[cairo-commit] src/cairo-xlib-surface.c

[Uli Schlachter]

 src/cairo-xlib-surface.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

New commits:
commit d69dd6b341594c338fa6c7b327fd7f201eb37bc1
Author: Uli Schlachter <psychon at znc.in>
Date:   Sat Jun 18 15:08:52 2016 +0200

    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 at znc.in>

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);
     }