Fix an use-after-free bug (#2522)

Fix the issue #2421 (https://github.com/i3/i3/issues/2421).

floating_enable() invokes tree_close_internal() to free con->parent.
After con->parent is freed in tree_close_internal() but before con->parent is reassigned by the caller, con->parent may be dereferenced and causes i3 crash.

The backtrace below is an example.
The already-freed pointer is dereferenced again through the pointer "focused" in x_push_changes().

Reassign con->parent before calling tree_close_internal() to fix this use-after-free bug.

0x0000000000416372 in con_get_workspace (con=0x7ab9c0) at ../i3/src/con.c:375
0x0000000000416103 in con_has_managed_window (con=0x7ab9c0) at ../i3/src/con.c:266
0x000000000042b413 in x_push_changes (con=0x78d190) at ../i3/src/x.c:1132
0x0000000l0004533e8 in tree_render () at ../i3/src/tree.c:504
0x0000000000452b4f in tree_close_internal (con=0x7b67c0, kill_window=DONT_KILL_WINDOW, dont_kill_parent=false, force_set_focus=false)
../i3/src/tree.c:314
0x00000000004196f0 in con_on_remove_child (con=0x7b67c0) at ../i3/src/con.c:1801
0x0000000000452eb7 in tree_close_internal (con=0x783840, kill_window=DONT_KILL_WINDOW, dont_kill_parent=false, force_set_focus=false)
../i3/src/tree.c:364
0x0000000000431516 in floating_enable (con=0x7ab9c0, automatic=false) at ../i3/src/floating.c:183
0x0000000000431eed in toggle_floating_mode (con=0x7ab9c0, automatic=false) at ../i3/src/floating.c:379
0x0000000000420d92 in cmd_floating (current_match=0x679a20 , cmd_output=0x679aa0 , floating_mode=0x7ab8c0 "toggle")
../i3/src/commands.c:1088
0x000000000043e5ae in GENERATED_call (call_identifier=60, result=0x679aa0 ) at include/GENERATED_command_call.h:486
0x000000000043ee19 in next_state (token=0x675d70 ) at ../i3/src/commands_parser.c:187
0x000000000043f2fb in parse_command (input=0x7b4fe0 "floating toggle", gen=0x0) at ../i3/src/commands_parser.c:308
0x00000000004125f8 in run_binding (bind=0x784260, con=0x0) at ../i3/src/bindings.c:792
0x000000000042bace in handle_key_press (event=0x7a01a0) at ../i3/src/key_press.c:33
0x000000000044e6aa in handle_event (type=2, event=0x7a01a0) at ../i3/src/handlers.c:1420
0x0000000000439533 in xcb_check_cb (loop=0x7ffff532f8e0, w=0x68c140, revents=32768) at ../i3/src/main.c:133
0x00007ffff5125d73 in ev_invoke_pending () from /usr/lib/x86_64-linux-gnu/libev.so.4
0x00007ffff51293de in ev_run () from /usr/lib/x86_64-linux-gnu/libev.so.4
0x0000000000439418 in ev_loop (loop=0x7ffff532f8e0, flags=0) at /usr/include/ev.h:835
0x000000000043d51d in main (argc=3, argv=0x7fffffffe0a8) at ../i3/src/main.c:913

diff --git a/src/floating.c b/src/floating.c
index 616602c02253a77c1459dbcabe8ca0037a6f2dca..0a8b6957c32635a5e0c7f7205d7db4584df28dc8 100644 (file)
--- a/src/floating.c
+++ b/src/floating.c
@@ -178,7 +178,10 @@ void floating_enable(Con *con, bool automatic) {
     if ((con->parent->type == CT_CON || con->parent->type == CT_FLOATING_CON) &&
         con_num_children(con->parent) == 0) {
         DLOG("Old container empty after setting this child to floating, closing\n");
-        tree_close_internal(con->parent, DONT_KILL_WINDOW, false, false);
+        Con *parent = con->parent;
+        /* clear the pointer before calling tree_close_internal in which the memory is freed */
+        con->parent = NULL;
+        tree_close_internal(parent, DONT_KILL_WINDOW, false, false);
     }
 
     char *name;
@@ -315,6 +318,7 @@ void floating_disable(Con *con, bool automatic) {
     const bool set_focus = (con == focused);
 
     Con *ws = con_get_workspace(con);
+    Con *parent = con->parent;
 
     /* 1: detach from parent container */
     TAILQ_REMOVE(&(con->parent->nodes_head), con, nodes);
@@ -323,7 +327,9 @@ void floating_disable(Con *con, bool automatic) {
     /* 2: kill parent container */
     TAILQ_REMOVE(&(con->parent->parent->floating_head), con->parent, floating_windows);
     TAILQ_REMOVE(&(con->parent->parent->focus_head), con->parent, focused);
-    tree_close_internal(con->parent, DONT_KILL_WINDOW, true, false);
+    /* clear the pointer before calling tree_close_internal in which the memory is freed */
+    con->parent = NULL;
+    tree_close_internal(parent, DONT_KILL_WINDOW, true, false);
 
     /* 3: re-attach to the parent of the currently focused con on the workspace
      * this floating con was on */

diff --git a/src/randr.c b/src/randr.c
index 51255619698fb7226b5b07d2293a0d57ea9ed136..e5dcddfc38146763b1c8e21043d73e1b09bcdef1 100644 (file)
--- a/src/randr.c
+++ b/src/randr.c
@@ -846,9 +846,11 @@ void randr_disable_output(Output *output) {
         }
 
         DLOG("destroying disappearing con %p\n", output->con);
-        tree_close_internal(output->con, DONT_KILL_WINDOW, true, false);
-        DLOG("Done. Should be fine now\n");
+        Con *con = output->con;
+        /* clear the pointer before calling tree_close_internal in which the memory is freed */
         output->con = NULL;
+        tree_close_internal(con, DONT_KILL_WINDOW, true, false);
+        DLOG("Done. Should be fine now\n");
     }
 
     output->to_be_disabled = false;