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

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Message-ID: <202308301646.8397A6A11@keescook>
Date:   Wed, 30 Aug 2023 16:48:00 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Dan Raymond <draymond@...valley.net>
Cc:     Azeem Shaikh <azeemshaikh38@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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 05:17:12PM -0600, Dan Raymond wrote:
> In my opinion strlcpy() is being used correctly here as a defensive
> precaution.  If the source string is larger than the destination buffer
> it will truncate rather than corrupt kernel memory.  However the
> return value of strlcpy() is being misused.  If truncation occurred
> the copy_to_user() call will corrupt user memory instead.
> 
> I also agree that this is not currently a bug.  It is fragile and it
> could break if someone added a very large string to the table.
> 
> Why not fix this by avoiding the redundant string copy?  How about
> something like this:
> 
> ptr = func_table[kb_func] ? : "";
> len = strlen(ptr);
> 
> if (len >= sizeof(user_kdgkb->kb_string))
> 	return -ENOSPC;
> 
> if (copy_to_user(user_kdgkb->kb_string, ptr, len + 1))
> 	return -EFAULT;

This would work if not for func_buf_lock. The bounce buffer is used to
avoid needing to hold the spin lock across copy_to_user.

-- 
Kees Cook