From c088ba1faab9579efdaed7a524124901a17801b0 Mon Sep 17 00:00:00 2001 From: Uli Schlachter Date: Sat, 18 Jun 2016 15:08:52 +0200 Subject: [PATCH] xlib: Fix double free in _get_image_surface() If XShmGetImage() fails, the code tries to continue with its normal, non-shared-memory path. However, the image variable, which was previously set to NULL, now points to an already-destroyed surface, causing a double-free when the function cleans up after itself (actually, its an assertion failure because the reference count of the surface is zero, but technically this is still a double free). Fix this by setting image=NULL after destroying the surface that this refers to, to make sure this surface will not be destroyed again. While we are here (multiple changes in a single commit are bad...), also fix the cleanup done in bail. In practice, &image->base should be safe when image==NULL, because this just adds some offset to the pointer (the offset here is actually zero, so this doesn't do anything at all). However, the C standard does not require this to be safe, so let's handle this case specially. Note that anything that is fixed by this change is still buggy, because the only reason why XShmGetImage() could fail would be BadDrawable, meaning that the target we draw to does not exist or was already destroyed. This patch will likely just cause X11 errors elsewhere and drawing to (possible) invalid drawables is not supported by cairo anyway. This means that if SHM fails, the following fallback code has a high chance of failing, too. Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=91967 Signed-off-by: Uli Schlachter --- src/cairo-xlib-surface.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/src/cairo-xlib-surface.c b/src/cairo-xlib-surface.c index 3f407c3..555c1fe 100644 --- a/src/cairo-xlib-surface.c +++ b/src/cairo-xlib-surface.c @@ -807,6 +807,7 @@ _get_image_surface (cairo_xlib_surface_t *surface, } cairo_surface_destroy (&image->base); + image = NULL; } } @@ -1011,7 +1012,8 @@ _get_image_surface (cairo_xlib_surface_t *surface, cairo_device_release (&display->base); if (unlikely (status)) { - cairo_surface_destroy (&image->base); + if (image) + cairo_surface_destroy (&image->base); return _cairo_surface_create_in_error (status); } -- 2.8.1