linux-hardening - [PATCH] VT: Protect KD_FONT_OP_GET

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Date:   Mon, 6 Mar 2023 10:49:21 +0100
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     gregkh@...uxfoundation.org
Cc:     linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org,
        Sanan Hasanov <sanan.hasanov@...ghts.ucf.edu>,
        Samuel Thibault <samuel.thibault@...-lyon.org>,
        keescook@...omium.org,
        syzbot+3af17071816b61e807ed@...kaller.appspotmail.com,
        akpm@...ux-foundation.org, linux-hardening@...r.kernel.org,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com,
        Jiri Slaby <jirislaby@...nel.org>
Subject: [PATCH] VT: Protect KD_FONT_OP_GET_TALL from unbound access

In ioctl(KD_FONT_OP_GET_TALL), userland tells through op->height which
vpitch should be used to copy over the font. In con_font_get, we were
not checking that it is within the maximum height value, and thus
userland could make the vc->vc_sw->con_font_get(vc, &font, vpitch);
call possibly overflow the allocated max_font_size bytes, and the
copy_to_user(op->data, font.data, c) call possibly read out of that
allocated buffer.

By checking vpitch against max_font_height, the max_font_size buffer
will always be large enough for the vc->vc_sw->con_font_get(vc, &font,
vpitch) call (since we already prevent loading a font larger than that),
and c = (font.width+7)/8 * vpitch * font.charcount will always remain
below max_font_size.

Fixes: 24d69384bcd3 ("VT: Add KD_FONT_OP_SET/GET_TALL operations")
Reported-by: syzbot+3af17071816b61e807ed@...kaller.appspotmail.com
Signed-off-by: Samuel Thibault <samuel.thibault@...-lyon.org>
Reviewed-by: Jiri Slaby <jirislaby@...nel.org>

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 57a5c23b51d4..3c2ea9c098f7 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -4545,6 +4545,9 @@ static int con_font_get(struct vc_data *vc, struct console_font_op *op)
 	int c;
 	unsigned int vpitch = op->op == KD_FONT_OP_GET_TALL ? op->height : 32;

+	if (vpitch > max_font_height)
+		return -EINVAL;
+
 	if (op->data) {
 		font.data = kvmalloc(max_font_size, GFP_KERNEL);
 		if (!font.data)