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

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Message-Id: <20200825164804.860743-1-yepeilin.cs@gmail.com>
Date:   Tue, 25 Aug 2020 12:48:04 -0400
From:   Peilin Ye <yepeilin.cs@...il.com>
To:     jslaby@...e.cz
Cc:     gregkh@...uxfoundation.org, 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

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?

Thank you,
Peilin Ye