Re: [PATCHv2 2/3] x86/tdx: Fix race between set_memory_encrypted() and load_unaligned_zeropad()

[Kirill A. Shutemov]

On Fri, May 26, 2023 at 03:10:56PM -0700, Sathyanarayanan Kuppuswamy wrote:
> 
> 
> On 5/26/23 5:02 AM, Kirill A. Shutemov wrote:
> > Touching privately mapped GPA that is not properly converted to private
> > with MapGPA and accepted leads to unrecoverable exit to VMM.
> > 
> > load_unaligned_zeropad() can touch memory that is not owned by the
> > caller, but just happened to next after the owned memory.
> 
> /s/to/to be ?

Yep, my bad.

> > This load_unaligned_zeropad() behaviour makes it important when kernel
> > asks VMM to convert a GPA from shared to private or back. Kernel must
> > never have a page mapped into direct mapping (and aliases) as private
> > when the GPA is already converted to shared or when GPA is not yet
> > converted to private.
> 
> I am wondering whether this issue exist in the AMD code? 
> 
> IMO, you can add some info on the window in set_memory_encrypted()
> where this race exists.

I don't think AMD affected by load_unaligned_zeropad() the same way as
Intel does. But I'm not sure.

Tom, do you have any comments?

> 
> > 
> > guest.enc_status_change_prepare() called before adjusting direct mapping
> > and therefore it is responsible for converting the memory to private.
> > 
> > guest.enc_status_change_finish() called after adjusting direct mapping
> > and it converts the memory to shared.
> > 
> > It is okay to have a shared mapping of memory that is not converted
> > properly. handle_mmio() knows how to deal with load_unaligned_zeropad()
> > stepping on it.
> 
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Fixes: 7dbde7631629 ("x86/mm/cpa: Add support for TDX shared memory")
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> >  arch/x86/coco/tdx/tdx.c | 56 ++++++++++++++++++++++++++++++++++++++---
> >  1 file changed, 53 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> > index e146b599260f..59cc13e41aa6 100644
> > --- a/arch/x86/coco/tdx/tdx.c
> > +++ b/arch/x86/coco/tdx/tdx.c
> > @@ -840,6 +840,30 @@ static bool tdx_enc_status_changed(unsigned long vaddr, int numpages, bool enc)
> >  	return true;
> >  }
> >  
> > +static bool tdx_enc_status_change_prepare(unsigned long vaddr, int numpages,
> > +					  bool enc)
> > +{
> > +	/*
> > +	 * Only handle shared->private conversion here.
> > +	 * See the comment in tdx_early_init().
> > +	 */
> > +	if (enc)
> > +		return tdx_enc_status_changed(vaddr, numpages, enc);
> > +	return true;
> > +}
> > +
> > +static bool tdx_enc_status_change_finish(unsigned long vaddr, int numpages,
> > +					 bool enc)
> > +{
> > +	/*
> > +	 * Only handle private->shared conversion here.
> > +	 * See the comment in tdx_early_init().
> > +	 */
> > +	if (!enc)
> > +		return tdx_enc_status_changed(vaddr, numpages, enc);
> > +	return true;
> > +}
> > +
> >  void __init tdx_early_init(void)
> >  {
> >  	u64 cc_mask;
> > @@ -867,9 +891,35 @@ void __init tdx_early_init(void)
> >  	 */
> >  	physical_mask &= cc_mask - 1;
> >  
> > -	x86_platform.guest.enc_cache_flush_required = tdx_cache_flush_required;
> > -	x86_platform.guest.enc_tlb_flush_required   = tdx_tlb_flush_required;
> 
> I think you don't need to change the order here.

I wanted to emphasise that the comment is for _prepare/_finish callbacks
and I hoped re-order would help with this.

> > -	x86_platform.guest.enc_status_change_finish = tdx_enc_status_changed;
> > +	/*
> > +	 * Touching privately mapped GPA that is not properly converted to
> > +	 * private with MapGPA and accepted leads to unrecoverable exit
> > +	 * to VMM.
> > +	 *
> > +	 * load_unaligned_zeropad() can touch memory that is not owned by
> > +	 * the caller, but just happened to next after the owned memory.
> > +	 * This load_unaligned_zeropad() behaviour makes it important when
> > +	 * kernel asks VMM to convert a GPA from shared to private or back.
> > +	 * Kernel must never have a page mapped into direct mapping (and
> > +	 * aliases) as private when the GPA is already converted to shared or
> > +	 * when GPA is not yet converted to private.
> > +	 *
> > +	 * guest.enc_status_change_prepare() called before adjusting direct
> > +	 * mapping and therefore it is responsible for converting the memory
> > +	 * to private.
> > +	 *
> > +	 * guest.enc_status_change_finish() called after adjusting direct
> > +	 * mapping and it converts the memory to shared.
> > +	 *
> > +	 * It is okay to have a shared mapping of memory that is not converted
> > +	 * properly. handle_mmio() knows how to deal with load_unaligned_zeropad()
> > +	 * stepping on it.
> > +	 */
> > +	x86_platform.guest.enc_status_change_prepare = tdx_enc_status_change_prepare;
> > +	x86_platform.guest.enc_status_change_finish  = tdx_enc_status_change_finish;
> > +
> > +	x86_platform.guest.enc_cache_flush_required  = tdx_cache_flush_required;
> > +	x86_platform.guest.enc_tlb_flush_required    = tdx_tlb_flush_required;
> >  
> >  	pr_info("Guest detected\n");
> >  }
> 
> -- 
> Sathyanarayanan Kuppuswamy
> Linux Kernel Developer

-- 
  Kiryl Shutsemau / Kirill A. Shutemov