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 <psychon@znc.in>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/cairo-xlib-surface.c b/src/cairo-xlib-surface.c
index 029a542b9..ffea476f6 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);
     }