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Date:   Tue, 5 Nov 2019 10:33:16 +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,
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        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:

> Because unicode uses 4 bytes. The issue is that there is no handling for
> unicode in vcs_write at all. (Compare with vcs_read.)

Exact.

----- >8

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;
+
 	con_buf = (char *) __get_free_page(GFP_KERNEL);
 	if (!con_buf)
 		return -ENOMEM;