tty: wipe buffer if not echoing data

BugLink: https://bugs.launchpad.net/bugs/1810967

commit b97b3d9f upstream.

If we are not echoing the data to userspace or the console is in icanon
mode, then perhaps it is a "secret" so we should wipe it once we are
done with it.

This mirrors the logic that the audit code has.
Reported-by: aszlig <aszlig@nix.build>
Tested-by: Milan Broz <gmazyland@gmail.com>
Tested-by: Daniel Zatovic <daniel.zatovic@gmail.com>
Tested-by: aszlig <aszlig@nix.build>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/tty/n_tty.c

@@ -165,15 +165,29 @@ static inline int tty_put_user(struct tty_struct *tty, unsigned char x,
 	return put_user(x, ptr);
 }
 
+/* If we are not echoing the data, perhaps this is a secret so erase it */
+static inline void zero_buffer(struct tty_struct *tty, u8 *buffer, int size)
+{
+	bool icanon = !!L_ICANON(tty);
+	bool no_echo = !L_ECHO(tty);
+
+	if (icanon && no_echo)
+		memset(buffer, 0x00, size);
+}
+
 static inline int tty_copy_to_user(struct tty_struct *tty,
 					void __user *to,
-					const void *from,
+					void *from,
 					unsigned long n)
 {
 	struct n_tty_data *ldata = tty->disc_data;
+	int retval;
 
 	tty_audit_add_data(tty, from, n, ldata->icanon);
-	return copy_to_user(to, from, n);
+	retval = copy_to_user(to, from, n);
+	if (!retval)
+		zero_buffer(tty, from, n);
+	return retval;
 }
 
 /**
@@ -2005,6 +2019,7 @@ static int copy_from_read_buf(struct tty_struct *tty,
 		is_eof = n == 1 && read_buf(ldata, tail) == EOF_CHAR(tty);
 		tty_audit_add_data(tty, read_buf_addr(ldata, tail), n,
 				ldata->icanon);
+		zero_buffer(tty, read_buf_addr(ldata, tail), n);
 		smp_store_release(&ldata->read_tail, ldata->read_tail + n);
 		/* Turn single EOF into zero-length read */
 		if (L_EXTPROC(tty) && ldata->icanon && is_eof &&