linux-kernel - Re: [PATCH 00/11] x86/ibt: FineIBT-BHI

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Message-ID: <CABCJKudA8aUf=SDsVOOsWX_Cq6LAcioOjgtpv+uq+WGwJbxFPA@mail.gmail.com>
Date: Mon, 10 Feb 2025 10:29:22 -0800
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, alyssa.milburn@...el.com, 
	scott.d.constable@...el.com, joao@...rdrivepizza.com, 
	andrew.cooper3@...rix.com, jpoimboe@...nel.org, jose.marchesi@...cle.com, 
	hjl.tools@...il.com, ndesaulniers@...gle.com, nathan@...nel.org, 
	ojeda@...nel.org, kees@...nel.org, alexei.starovoitov@...il.com, 
	mhiramat@...nel.org
Subject: Re: [PATCH 00/11] x86/ibt: FineIBT-BHI

Hi Peter,

On Fri, Feb 7, 2025 at 4:28 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> Hi!
>
> Respin of the FineIBT-BHI patches.
>
> Scott has managed to get LLVM bits merged:
>
>   https://github.com/llvm/llvm-project/commit/e223485c9b38a5579991b8cebb6a200153eee245
>
> Which prompted me to update these patches once again.
>
> They boot and build the next kernel on my ADL when booted with: cfi=fineibt+bhi
>
> Aside from the last two patches -- which implement the FineIBT-BHI scheme
> proper -- I'm planning on getting these patches merged 'soon'.
>
> Scott, what those last two patches need, aside from a lot more testing, is a
> coherent writeup of how the mitigation works and ideally also a few numbers
> proving the performance gains are worth it.
>
> Last version at:
>
>   https://lore.kernel.org/all/20240927194856.096003183@infradead.org/T/#u
>
> Current patches:
>
>   git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git x86/fineibt-bhi
>
> Patches apply on top of tip/master.

I gave this a spin with a ToT Clang; LKDTM 's CFI_FORWARD_PROTO test
now traps in __bhi_args_1 as expected, and the changes look good to
me. The is_endbr() clean-up also fixes the gendwarfksyms+ftrace build
issue reported earlier. Feel free to add:

Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>

One thing I realized is that CONFIG_CFI_PERMISSIVE doesn't actually do
anything when FineIBT is used since we lose track of CFI trap
locations. I'm not sure if that's worth fixing, but perhaps we could
disable FineIBT when permissive mode is enabled to avoid confusion?

Sami