[D/E][SRU][CVE-2019-19252][PATCH] vcs: prevent write access to vcsu devices

[Connor Kuehl]

From: Nicolas Pitre <nico at fluxnic.net>

CVE-2019-19252

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.

Reported-by: Or Cohen <orcohen at paloaltonetworks.com>
Signed-off-by: Nicolas Pitre <npitre at baylibre.com>
Cc: <stable at vger.kernel.org> # v4.19+
Cc: Jiri Slaby <jslaby at suse.com>
Fixes: d21b0be246bf ("vt: introduce unicode mode for /dev/vcs")
Link: https://lore.kernel.org/r/nycvar.YSQ.7.76.1911051030580.30289@knanqh.ubzr
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from 0c9acb1af77a3cb8707e43f45b72c95266903cee)
Signed-off-by: Connor Kuehl <connor.kuehl at canonical.com>
---
 drivers/tty/vt/vc_screen.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
index 2384ea85ffaf..2fb509d57e88 100644
--- a/drivers/tty/vt/vc_screen.c
+++ b/drivers/tty/vt/vc_screen.c
@@ -437,6 +437,9 @@ 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;
-- 
2.20.1