tty: 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@syzkaller.appspotmail.com
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230306094921.tik5ewne4ft6mfpo@begin
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 57a5c23..3c2ea9c 100644 (file)
--- 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)