Re: [PATCH 3/4] x86/tdx: Relax SEPT_VE_DISABLE check for debug TD

[Kirill A. Shutemov]

On Fri, Dec 09, 2022 at 07:45:34AM -0800, Sathyanarayanan Kuppuswamy wrote:
> 
> 
> On 12/9/22 5:25 AM, 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.
> > 
> > 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@xxxxxxxxxxxxxxx>
> > ---
> >  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.");
> 
> Why add this change part of TD debug check? Should this be a separate patch?

This code is never reachable if ATTR_SEPT_VE_DISABLE is set. And the panic
provides backtrace useful for debug.


> 
> >  		return handle_mmio(regs, ve);
> >  	case EXIT_REASON_IO_INSTRUCTION:
> >  		return handle_io(regs, ve);
> 
> -- 
> Sathyanarayanan Kuppuswamy
> Linux Kernel Developer

-- 
  Kiryl Shutsemau / Kirill A. Shutemov