> > a source string is not NUL-terminated [1].
>
> But that's not the case here, right?  So your "potential read overflow"
> isn't relevant here.
>
> > The copy_to_user() call uses @len returned from strlcpy() directly
> > without checking its value. This could potentially lead to read
> > overflow.
>
> But can it?  How?
>

The case I was considering is when the null-terminated hardcoded
string @func_table[kb_func] has length @new_len > @len. In this case,
strlcpy() will assign @len = @new_len and copy_to_user() would read
@new_len from the kmalloc-ed memory of @len. This is the potential
read overflow I was referring to. Let me know if I'm mistaken.