linux-kernel - Re: Hang when booting guest kernels compiled with clang after SRSO mitigations

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Date:   Thu, 10 Aug 2023 06:40:56 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: Hang when booting guest kernels compiled with clang after SRSO
 mitigations

On Thu, Aug 10, 2023 at 03:32:16PM +0200, Borislav Petkov wrote:
> On Thu, Aug 10, 2023 at 06:27:06AM -0700, Nathan Chancellor wrote:
> > But my host kernel was compiled using GCC 13.2.0 from kernel.org for the
> > sake of testing to see if the compiler used to build the host kernel had
> > an impact on the problem and it did not.
> 
> Ok, now I'm confused.

Heh, so was I at first when I was doing my regular build and boot tests
of -next :P

> Lemme see if I understand it correctly:
> 
> host kernel:
>  - SRSO enabled
>  - built with gcc
> 
> guest kernel:
>  - built with clang
>  - SRSO not necessary
> 
> Is that the scenario?

Yes, that should be correct.

Host kernel string:

  6.5.0-rc5-00039-g138bcddb86d8 (nathan@...-arch.thelio-3990X) (x86_64-linux-gcc (GCC) 13.2.0, GNU ld (GNU Binutils) 2.41) #1 SMP PREEMPT_DYNAMIC Wed Aug  9 17:34:43 MST 2023

Guest kernel string:

  6.5.0-rc5 (nathan@...-arch.thelio-3990X) (ClangBuiltLinux clang version 16.0.6 (https://github.com/llvm/llvm-project 7cbf1a2591520c2491aa35339f227775f4d3adf6), GNU ld (GNU Binutils) 2.41.50.20230809) #1 SMP PREEMPT_DYNAMIC Wed Aug  9 16:54:33 MST 2023

> Anything else?

Shouldn't be. As I noted in the original email, it seems something
specific to the safe-ret mitigation as I don't see the problem with
ibpb, that would be a good canary for making sure that you see the same
behavior.

Cheers,
Nathan