From df3f307b8e013814384723371dbfc549542a3a8f Mon Sep 17 00:00:00 2001
From: Kenny Levinsen <kl@kl.wtf>
Date: Fri, 23 Apr 2021 00:17:19 +0200
Subject: seat: Avoid holding a tty fd

The kernel Secure Attention Key killer, triggered by SysRq+k, kills all
processes that hold an fd referencing the tty.

To avoid its attention, we stop storing the fd for the currently active
VT in seat state. This has the added benefit of simplifying state a bit.
---
 include/seat.h |  1 -
 seatd/seat.c   | 51 ++++++++++++++++-----------------------------------
 2 files changed, 16 insertions(+), 36 deletions(-)

diff --git a/include/seat.h b/include/seat.h
index fa5873c..cc243b6 100644
--- a/include/seat.h
+++ b/include/seat.h
@@ -33,7 +33,6 @@ struct seat {
 	struct client *next_client;
 
 	bool vt_bound;
-	int cur_ttyfd;
 	int cur_vt;
 	int session_cnt;
 };
diff --git a/seatd/seat.c b/seatd/seat.c
index 9ad4af2..842928e 100644
--- a/seatd/seat.c
+++ b/seatd/seat.c
@@ -19,7 +19,7 @@
 #include "terminal.h"
 
 static int seat_close_client(struct client *client);
-static void vt_close(struct seat *seat);
+static int vt_close(int vt);
 
 struct seat *seat_create(const char *seat_name, bool vt_bound) {
 	struct seat *seat = calloc(1, sizeof(struct seat));
@@ -30,7 +30,6 @@ struct seat *seat_create(const char *seat_name, bool vt_bound) {
 	seat->vt_bound = vt_bound;
 	seat->seat_name = strdup(seat_name);
 	seat->cur_vt = 0;
-	seat->cur_ttyfd = -1;
 	if (seat->seat_name == NULL) {
 		free(seat);
 		return NULL;
@@ -50,7 +49,6 @@ void seat_destroy(struct seat *seat) {
 		assert(client->seat == seat);
 		client_destroy(client);
 	}
-	vt_close(seat);
 	linked_list_remove(&seat->link);
 	free(seat->seat_name);
 	free(seat);
@@ -66,47 +64,30 @@ static void seat_update_vt(struct seat *seat) {
 	close(tty0fd);
 }
 
-static int vt_open(struct seat *seat, int vt) {
+static int vt_open(int vt) {
 	assert(vt != -1);
-	if (seat->cur_ttyfd != -1) {
-		terminal_set_process_switching(seat->cur_ttyfd, true);
-		close(seat->cur_ttyfd);
-	}
-	seat->cur_ttyfd = terminal_open(vt);
-	if (seat->cur_ttyfd == -1) {
+	int ttyfd = terminal_open(vt);
+	if (ttyfd == -1) {
 		log_errorf("Could not open terminal for VT %d: %s", vt, strerror(errno));
 		return -1;
 	}
 
-	terminal_set_process_switching(seat->cur_ttyfd, true);
-	terminal_set_keyboard(seat->cur_ttyfd, false);
-	terminal_set_graphics(seat->cur_ttyfd, true);
+	terminal_set_process_switching(ttyfd, true);
+	terminal_set_keyboard(ttyfd, false);
+	terminal_set_graphics(ttyfd, true);
+	close(ttyfd);
 	return 0;
 }
 
-static void vt_close_fd(int fd) {
-	terminal_set_process_switching(fd, true);
-	terminal_set_keyboard(fd, true);
-	terminal_set_graphics(fd, false);
-}
-
-static void vt_close(struct seat *seat) {
-	if (seat->cur_ttyfd == -1) {
-		return;
-	}
-
-	vt_close_fd(seat->cur_ttyfd);
-	close(seat->cur_ttyfd);
-	seat->cur_ttyfd = -1;
-}
-
-static int vt_close_num(int vt) {
+static int vt_close(int vt) {
 	int ttyfd = terminal_open(vt);
 	if (ttyfd == -1) {
 		log_errorf("Could not open terminal to clean up VT %d: %s", vt, strerror(errno));
 		return -1;
 	}
-	vt_close_fd(ttyfd);
+	terminal_set_process_switching(ttyfd, true);
+	terminal_set_keyboard(ttyfd, true);
+	terminal_set_graphics(ttyfd, false);
 	close(ttyfd);
 	return 0;
 }
@@ -452,7 +433,7 @@ int seat_open_client(struct seat *seat, struct client *client) {
 		return -1;
 	}
 
-	if (seat->vt_bound && vt_open(seat, client->session) == -1) {
+	if (seat->vt_bound && vt_open(client->session) == -1) {
 		log_error("Could not open VT for client");
 		goto error;
 	}
@@ -477,7 +458,7 @@ int seat_open_client(struct seat *seat, struct client *client) {
 
 error:
 	if (seat->vt_bound) {
-		vt_close(seat);
+		vt_close(seat->cur_vt);
 	}
 	return -1;
 }
@@ -506,12 +487,12 @@ static int seat_close_client(struct client *client) {
 			// This client was current, but there were no clients
 			// waiting to take this VT, so clean it up.
 			log_debug("Closing active VT");
-			vt_close(seat);
+			vt_close(seat->cur_vt);
 		} else if (!was_current && client->state != CLIENT_CLOSED) {
 			// This client was not current, but as the client was
 			// running, we need to clean up the VT.
 			log_debug("Closing inactive VT");
-			vt_close_num(client->session);
+			vt_close(client->session);
 		}
 	}
 
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