Subject: Fixes a crash in the Haiku font driver for daemon mode

From: Kyle Ambroff-Kao <[email-masked]>
Date: Wed, 02 Apr 2025 23:33:54 -0700

Tags: patch

Fix use-after-free bug in the Haiku font driver

* src/haikufont.c: Set objects freed with haikufont_close to NULL so
  they will not be reused, which seems to happen in daemon mode when all
  frames have been closed and fonts are garbage collected.

In GNU Emacs 30.1 (build 2, amd64-portbld-freebsd15.0, GTK+ Version
3.24.48, cairo version 1.18.2)
System Description: 15.0-CURRENT

Configured using:
 'configure --disable-build-details --localstatedir=/var --without-gconf
 --without-libsystemd --without-selinux --with-x --enable-acl
 --with-cairo --with-dbus --with-gif --with-gnutls --with-gsettings
 --with-x-toolkit=gtk3 --with-harfbuzz --with-jpeg
 --with-file-notification=kqueue --with-lcms2 --without-m17n-flt
 --without-imagemagick --with-mailutils --with-modules
 --with-native-compilation=aot --with-sound=oss --without-libotf
 --without-pgtk --with-png --with-toolkit-scroll-bars --with-sqlite3
 --with-rsvg --with-threads --with-tiff --with-tree-sitter --with-webp
 --without-xft --with-xim --with-xml2 --with-xpm --without-xwidgets
 --x-libraries=/usr/local/lib --x-includes=/usr/local/include
 --prefix=/usr/local --mandir=/usr/local/share/man
 --disable-silent-rules --infodir=/usr/local/share/emacs/info/
 --build=amd64-portbld-freebsd15.0 'CFLAGS=-O2 -pipe
 -fstack-protector-strong -Wl,-rpath=/usr/local/lib/gcc13 -isystem
 /usr/local/include -fno-strict-aliasing ' 'CPPFLAGS=-isystem
 /usr/local/include' 'LDFLAGS= -fstack-protector-strong
 -Wl,-rpath=/usr/local/lib/gcc13 -L/usr/local/lib/gcc13 -L/usr/local/lib
 ''

commit 05846e17841fce3dbbb8e15fe11d38fe44b3e5e5
Author: Kyle Ambroff-Kao <[email-masked]>
Date:   Wed Apr 2 22:34:26 2025 -0700

    Fix use-after-free bug in the Haiku font driver
    
    This fixes a bug in Emacs daemon mode on Haiku. To reproduce:
    
    1. Start emacs with "emacs --daemon"
    2. Create a new frame with "emacsclient -c" and then close it.
    3. Create a new frame with "emacsclient -c"
    
    Step 3 will cause the Emacs daemon to crash.
    
      KERN: debug_server: Thread 3616 entered the debugger: Debugger call:
      `tried to free 0xb960bc9fd0 which points at page 232 which is not an
      allocation first page'
    
    The backtrace from Emacs:
       heap_free(void*) + 0x35
       BFont_close + 0x4d
       haikufont_close(font*) + 0x29 (/Code/emacs/src/haikufont.c:893)
       sweep_vectors(void) + 0x1af (/Code/emacs/src/alloc.c:3242)
       garbage_collect(void) + 0x7b3 (/Code/emacs/src/alloc.c:7247)
       Ffuncall(ptrdiff_t, Lisp_Object*) + 0x194 (/Code/emacs/src/eval.c:3084)
       internal_condition_case_n(*, ptrdiff_t, Lisp_Object*, Lisp_Object, *)
       + 0x6c (/Code/emacs/src/eval.c:1699)
       safe_funcall(ptrdiff_t, Lisp_Object*) + 0x50 (/Code/emacs/src/eval.c:3114)
       map_keymap_canonical(Lisp_Object,map_keymap_function_t,Lisp_Object,void*)
       + 0x2b (/Code/emacs/src/keymap.c:608)
       ...
    
    It appears that the BFont has already been closed. I think that the
    driver is holding on to the pointer to the freed BFont
    (into->be_font). This patch addresses this by setting be_font to NULL so
    that this pointer will not be freed again.
    
    The same thing applies to info->metrics and info->glyphs, since just
    making this change to be_font wasn't enough to avoid crashes.
    
    With this patch I can open and close as many frames as I want without
    crashing.
    
    I don't totally understand the interactions here, and I see there are
    similar bugs in other font drivers with different workarounds. For
    example, in https://debbugs.gnu.org/cgi/bugreport.cgi?bug=16069 which I
    found from xfont.c:xfont_close, it seems like there is an attempt to
    just not free the fonts when GC is invoked.
    
    I think the solution in this patch seems a little simpler, but possibly
    means that the fonts are initialized every time the frame count goes
    from 0 to 1 or more instead of just once for the life of the daemon.

diff --git a/src/haikufont.c b/src/haikufont.c
index 7522b92207fa..72dfcc4aa3bf 100644
--- a/src/haikufont.c
+++ b/src/haikufont.c
@@ -890,25 +890,45 @@ haikufont_close (struct font *font)
     return;
 
   block_input ();
-  if (info && info->be_font)
-    BFont_close (info->be_font);
 
-  for (i = 0; i < info->metrics_nrows; i++)
+  if (info)
     {
-      if (info->metrics[i])
-	xfree (info->metrics[i]);
+      if (info->be_font)
+        {
+          BFont_close (info->be_font);
+          info->be_font = NULL;
+        }
+
+      if (info->metrics)
+        {
+          for (i = 0; i < info->metrics_nrows; i++)
+            {
+              if (info->metrics[i])
+                {
+                  xfree (info->metrics[i]);
+                  info->metrics[i] = NULL;
+                }
+            }
+
+          xfree (info->metrics);
+          info->metrics = NULL;
+        }
+
+      if (info->glyphs)
+        {
+          for (i = 0; i < 0x100; ++i)
+            {
+              if (info->glyphs[i])
+                {
+                  xfree (info->glyphs[i]);
+                }
+            }
+
+          xfree (info->glyphs);
+          info->glyphs = NULL;
+        }
     }
 
-  if (info->metrics)
-    xfree (info->metrics);
-
-  for (i = 0; i < 0x100; ++i)
-    {
-      if (info->glyphs[i])
-	xfree (info->glyphs[i]);
-    }
-
-  xfree (info->glyphs);
   unblock_input ();
 }