linux-kernel - Re: [PATCH v7 10/13] kexec: Secure Launch kexec SEXIT support

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Message-ID: <ZU6_uUe45qAx52mI@google.com>
Date:   Fri, 10 Nov 2023 15:41:45 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ross Philipson <ross.philipson@...cle.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-crypto@...r.kernel.org, iommu@...ts.linux-foundation.org,
        kexec@...ts.infradead.org, linux-efi@...r.kernel.org,
        dpsmith@...rtussolutions.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, ardb@...nel.org, mjg59@...f.ucam.org,
        James.Bottomley@...senpartnership.com, luto@...capital.net,
        nivedita@...m.mit.edu, kanth.ghatraju@...cle.com,
        trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v7 10/13] kexec: Secure Launch kexec SEXIT support

On Fri, Nov 10, 2023, Ross Philipson wrote:
> Prior to running the next kernel via kexec, the Secure Launch code
> closes down private SMX resources and does an SEXIT. This allows the
> next kernel to start normally without any issues starting the APs etc.
> 
> Signed-off-by: Ross Philipson <ross.philipson@...cle.com>
> ---
>  arch/x86/kernel/slaunch.c | 73 +++++++++++++++++++++++++++++++++++++++
>  kernel/kexec_core.c       |  4 +++
>  2 files changed, 77 insertions(+)
> 
> diff --git a/arch/x86/kernel/slaunch.c b/arch/x86/kernel/slaunch.c
> index cd5aa34e395c..32b0c24a6484 100644
> --- a/arch/x86/kernel/slaunch.c
> +++ b/arch/x86/kernel/slaunch.c
> @@ -523,3 +523,76 @@ void __init slaunch_setup_txt(void)
>  
>  	pr_info("Intel TXT setup complete\n");
>  }
> +
> +static inline void smx_getsec_sexit(void)
> +{
> +	asm volatile (".byte 0x0f,0x37\n"
> +		      : : "a" (SMX_X86_GETSEC_SEXIT));

SMX has been around for what, two decades?  Is open coding getsec actually necessary?

> +	/* Disable SMX mode */

Heh, the code and the comment don't really agree.  I'm guessing the intent of the
comment is referring to leaving the measured environment, but it looks odd.   If
manually setting SMXE is necessary, I'd just delete this comment, or maybe move
it to above SEXIT.

> +	cr4_set_bits(X86_CR4_SMXE);

Is it actually legal to clear CR4.SMXE while post-SENTER?  I don't see anything
in the SDM that says it's illegal, but allowing software to clear SMXE in that
case seems all kinds of odd.

> +
> +	/* Do the SEXIT SMX operation */
> +	smx_getsec_sexit();
> +
> +	pr_info("TXT SEXIT complete.\n");
> +}