Messages in this thread | | | Date | Fri, 16 Apr 2021 15:37:47 -0700 | From | Kees Cook <> | Subject | Re: [PATCH 05/15] x86: Implement function_nocfi |
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On Fri, Apr 16, 2021 at 03:20:17PM -0700, Andy Lutomirski wrote: > But obviously there is code that needs real function pointers. How > about making this a first-class feature, or at least hacking around it > more cleanly. For example, what does this do: > > char entry_whatever[]; > wrmsrl(..., (unsigned long)entry_whatever);
This is just casting. It'll still resolve to the jump table entry.
> or, alternatively, > > extern void func() __attribute__((nocfi));
__nocfi says func() should not perform checking of correct jump table membership for indirect calls.
But we don't want a global marking for a function to be ignored by CFI; we don't want functions to escape CFI -- we want specific _users_ to either not check CFI for indirect calls (__nocfi) or we want specific passed addresses to avoid going through the jump table (function_nocfi()).
So, instead of a cast, a wrapper is used to bypass instrumentation in the very few cases its needed.
(Note that such a wrapper is no-op without CFI enabled.)
-- Kees Cook
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