Re: [PATCH v2 00/14] x86: Add support for Clang CFI

[Sami Tolvanen]

On Tue, Aug 24, 2021 at 12:47 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Aug 23, 2021 at 10:13:04AM -0700, Sami Tolvanen wrote:
> > This series adds support for Clang's Control-Flow Integrity (CFI)
> > checking to x86_64. With CFI, the compiler injects a runtime
> > check before each indirect function call to ensure the target is
> > a valid function with the correct static type. This restricts
> > possible call targets and makes it more difficult for an attacker
> > to exploit bugs that allow the modification of stored function
> > pointers. For more details, see:
>
> If I understand this right; tp_stub_func() in kernel/tracepoint.c
> violates this (as would much of the HAVE_STATIC_CALL=n code, luckily
> that is not a valid x86_64 configuration).
>
> Specifically, we assign &tp_stub_func to tracepoint_func::func, but that
> function pointer is only ever indirectly called when cast to the
> tracepoint prototype:
>
>   ((void(*)(void *, proto))(it_func))(__data, args);
>
> (see DEFINE_TRACE_FN() in linux/tracepoint.h)
>
> This means the indirect function type and the target function type
> mismatch, resulting in that runtime check you added to trigger.

Thanks for pointing this out. Yes, that would clearly trip CFI.

Any concerns about just writing a magic value to the slot instead of
pointing it to a stub function, and checking for it before the call?

> Hitting tp_stub_func() at runtime is exceedingly rare, but possible.
>
> I realize this is strictly UB per C, but realistically any CDECL ABI
> requires that any function with arbitrary signature:
>
>   void foo(...)
>   {
>   }
>
> translates to the exact same code. Specifically on x86-64, the super
> impressive:
>
> foo:
>         RET
>
> And as such this works just fine. Except now you wrecked it.

Sure. Another option is to disable CFI for the functions that perform
the call, but I would rather avoid that whenever possible.

Sami