linux-kernel - Re: [PATCH 15/16] vc_screen: extract vcs_read_buf

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Message-ID: <20200825165935.GA1527003@kroah.com>
Date:   Tue, 25 Aug 2020 18:59:35 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Peilin Ye <yepeilin.cs@...il.com>
Cc:     jslaby@...e.cz, linux-kernel@...r.kernel.org,
        linux-serial@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH 15/16] vc_screen: extract vcs_read_buf_header

On Tue, Aug 25, 2020 at 12:48:04PM -0400, Peilin Ye wrote:
> Hi all,
> 
> Link: https://syzkaller.appspot.com/bug?id=f332576321998d36cd07d09c9c1268cfed1895c9
> 
> As reported by syzbot, vcs_read_buf() is overflowing `con_buf16`, since
> this patch removed the following check:
> 
> -		if (count > CON_BUF_SIZE) {
> -			count = CON_BUF_SIZE;
> -			filled = count - pos;
> -		}
> 
> Decreasing `count` by `min(HEADER_SIZE - pos, count)` bypasses this check.
> Additionally, this patch also removed updates to `skip` and `filled`.
> 
> What should we do in order to fix it?

This patch is already reverted, and it has been discussed a bit as to
how to do this properly if you look at the email where this was reported
to us.

thanks,

greg k-h