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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
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