[PATCH 1/2] win32u: Guard font unused_entry against race condition.

[Rémi Bernon]

There is a race condition otherwise between release_gdi_font and
find_cached_gdi_font, leading to invalid memory access:

One thread calling release_gdi_font may decrement refcount to 0,
then try to enter font_lock. At the same time, another thread may be
calling find_cached_gdi_font through select_font, holding the font_lock.

This second thread would find refcount set to 0, and then try to remove
unused_entry from its list, although it hasn't been added yet to the
unused list.

Signed-off-by: Rémi Bernon <rbernon at codeweavers.com>
---

The crash can be easily reproduced by running a simple program which
only creates a window then exits, in a loop, shutting down the prefix
every time. It randomly causes a segmentation fault.

For some reason, this race condition is only present after
fd675485be8686ec5e3b4667d3f3e9739a4a07ce, possibly because of the
regression described in next patch.

 dlls/win32u/font.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/dlls/win32u/font.c b/dlls/win32u/font.c
index d2f61526540..f9a0359e92c 100644
--- a/dlls/win32u/font.c
+++ b/dlls/win32u/font.c
@@ -2576,22 +2576,24 @@ static struct gdi_font *find_cached_gdi_font( const LOGFONTW *lf, const FMAT2 *m
 static void release_gdi_font( struct gdi_font *font )
 {
     if (!font) return;
-    if (--font->refcount) return;
 
     TRACE( "font %p\n", font );
 
     /* add it to the unused list */
     pthread_mutex_lock( &font_lock );
-    list_add_head( &unused_gdi_font_list, &font->unused_entry );
-    if (unused_font_count > UNUSED_CACHE_SIZE)
+    if (!--font->refcount)
     {
-        font = LIST_ENTRY( list_tail( &unused_gdi_font_list ), struct gdi_font, unused_entry );
-        TRACE( "freeing %p\n", font );
-        list_remove( &font->entry );
-        list_remove( &font->unused_entry );
-        free_gdi_font( font );
+        list_add_head( &unused_gdi_font_list, &font->unused_entry );
+        if (unused_font_count > UNUSED_CACHE_SIZE)
+        {
+            font = LIST_ENTRY( list_tail( &unused_gdi_font_list ), struct gdi_font, unused_entry );
+            TRACE( "freeing %p\n", font );
+            list_remove( &font->entry );
+            list_remove( &font->unused_entry );
+            free_gdi_font( font );
+        }
+        else unused_font_count++;
     }
-    else unused_font_count++;
     pthread_mutex_unlock( &font_lock );
 }
 
-- 
2.33.1