[PATCH] tty: vt: selection: fix soft lockup in paste_selection()

[Yi Yang <yiyang13@xxxxxxxxxx>]

Soft lockup occurs when vt device used n_null ldisc, n_null_receivebuf()
is not implemented in null_ldisc. So tty_ldisc_receive_buf always return
0 in paste_selection(), this cause deadloop and cause soft lockup.

This can be reproduced as follows:
  int ldisc = 0x1b; // 0x1b is n_null
  struct{
  	char subcode;
  	struct tiocl_selection sel;
  } data;
  date.subcode = TIOCL_SETSEL;
  data.sel.xs = 0;
  data.sel.xe = 1;
  data.sel.ys = 0;
  data.sel.ye = 1;
  data.sel.sel_mode = TIOCL_SELCHAR;
  char bytes[2] = {TIOCL_PASTESEL, 0};
  open("ttyxx", O_RDWR) // open a vt device
  ioctl(fd, TIOCSETD, &ldisc) // set ldisc to n_null
  ioctl(fd, TIOCLINUX, &data.subcode);
  ioctl(fd, TIOCLINUX, bytes); // cause deadloop

Fix soft lockup by check if ldisc in paste_selection() is n_null.

Link: https://lore.kernel.org/all/000000000000fe769905d315a1b7@xxxxxxxxxx/
Fixes: 8a8dabf2dd68 ("tty: handle the case where we cannot restore a line discipline")
Signed-off-by: Yi Yang <yiyang13@xxxxxxxxxx>
---
 drivers/tty/vt/selection.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
index 6ef22f01cc51..9ba7f66fcf05 100644
--- a/drivers/tty/vt/selection.c
+++ b/drivers/tty/vt/selection.c
@@ -388,6 +388,12 @@ int paste_selection(struct tty_struct *tty)
 	ld = tty_ldisc_ref_wait(tty);
 	if (!ld)
 		return -EIO;	/* ldisc was hung up */
+
+	/* tty_ldisc_receive_buf() will not do anything when ldisc is n_null*/
+	if (ld->ops->num == N_NULL) {
+		tty_ldisc_deref(ld);
+		return -EIO;
+	}
 	tty_buffer_lock_exclusive(&vc->port);
 
 	add_wait_queue(&vc->paste_wait, &wait);
-- 
2.17.1