[PATCH] tty: vt: add some NULL checks for vc_data

[Hang Zhang <zh.nvgt@gmail.com>]

vc_selection(), do_blank_screen() and scrollfront() all access "vc_data"
structures obtained from the global "vc_cons[fg_console].d", which can
be freed and nullified (e.g., in the error path of vc_allocate()). But
these functions don't have any NULL checks against the pointers before
dereferencing them, causing potentially use-after-free or null pointer
dereference.

Prevent these potential issues by placing NULL checks in these functions
before accessing "vc_data" structures. Similar checks can be found in
other functions like vt_console_print() and poke_blanked_console().

Signed-off-by: Hang Zhang <zh.nvgt@gmail.com>
---
 drivers/tty/vt/selection.c | 3 +++
 drivers/tty/vt/vt.c        | 5 +++++
 2 files changed, 8 insertions(+)

diff --git a/drivers/tty/vt/selection.c b/drivers/tty/vt/selection.c
index 6ef22f01cc51..c727fd947683 100644
--- a/drivers/tty/vt/selection.c
+++ b/drivers/tty/vt/selection.c
@@ -319,6 +319,9 @@ static int vc_selection(struct vc_data *vc, struct tiocl_selection *v,
 {
 	int ps, pe;
 
+	if (!vc)
+		return 0;
+
 	poke_blanked_console();
 
 	if (v->sel_mode == TIOCL_SELCLEAR) {
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
index 981d2bfcf9a5..00f8fdc61e9f 100644
--- a/drivers/tty/vt/vt.c
+++ b/drivers/tty/vt/vt.c
@@ -1493,6 +1493,8 @@ void scrollback(struct vc_data *vc)
 
 void scrollfront(struct vc_data *vc, int lines)
 {
+	if (!vc)
+		return;
 	if (!lines)
 		lines = vc->vc_rows / 2;
 	scrolldelta(lines);
@@ -4346,6 +4348,9 @@ void do_blank_screen(int entering_gfx)
 	struct vc_data *vc = vc_cons[fg_console].d;
 	int i;
 
+	if (!vc)
+		return;
+
 	might_sleep();
 
 	WARN_CONSOLE_UNLOCKED();
-- 
2.39.0