linux-kernel - Re: [PATCH 3/4] x86/tdx: Relax SEPT_VE

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Date:   Tue, 13 Dec 2022 15:13:43 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Elena Reshetova <elena.reshetova@...el.com>, x86@...nel.org,
        linux-coco@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] x86/tdx: Relax SEPT_VE_DISABLE check for debug TD

On 12/9/22 05:25, Kirill A. Shutemov wrote:
> SEPT_VE_DISABLE check is required to keep the TD protected from VMM
> attacks, but it makes harder to debug guest kernel bugs. If guest
> touches unaccepted memory the TD will get terminated without any
> traces on what has happened.

This is a bit sparse.

--

A "SEPT #VE" occurs when a TDX guest touches memory that is not properly
mapped into the "secure EPT".  This can be the result of hypervisor
attacks or bugs, *OR* guest bugs.  Most notably, buggy guests might
touch unaccepted memory for lots of different memory safety bugs like
buffer overflows.

TDX guests do not want to continue in the face of hypervisor attacks or
hypervisor bugs.  They want to terminate as fast and safely as possible.
 SEPT_VE_DISABLE ensures that TDX guests *can't* continue in the face of
these kinds of issues.

But, that causes a problem.  TDX guests that can't continue can't spit
out oopses or other debugging info.  In essence SEPT_VE_DISABLE=1 guests
are not debuggable.  That's a problem.

--

Eh?

> Relax the SEPT_VE_DISABLE check to warning on debug TD and panic() in
> the #VE handler on EPT-violation on private memory. It will produce
> useful backtrace.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> ---
>  arch/x86/coco/tdx/tdx.c | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 8ad04d101270..0e47846ff8ff 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -38,6 +38,7 @@
>  #define VE_GET_PORT_NUM(e)	((e) >> 16)
>  #define VE_IS_IO_STRING(e)	((e) & BIT(4))
>  
> +#define ATTR_DEBUG		BIT(0)
>  #define ATTR_SEPT_VE_DISABLE	BIT(28)
>  
>  /* TDX Module call error codes */
> @@ -207,8 +208,15 @@ static void tdx_parse_tdinfo(u64 *cc_mask)
>  	 * TD-private memory.  Only VMM-shared memory (MMIO) will #VE.
>  	 */
>  	td_attr = out.rdx;
> -	if (!(td_attr & ATTR_SEPT_VE_DISABLE))
> -		tdx_panic("TD misconfiguration: SEPT_VE_DISABLE attribute must be set.");
> +	if (!(td_attr & ATTR_SEPT_VE_DISABLE)) {
> +		const char *msg = "TD misconfiguration: SEPT_VE_DISABLE attribute must be set.";
> +
> +		/* Relax SEPT_VE_DISABLE check for debug TD. */
> +		if (td_attr & ATTR_DEBUG)
> +			pr_warn("%s\n", msg);
> +		else
> +			tdx_panic(msg);
> +	}
>  }
>  
>  /*
> @@ -682,6 +690,8 @@ static int virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
>  	case EXIT_REASON_CPUID:
>  		return handle_cpuid(regs, ve);
>  	case EXIT_REASON_EPT_VIOLATION:
> +		if (ve->gpa != cc_mkdec(ve->gpa))
> +			panic("Unexpected EPT-violation on private memory.");

What's the cc_mkdec() doing?