riched20: Avoid releasing a non-existent interface.

[Nikolay Sivov]

On 4/16/2014 17:37, Jactry Zeng wrote:
> Hi Nikolay,
>
> Thanks for your review!
>
> The crash can be reproduced follow this:
> - first release the ITextSelection or IOleClientSite 
> interfaces completely;
> - release ITextDocument interface;
> - try to release the IRichEditOle (crash happen)
>
> And this patch try to fix it.
>
> +  /* hres = ITextDocument_GetSelection(txtDoc, &txtSel); */
> +  /* ok(hres == S_OK, "ITextDocument_GetSelection\n"); */
> +  /* while(ITextSelection_Release(txtSel)); */
What you're doing is a violation of refcount handling. The rule is to 
release what you got, without relying on internals like that. In this 
case GetSelection() returns interface pointer and you're responsible in 
exactly one Release() on it.

Interesting thing to test would be to check if GetSelection() returns 
new instance every time it's called. If this is a case it will justify 
some code changes to support this, right now patch is wrong.

If it actually returns same interface pointer you can't protect from 
use-after-free because I can grab multiple references with several 
GetSelection() calls, and when I'll try to release them it will be freed 
already by a loop like that.