AXObjectCache::characterOffsetFromVisiblePosition can deref a nullptr when underlying renderer is destroyed

[Tyler Wilcock]

AXObjectCache::characterOffsetFromVisiblePosition creates an AX object from the node backing a VisiblePosition at the beginning of the method. Then, it does non-trivial work that could cause the renderer backing the AX object to be destroyed, and afterwards unconditionally deferences that AX object's node() at the end of the method. This can cause a null pointer dereference crash (because AccessibilityRenderObject::node() depends on a non-null renderer), and is generally poor pointer hygiene.

[Radar WebKit Bug Importer]

<rdar://problem/107459184>

[Tyler Wilcock]

rdar://103456792

[Tyler Wilcock]

Created attachment 465696 [details]
Patch

[chris fleizach]

Do we still want to check that this is NOT null first?

 deepPos.deprecatedNode();

[Tyler Wilcock]

(In reply to chris fleizach from comment #4)
> Do we still want to check that this is NOT null first?
> 
>  deepPos.deprecatedNode();
We should be safe because if `deepPos.deprecatedNode()` were null, this check just above dereferencing it would return:

if (visiblePos.isNull())
   return CharacterOffset();

[EWS]

Committed 262432@main (7d93b07962d5): <https://commits.webkit.org/262432@main>

All reviewed patches have been landed. Closing bug and clearing flags on attachment 465696 [details].