Messages in this thread | | | Date | Mon, 1 Nov 2021 10:01:55 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] static_call,x86: Robustify trampoline patching |
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On Mon, Nov 01, 2021 at 12:36:18AM +0100, Ard Biesheuvel wrote: > On Sun, 31 Oct 2021 at 21:45, Peter Zijlstra <peterz@infradead.org> wrote: > > > > On Sun, Oct 31, 2021 at 09:21:56PM +0100, Ard Biesheuvel wrote: > > > > > That means we can support static calls on arm64 now without breaking > > > Clang CFI, and work on a solution for the redundant jumps on a more > > > relaxed schedule. > > > > Yes, arm64 has a 'problem' with having already merged the clang-cfi > > stuff :/ > > > > I'm hoping the x86 solution can be an alternative CFI scheme, I'm > > starting to really hate this one. And I'm not at all convinced the > > proposed scheme is the best possible scheme given the constraints of > > kernel code. AFAICT it's a compromise made in userspace. > > Your scheme only works with IBT: the value of %r11 is under the > adversary's control so it could just point it at 'foo+0x10' if it > wants to call foo indirectly, and circumvent the check. So without IBT > (or BTI), I think the check fundamentally belongs in the caller, not > in the callee.
How is that not true for the jump table approach? Like I showed earlier, it is *trivial* to reconstruct the actual function pointer from a jump-table entry pointer.
In any case, I really want the discussion to start at square one, and show/explain why any chosen CFI scheme is actually good for the kernel. Just because clang happened to have implemented it, doesn't make it the most suitable scheme for the kernel.
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