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

[Kirill A. Shutemov]

On Tue, Dec 13, 2022 at 03:13:43PM -0800, Dave Hansen wrote:
> 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?

Thanks!

> > 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.");
> 
> What's the cc_mkdec() doing?

Checks if the GPA is private. I will move it to helper. Like this:

static inline bool is_private_gpa(u64 gpa)
{
	return gpa == cc_mkenc(gpa);
}

-- 
  Kiryl Shutsemau / Kirill A. Shutemov