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From:   George Kennedy <george.kennedy@oracle.com>
To:     george.kennedy@oracle.com, gregkh@linuxfoundation.org,
        jslaby@suse.com, ebiggers@google.com, linux-kernel@vger.kernel.org,
        dan.carpenter@oracle.com, dhaval.giani@oracle.com
Subject: [PATCH 1/1] vt_ioctl: prevent VT_RESIZEX font height change from causing potential out-of-bounds access
Date:   Wed, 29 Jul 2020 08:39:41 -0400
Message-Id: <1596026381-5013-1-git-send-email-george.kennedy@oracle.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Add a VT_RESIZEX check to ensure that changing the font height will not
cause a potential out-of-bounds access. The candidate font height contained
in "v_clin", though below the max, could still result in accesses beyond
the allocated font data size.

Signed-off-by: George Kennedy <george.kennedy@oracle.com>
Reported-by: syzbot+38a3699c7eaf165b97a6@syzkaller.appspotmail.com
---
 drivers/tty/vt/vt_ioctl.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/vt/vt_ioctl.c b/drivers/tty/vt/vt_ioctl.c
index daf61c2..6185f1a 100644
--- a/drivers/tty/vt/vt_ioctl.c
+++ b/drivers/tty/vt/vt_ioctl.c
@@ -342,6 +342,9 @@ static void vt_disallocate_all(void)
 	}
 }
 
+/* from fbcon.c */
+#define FNTSIZE(fd) (((int *)(fd))[-2])
+#define FNTCHARCNT(fd) (((int *)(fd))[-3])
 
 /*
  * We handle the console-specific ioctl's here.  We allow the
@@ -895,8 +898,23 @@ int vt_ioctl(struct tty_struct *tty,
 			if (vcp) {
 				if (v.v_vlin)
 					vcp->vc_scan_lines = v.v_vlin;
-				if (v.v_clin)
+				if (v.v_clin) {
+					int width, pitch, size;
+
+					width = (vcp->vc_font.width > 8) ? 8 : vcp->vc_font.width;
+					pitch = (width + 7) >> 3;
+
+					pitch = (pitch) ? pitch : 1;
+
+					/* font size = height * pitch * charcount */
+					size = v.v_clin * pitch * FNTCHARCNT(vcp->vc_font.data);
+
+					if (size > FNTSIZE(vcp->vc_font.data)) {
+						console_unlock();
+						return -EINVAL;
+					}
 					vcp->vc_font.height = v.v_clin;
+				}
 				vcp->vc_resize_user = 1;
 				vc_resize(vcp, v.v_cols, v.v_rows);
 			}
-- 
1.8.3.1