linux-kernel - Re: [PATCH v2 5/8] x86/sev-es: Leave NMI-mode before sending signals

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Date:   Wed, 19 May 2021 19:54:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     x86@...nel.org, Hyunwook Baek <baekhw@...gle.com>,
        Joerg Roedel <jroedel@...e.de>, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Martin Radev <martin.b.radev@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v2 5/8] x86/sev-es: Leave NMI-mode before sending signals

On Wed, May 19, 2021 at 03:52:48PM +0200, Joerg Roedel wrote:
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -1343,9 +1343,10 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
>  		return;
>  	}
>  
> +	instrumentation_begin();
> +
>  	irq_state = irqentry_nmi_enter(regs);
>  	lockdep_assert_irqs_disabled();
> -	instrumentation_begin();
>  
>  	/*
>  	 * This is invoked through an interrupt gate, so IRQs are disabled. The

That's just plain wrong. No instrumentation is allowed before you enter
the exception context.

> @@ -1395,13 +1396,19 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
>  		BUG();
>  	}
>  
> -out:
> -	instrumentation_end();
>  	irqentry_nmi_exit(regs, irq_state);
> +	instrumentation_end();

And this can't be right either, same issue, no instrumentation is
allowed after you leave the exception context.

>  
>  	return;
>  
>  fail:
> +	/*
> +	 * Leave NMI mode - the GHCB is not busy anymore and depending on where
> +	 * the #VC came from this code is about to either kill the task (when in
> +	 * task context) or kill the machine.
> +	 */
> +	irqentry_nmi_exit(regs, irq_state);
> +

And this is wrong too; because at this point the handler doesn't run in
_any_ context anymore, certainly not one you can call regular C code
from.

>  	if (user_mode(regs)) {
>  		/*
>  		 * Do not kill the machine if user-space triggered the
> @@ -1423,7 +1430,9 @@ DEFINE_IDTENTRY_VC_SAFE_STACK(exc_vmm_communication)
>  		panic("Returned from Terminate-Request to Hypervisor\n");
>  	}
>  
> -	goto out;
> +	instrumentation_end();
> +
> +	return;
>  }


You either get to do what MCE does, or what MCE does. That is, either
use task_work or MCE_USER and have the _user() handler use
irqentry_enter_from_user_mode().

The above is an absolute no-go.