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Date:   Tue, 5 Nov 2019 14:44:13 +0100 (CET)
From:   Nicolas Pitre <nico@fluxnic.net>
To:     Jiri Slaby <jslaby@suse.com>
cc:     Or Cohen <orcohen@paloaltonetworks.com>,
        Greg KH <gregkh@linuxfoundation.org>, textshell@uchuujin.de,
        Daniel Vetter <daniel.vetter@ffwll.ch>, sam@ravnborg.org,
        mpatocka@redhat.com, ghalat@redhat.com,
        linux-kernel@vger.kernel.org, jwilk@jwilk.net,
        Nadav Markus <nmarkus@paloaltonetworks.com>,
        syzkaller@googlegroups.com
Subject: Re: Bug report - slab-out-of-bounds in vcs_scr_readw
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On Tue, 5 Nov 2019, Jiri Slaby wrote:

> On 05. 11. 19, 10:33, Nicolas Pitre wrote:
> > Subject: [PATCH] vcs: prevent write access to vcsu devices
> > 
> > Commit d21b0be246bf ("vt: introduce unicode mode for /dev/vcs") guarded
> > against using devices containing attributes as this is not yet
> > implemented. It however failed to guard against writes to any devices
> > as this is also unimplemented.
> > 
> > Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
> > Cc: <stable@vger.kernel.org> # v4.19+
> > 
> > diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
> > index fa07d79027..ef19b95b73 100644
> > --- a/drivers/tty/vt/vc_screen.c
> > +++ b/drivers/tty/vt/vc_screen.c
> > @@ -456,6 +456,9 @@ vcs_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
> >  	size_t ret;
> >  	char *con_buf;
> >  
> > +	if (use_unicode(inode))
> > +		return -EOPNOTSUPP;
> 
> Looks good to me. I am also thinking about a ban directly in open:
> 
> if (use_unicode(inode) && (filp->f_flags & O_ACCMODE) != O_RDONLY)
>   return -EOPNOTSUPP;
> 
> Would that break the unicode users?

The user I know about uses a common helper that uses O_RDWR.
So yes, in that case that would break it.


Nicolas